Atomic Structure & Chemical Bonding - Questions & Answers

1. PREVIOUS QUESTIONS

Short Answer Questions

1. From the symbol ⁴₂He for the element helium write – the mass number & the atomic number of the element.
Mass number = 4, Atomic number = 2.

2. Name the element – which does not contain any neutrons in its nucleus.
Hydrogen (Protium).

3. Elements X, Y, Z – have atomic numbers 6, 9 and 12 respectively. Which one : i] forms anion - negative ion; ii] forms cation - positive ion; iii] has 4 electrons in the outermost orbit.
i] Y (forms anion, valency -1)
ii] Z (forms cation, valency +2)
iii] X (has 4 electrons in its outermost orbit).

Long Answer Questions

1. Define the terms: atomic number, mass number and electron.
Atomic number: The number of protons present in the nucleus of an atom.
Mass number: The total number of protons and neutrons collectively present in the nucleus of an atom.
Electron: A negatively charged subatomic particle that revolves around the nucleus of an atom.

2. Sulphur has an atomic number of 16 and a mass number of 32. State the number of protons and neutrons present in the nucleus of sulphur.
Number of protons = 16.
Number of neutrons = Mass number - Atomic number = 32 - 16 = 16.

3. Give a simple diagram – to show the arrangement of the electrons in an atom of sulphur.
The electronic configuration of sulphur is 2, 8, 6. The atomic diagram features a central nucleus with 3 surrounding shells: the K shell has 2 electrons, the L shell has 8 electrons, and the M shell has 6 electrons.

4. ²⁴₁₂Mg & ²⁶₁₂Mg – are two isotopes of magnesium. Compare the atoms of these isotopes with respect to i] the composition of their nuclei. ii] their electronic configurations. iii] give the reason why the two isotopes of magnesium have different mass numbers.
i] ²⁴₁₂Mg has 12 protons and 12 neutrons. ²⁶₁₂Mg has 12 protons and 14 neutrons.
ii] Both have the same electronic configuration: 2, 8, 2.
iii] They have different mass numbers because their nuclei contain a different number of neutrons (12 and 14 respectively).

5. Chlorine is an element – of atomic number 17. It is a mixture of two isotopes having mass number of 35 & 37. i] What is meant by "atomic number of an element". What do you understand by an 'atom'. ii] Write down the electronic configuration of the chlorine atom. iii] State the number of protons, electrons and neutrons in the following isotopes: ³⁵₁₇Cl, ³⁷₁₇Cl. iv] Explain why the two atoms in (iii) above have the same chemical reactions. v] If molten magnesium chloride is electrolysed suggest a suitable electrode [anode].
i] Atomic number is the total number of protons present in the nucleus of an atom. An atom is the smallest particle of an element which can take part in a chemical reaction.
ii] 2, 8, 7.
iii] ³⁵₁₇Cl: Protons = 17, Electrons = 17, Neutrons = 18. ³⁷₁₇Cl: Protons = 17, Electrons = 17, Neutrons = 20.
iv] Both isotopes have the same number of valence electrons (7) and identical electronic configurations, determining identical chemical properties.
v] Carbon (graphite) electrode.

6. Five atoms are labelled V to Z. (V: Mass 40, At. No. 20; W: Mass 19, At. No. 9; X: Mass 7, At. No. 3; Y: Mass 16, At. No. 8; Z: Mass 28, At. No. 14). i] Which one of these atoms has an electronic configuration 2,7. ii] Write down the formula of the compound formed by atoms X & Y.
i] Atom W (Atomic number 9).
ii] X has valency +1 (Li), Y has valency -2 (O). The compound formula is X₂Y.

7. Ordinary chlorine gas has two isotopes : ³⁵₁₇Cl & ³⁷₁₇Cl in the ratio of 3:1. Calculate the relative atomic mass [atomic weight] of chlorine.
Relative atomic mass = [(35 × 3) + (37 × 1)] / 4 = (105 + 37) / 4 = 142 / 4 = 35.5.

8. The atom of aluminium is represented by ²⁷₁₃Al. State the number of a] electrons, b] protons, c] neutrons & d] the arrangement of electrons in the different orbits or shells in one atom of aluminium.
a] Electrons = 13
b] Protons = 13
c] Neutrons = 27 - 13 = 14
d] Arrangement: 2, 8, 3 (K=2, L=8, M=3).

9. Elements A, B & C – have atomic numbers 9, 20 & 10 respectively. i] State which one is (1) a nonmetal; (2) a metal, (3) chemically inert. ii] Write down the formula of the compound formed by two of the above elements.
i] (1) A (Fluorine) is a nonmetal. (2) B (Calcium) is a metal. (3) C (Neon) is chemically inert.
ii] Compound formed by A (valency -1) and B (valency +2) is BA₂.

10. Define : Proton, Electron, Neutron.
Proton: A positively charged subatomic particle present inside the nucleus of an atom.
Electron: A negatively charged subatomic particle revolving around the nucleus in fixed orbits.
Neutron: A neutral subatomic particle present in the nucleus, having a mass almost equal to that of a proton.

11. Write down the electronic configuration of the following : i] ²⁷₁₃X, ii] ³⁵₁₇Y. Write down the no. of electrons in X, the neutrons in Y & the formula of the compound formed by X & Y.
i] Configuration of X = 2, 8, 3.
ii] Configuration of Y = 2, 8, 7.
Number of electrons in X = 13. Number of neutrons in Y = 35 - 17 = 18. Formula of the compound formed = XY₃.

12. According to the Dalton's Atomic Theory – atoms of the same element are identical in all respects. But according to the Modern Atomic Theory, this postulate is proved wrong. Explain.
Modern Atomic Theory discovered isotopes, which are atoms of the same element that have the same atomic number but different mass numbers due to a different number of neutrons. Therefore, atoms of the same element are not identical in all physical respects (mass).

13. State what are isotopes.
Isotopes are atoms of the same element having the same atomic number but different mass numbers.

14. Write down : i] the mass number of the atom having 20 neutrons & 15 protons. ii] the number of neutrons in the nucleus of an atom having atomic number 17 & mass number 37.
i] Mass number = Protons + Neutrons = 15 + 20 = 35.
ii] Number of neutrons = Mass number - Atomic number = 37 - 17 = 20.

15. State what is a proton. What is the significance of the number of protons found in the atoms of each of the different elements.
A proton is a positively charged subatomic particle in the nucleus. The number of protons represents the atomic number, which uniquely identifies an element and governs its chemical properties via the number of electrons.

16. State the relation between – the number of protons and the number of electrons in an atom.
In a neutral atom, the number of protons is exactly equal to the number of electrons.

17. The electronic structure [configuration] of fluorine can be written as 2, 7. In a similar way give the electronic configuration of : i] aluminium, ii] phosphorus
i] Aluminium (Atomic No. 13) = 2, 8, 3.
ii] Phosphorus (Atomic No. 15) = 2, 8, 5.

18. State what would be the reason – for an element to have atoms with differing mass numbers.
The reason is the presence of differing numbers of neutrons in the nuclei of these atoms, forming isotopes.

19. Copy & complete the following table – relating to the atomic structure of some elements:
- Beryllium: Atomic Number = 4, Mass Number = 9, Protons = 4, Neutrons = 5, Electrons = 4.
- Fluorine: Atomic Number = 9, Mass Number = 19, Protons = 9, Neutrons = 10, Electrons = 9.
- Sodium: Atomic Number = 11, Mass Number = 23, Protons = 11, Neutrons = 12, Electrons = 11.
- Aluminium: Atomic Number = 13, Mass Number = 27, Protons = 13, Neutrons = 14, Electrons = 13.
- Phosphorus: Atomic Number = 15, Mass Number = 31, Protons = 15, Neutrons = 16, Electrons = 15.

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2. ADDITIONAL QUESTIONS

Short Answer Questions

1. Represent each of the following : (a) a proton 'p', (b) an electron 'e', (c) a neutron 'n' - in terms of its symbols showing the subscript and superscript values.
(a) ¹₊₁p    (b) ⁰_-1e    (c) ¹₀n

Long Answer Questions

1. State the main postulates – of Dalton's atomic theory. How does the modern atomic theory contradict and correlate with Dalton's atomic theory.
Postulates: Matter consists of indivisible atoms; atoms can neither be created nor destroyed; atoms of the same element are identical. Contradiction: Modern theory proves atoms are divisible into protons, neutrons, and electrons; atoms of the same element can differ in mass (isotopes). Correlation: Atoms are still recognized as the smallest units of matter taking part in chemical reactions.

2. State the experimental proofs which led to the discovery of – (a) electrons, (b) protons, (c) atomic nucleus, (d) neutrons.
(a) Electrons: Cathode ray tube experiments by William Crookes and J.J. Thomson.
(b) Protons: Discovery of positive (canal) rays in a discharge tube by Goldstein.
(c) Atomic nucleus: Alpha-particle scattering experiment on gold foil by Lord Rutherford.
(d) Neutrons: Experimental establishment by James Chadwick showing neutral particles with mass equal to a proton exist in the nucleus.

3. State the drawbacks – of Rutherford's atomic model correlating them with the postulates of Bohr's atomic model.
Rutherford's model could not explain the stability of an atom; an electron moving in a circular orbit should radiate energy and collapse into the nucleus. Bohr resolved this by proposing that electrons revolve in fixed 'energy levels' or orbits, without radiating or losing energy while remaining in those specific orbits.

4. State what is meant by the terms : (a) subatomic particles, (b) nucleus, (c) orbits, (d) atomic number, (e) mass number with reference to an atom.
(a) Subatomic particles: The basic constituents of an atom (protons, electrons, neutrons).
(b) Nucleus: The dense, positively charged central region of an atom containing protons and neutrons.
(c) Orbits: Definite imaginary paths around the nucleus where electrons revolve.
(d) Atomic number: The number of protons in the nucleus of an atom.
(e) Mass number: The total number of protons and neutrons in the nucleus.

5. State what are 'energy levels'? Explain the arrangement & distribution of electrons in the various shells with reference to an atom in general & to an atom of potassium ³⁹₁₉K with special reference to the 2n² rule.
Energy levels are fixed orbits around the nucleus where electrons revolve possessing a certain amount of energy. According to the 2n² rule, the maximum number of electrons in a shell is governed by the formula (K=2, L=8, M=18, N=32), but the outermost shell cannot hold more than 8 electrons. For Potassium (Atomic No. 19), the arrangement is K=2, L=8, M=8, N=1 (configuration 2, 8, 8, 1) rather than putting 9 in the M shell to satisfy the outer octet rule.

6. An element 'A' - has mass number 23 & atomic number 11. State the — (a) no. of neutrons in its shell, (b) electronic configuration of the element 'A'.
(a) Number of neutrons (in nucleus, not shell) = 23 - 11 = 12.
(b) Electronic configuration = 2, 8, 1.

7. The following elements U to Z are given : ₃U, ₆V, ₉W, ₁₄X, ₁₈Y, ₂₀Z. State the electronic configuration of each & state which are metals, non-metals or inert gases.
U (At. no. 3): 2, 1 (Metal)
V (At. no. 6): 2, 4 (Non-metal)
W (At. no. 9): 2, 7 (Non-metal)
X (At. no. 14): 2, 8, 4 (Metalloid/Non-metal)
Y (At. no. 18): 2, 8, 8 (Inert gas)
Z (At. no. 20): 2, 8, 8, 2 (Metal)

8. Draw the geometric atomic structure – of each of the following atoms showing the number of electrons, protons & neutrons in each of them : (a) ¹²₆C, (b) ²³₁₁Na, (c) ³¹₁₅P, (d) ³⁹₁₉K, (e) ⁴⁰₂₀Ca.
(a) C: p=6, n=6, e=6 (Config: 2, 4).
(b) Na: p=11, n=12, e=11 (Config: 2, 8, 1).
(c) P: p=15, n=16, e=15 (Config: 2, 8, 5).
(d) K: p=19, n=20, e=19 (Config: 2, 8, 8, 1).
(e) Ca: p=20, n=20, e=20 (Config: 2, 8, 8, 2).
(Geometric structure requires drawing central nucleus with corresponding p & n counts, surrounded by concentric circular shells distributing the respective electrons).

9. Define an 'isotope'. Give reasons why isotopes have same chemical but different physical properties.
Isotopes are atoms of the same element having the same atomic number but different mass numbers. They exhibit identical chemical properties because they have the same electronic configuration (same number of valence electrons). They show different physical properties due to differing atomic masses caused by a different number of neutrons.

10. Draw the geometric atomic structure – of the three isotopes of hydrogen & the two isotopes of chlorine.
Hydrogen Isotopes: Protium (nucleus p=1, orbit e=1), Deuterium (nucleus p=1, n=1, orbit e=1), Tritium (nucleus p=1, n=2, orbit e=1).
Chlorine Isotopes: ³⁵Cl (nucleus p=17, n=18, orbits 2, 8, 7), ³⁷Cl (nucleus p=17, n=20, orbits 2, 8, 7). (Diagrams depict corresponding central nucleus and electron shells).

11. State what are noble gases. Give a reason why noble gases have stable electronic configuration.
Noble gases are elements that have completely filled outermost shells. They are stable because they possess the maximum capacity of 8 electrons (octet) or 2 for helium (duplet) in their valence shell, rendering them chemically unreactive as they do not need to gain, lose, or share electrons.

12. Explain the reason for – chemical activity of an atom with reference to its electronic configuration.
Atoms of most elements have an unstable (incomplete) electronic configuration. The driving force for their chemical activity is the tendency to lose, gain, or share electrons to attain the stable electronic configuration (octet/duplet) of the nearest noble gas.

13. Differentiate between the terms — (a) Stable & unstable electronic configuration (b) Duplet & octet rule.
(a) A stable configuration has a complete valence shell (like noble gases), whereas an unstable configuration has an incomplete valence shell.
(b) The duplet rule refers to atoms attaining 2 electrons in their first shell (K shell) for stability. The octet rule refers to atoms attaining 8 electrons in their outermost shell for stability.

14. Explain the octet rule for formation of — (a) Sodium chloride from a sodium atom and a chlorine atom. (b) Nitrogen molecule from two nitrogen atoms.
(a) Sodium (2, 8, 1) transfers its 1 valence electron to Chlorine (2, 8, 7). This allows Sodium to attain a stable octet (2, 8) as a cation, and Chlorine to attain a stable octet (2, 8, 8) as an anion, forming an electrovalent bond.
(b) Two Nitrogen atoms (2, 5) each need 3 electrons to complete an octet. They achieve this by mutually sharing three pairs of electrons, forming a triple covalent bond resulting in a stable N₂ molecule.

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Multiple Choice Questions [MCQs] - Select the correct answer

1. The nucleus of element with 2 electrons in 'K' shell & 1 electron in 'M' shell .
(a) is ³⁹₁₉X (b) has 12 neutrons (c) has 19 protons (d) has equal no. of protons & neutrons
(b) has 12 neutrons (The element is Sodium with configuration 2, 8, 1, Atomic No. 11, Mass No. 23. Neutrons = 23 - 11 = 12).

2. Which of the following is correct in an element ^A_ZY.
(a) Number of neutrons = Z–A. (b) Number of protons = A (c) Z = sum of protons & electrons (d) A = sum of neutrons & electrons
None of the printed options perfectly match standard formulas except an intended misprint or adaptation. Generally, Number of neutrons = A - Z. Assuming (a) meant A - Z, but strictly, no option is perfectly accurate as printed. If standard convention applies, Z is protons, A is mass (protons+neutrons).

3. Assertion (A) : Elements ¹₁H, ²₁H, ³₁H - have different number of neutrons. Reason (R) : Isotopes have the same - atomic number.
(a) Both A & R are true – and R is the correct explanation of A.
(b) Both A & R are true – but R is not the correct explanation of A.
(c) A is true – but R is false
(d) A is false – but R is true.
(b) Both A & R are true – but R is not the correct explanation of A.

4. The element 'X' has same number of protons, neutrons & electrons, with same number of electrons in 'K' shell & 'N' shell.
The element –
(a) Is a non-metal with valency –2.
(b) Attains stable octet structure of the nearest inert gas 'Ar', by gain of two electrons in its valence shell.
(c) Is a metal with valency +2.
(d) X²⁺ has electronic configuration – 2, 8.
(c) Is a metal with valency +2 (Element is Calcium, 2, 8, 8, 2).

5. Elements ²⁰₁₀X & ⁴⁰₁₈Y
(a) Are highly reactive.
(b) Both have 8 electrons in 'M' shell.
(c) Are isotopes of the same element.
(d) Do not gain, lose or share electrons.
(d) Do not gain, lose or share electrons.

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High Order Thinking Questions

1. Four elements A, B, C, D are given :
A shows the presence of 20 neutrons, 17 protons and 17 electrons.
B shows the presence of 18 neutrons, 17 protons and 17 electrons.
C shows the presence of 10 neutrons, 9 protons and 10 electrons.
D shows the presence of 4 neutrons, 3 protons and 2 electrons.
State which of the above is — (a) an anion, (b) a cation, (c) a pair of isotopes.
Write the formula of the compound formed between D and C.
(a) Anion: C (has more electrons than protons).
(b) Cation: D (has more protons than electrons).
(c) Pair of isotopes: A and B (same protons, different neutrons).
Formula of compound: DC.

2. State the incorrect statement from the list given below:
(a) Modern atomic theory differs from Dalton's atomic theory – with inference to isotopes.
(b) On supplying energy initially to an atom, an electron may move from K shell to L shell of the atom.
(c) An atom 'X' having equal no. of protons & neutrons in its atom, has electronic configuration – 2,8,10.
(d) Atoms of the same element – having atomic mass 39 & 41 respectively, have the same electronic configuration.
(c) An atom 'X' having equal no. of protons & neutrons in its atom, has electronic configuration – 2,8,10 is incorrect.

3. Give reasons:
(a) An electron in K shell has the minimum energy.
Because the K shell is the closest orbit to the nucleus and electrons in shells closer to the nucleus possess lower energy.
(b) Mass number is slightly less than the actual atomic mass.
Because the actual atomic mass accounts for the fractional masses of protons, neutrons, and electrons, and represents an average taking naturally occurring isotopes into account.
(c) Noble gases are almost inert to chemical reactions.
Because their outermost valence shells are completely filled (stable octet or duplet), meaning they do not need to share, lose, or gain electrons.
(d) Nitrogen molecule has no lone pair of electrons, while ammonia molecule has one lone pair.
In ammonia (NH₃), the nitrogen atom shares 3 of its 5 valence electrons, leaving one unshared lone pair. In a nitrogen molecule (N₂), the atoms share 3 pairs of electrons, leaving one lone pair on each atom (if the text implies otherwise, it is a simplification error, but based on strict definitions, each N in N₂ has a lone pair).
(e) The octet & duplet arrangement seem to represent stability.
Because elements possessing these arrangements (noble gases) are chemically unreactive, indicating these configurations reflect the lowest, most stable energy state.

4. Match Column A with the correct reason in Column B.
Column A
(a) ²⁷₁₃Al
(b) ¹₁H
(c) Kr [Krypton]
(d) ³⁷₁₇Cl
(e) ³⁹₁₉K
Column B
1. Has 18 electrons in M shell
2. Has 7 electrons in M shell
3. Has 27 nucleons in its nucleus
4. Has 8 electrons in M shell
5. Has no neutrons in its nucleus
(a) Matches with 3.
(b) Matches with 5.
(c) Matches with 1.
(d) Matches with 2.
(e) Matches with 4.

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3. TEST – HIGH ORDER & CRITICAL THINKING QUESTIONS
UNIT TEST PAPER – 4 (Page 65)

Q.1 Select the correct answer from the answers in brackets to complete each sentence.
1. An element has electronic configuration 2,8,1 and 12 neutrons. Its mass no. is _______. [11/23/12]
An element has electronic configuration 2,8,1 and 12 neutrons. Its mass no. is 23.

2. The maximum number of electrons in M-shell is _______. [8/32/18]
The maximum number of electrons in M-shell is 18.

3. Isotopes have same _______. [no. of neutrons/electronic configuration/atomic masses].
Isotopes have same electronic configuration.

4. An _______. [atom/ion] is capable of independent existence in solution.
An ion is capable of independent existence in solution.

5. An atom with electronic configuration 2, 7 & mass number 19 will have _______ neutrons. [8/10/12]
An atom with electronic configuration 2, 7 & mass number 19 will have 10 neutrons.

Q.2 Give reasons for the following.
1. The physical properties of isotopes of the same element are not identical.
Because isotopes have different mass numbers (differing number of neutrons), and physical properties depend on atomic mass.

2. The mass number of an atom is slightly less than the actual atomic mass.
Because mass number is just the whole number sum of protons and neutrons, while actual atomic mass is the exact measured mass accounting for isotopic fractions and tiny masses of electrons.

3. The shells surrounding the nucleus of an atom are also called 'energy levels'.
Because an electron revolving in a specific shell possesses a certain, fixed amount of energy.

4. Helium is chemically extremely unreactive.
Because its only shell (K shell) is completely filled with 2 electrons (stable duplet structure).

5. Mass number is slightly less than the actual atomic mass.
(Same as Q.2.2) Because atomic mass is the weighted fractional average of all isotopes, while mass number is a simple integer approximation for a single atom.

Q.3 Differentiate between the following terms.
1. Electron and proton
An electron is a negatively charged particle revolving outside the nucleus. A proton is a positively charged particle located inside the nucleus.

2. Atomic number and mass number
Atomic number is the total number of protons in an atom. Mass number is the total sum of protons and neutrons in the nucleus.

3. Nucleus and nucleons
The nucleus is the dense central region of an atom. Nucleons are the particles (protons and neutrons) residing within the nucleus.

4. Valence shell and penultimate shell
The valence shell is the outermost electron shell of an atom. The penultimate shell is the shell immediately preceding (inside) the outermost shell.

5. Octet and duplet
Octet is a stable arrangement of 8 electrons in the outermost shell. Duplet is a stable arrangement of 2 electrons in the outermost (and first) shell, like in Helium.

Q.4 Name or state the following.
1. The three isotopes of hydrogen.
Protium, Deuterium, and Tritium.

2. Two elements having same number of protons and electrons but different number of neutrons.
Isotopes of an element (e.g., Chlorine-35 and Chlorine-37).

3. The valency of an element whose electronic configuration is 2, 8, 3.
+3.

4. The shell closest to the nucleus of an atom.
K shell (or first shell).

5. An element having valency 'zero'.
Helium (or Neon, Argon - any noble gas).

Q.5 State the number of neutrons in each of the atoms A to E. Also state which of the atoms A to E is a metal.
A: At. no.1; Mass no.1
B: At. no.8; Mass no.16
C: At. no.7; Mass no.14
D: At. no.11; Mass no.23
E: At. no.17; Mass no.35
Neutrons in A = 0; B = 8; C = 7; D = 12; E = 18.
Atom D (Sodium) is a metal.

Q.6 Match the elements A to E in List 1 with their valencies in List 2 and with their nature in List 3.
List 1 [Elements]
A: At no. 7, Mass no. 14
B: Elec. config. 2,8
C: Neutrons 14, electrons 13
D: Neutrons 22, protons 18
E: Elec. config. 2,8,1
List 2 [Valency]: 1. -3   2. +1   3. +3   4. +2   5. 0
List 3 [Nature]: X: Metal   Y: Non-metal   Z: Noble gas
A matches with 1 (-3) and Y (Non-metal).
B matches with 5 (0) and Z (Noble gas).
C matches with 3 (+3) and X (Metal).
D matches with 5 (0) and Z (Noble gas).
E matches with 2 (+1) and X (Metal).

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1. PREVIOUS QUESTIONS (Page 77)

Short Answer Questions

1. Compound X consists of molecules. i] Bonding in X will be:– A: ionic. B: electrovalent. C: covalent. D: molecular.
C: covalent.

2. State what is the term defined in :– a bond form by a shared pair electrons each bonding atom contributing one electron to the pair.
Covalent bond.

3. The molecule with a triple covalent bond is: A: ammonia B: methane C: water D: nitrogen
D: nitrogen.

Long Answer Questions

1. State what is a – lone pair of electrons.
A lone pair of electrons is a pair of valence electrons that are not shared with another atom and thus do not participate in bond formation.

2. The electronic configuration of nitrogen is 2, 5. How many electrons in the outer shell of a nitrogen atom are not involved in the formation of a nitrogen molecule.
Two electrons (one lone pair) per nitrogen atom are not involved in bonding.

3. Draw the atomic orbit structure of – carbon tetrachloride & state the type of bond present in it.
(Diagram features one central Carbon atom sharing electron pairs with four surrounding Chlorine atoms). The type of bond present is a single covalent bond (four of them).

4. Choose the answer - i] Sodium chloride - covalent bond/ionic bond/covalent & coordinate bond. ii] Carbon tetrachloride - covalent bond/ionic bond/covalent and coordinate bond.
i] ionic bond.
ii] covalent bond.

5. i] In covalent compounds, the bond is formed due to _______ [sharing/transfer] of electrons. ii] A molecule of _______ contains a triple bond. [hydrogen, ammonia, nitrogen].
i] In covalent compounds, the bond is formed due to sharing of electrons.
ii] A molecule of nitrogen contains a triple bond.

6. State the bonding in — methane molecule
Methane contains four single covalent bonds formed by sharing of electron pairs between one carbon atom and four hydrogen atoms.

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2. ADDITIONAL QUESTIONS (Page 77)

Short Answer Questions

1. Name – (a) an atom with elec. configuration 2, 8, 6. (b) A gaseous covalent molecule – with four covalent bonds.
(a) Sulphur.
(b) Methane.

Long Answer Questions

1. Define the term – 'chemical bond' & 'chemical bonding'.
Chemical bond: The strong force of attraction which holds atoms together in a molecule. Chemical bonding: The process of atoms uniting by sharing or transferring electrons to form a stable molecule.

2. Give a reason why – noble gases have stable electronic configuration, while atoms of other elements have unstable electronic configuration.
Noble gases have fully complete outermost shells (a stable octet or duplet). Other elements have incomplete outermost shells, leaving them unstable and reactive.

3. State how – atoms of elements other than noble gases, attain stable elec. config. of the nearest noble gas.
By losing, gaining, or sharing their valence electrons with other atoms.

4. Define each of the following terms – (a) electrovalent bond (b) electrovalent compound (c) covalent bond (d) covalent compound – with suitable examples wherever required.
(a) Electrovalent bond: A bond formed by the complete transfer of one or more electrons from a metal to a non-metal (e.g., in NaCl).
(b) Electrovalent compound: A compound formed by electrovalent bonding (e.g., Calcium Oxide).
(c) Covalent bond: A bond formed by mutual sharing of electrons between non-metallic atoms (e.g., in H₂).
(d) Covalent compound: A compound formed by covalent bonding (e.g., Water, H₂O).

5. State what is meant by – (a) duplet rule (b) octet rule.
(a) Duplet rule: The tendency of atoms to attain a stable configuration of two electrons in their outermost K shell.
(b) Octet rule: The tendency of atoms to attain a stable configuration of eight electrons in their outermost shell.

6. Sodium chloride is formed as a result of – transfer of electrons from metallic sodium atom to non-metallic chlorine atom. With the help of atomic orbit structure diagram – show the formation of sodium chloride.
(Diagram shows a Sodium atom (2,8,1) transferring its outermost single electron to a Chlorine atom (2,8,7). It results in a positively charged Na⁺ ion (2,8) and negatively charged Cl^- ion (2,8,8), which attract electrostatically).

7. At. no. of calcium is 20 & of oxygen is 8. State the number of electrons – calcium loses & oxygen gains to attain stable electronic configuration of the nearest noble gas – during formation of electrovalent molecule, calcium oxide.
Calcium loses 2 electrons. Oxygen gains 2 electrons.

8. Explain with the help of atomic orbit structure diagram the formation of – calcium oxide.
(Diagram depicts Calcium (2,8,8,2) transferring its 2 outermost electrons to Oxygen (2,6). This forms Ca²⁺ (2,8,8) and O^2- (2,8), held together by a strong electrovalent bond).

9. Give a reason why – in the formation of electrovalent compound – magnesium chloride, one magnesium atom combines with two chlorine atoms to give magnesium chloride. [at. no.: Mg = 12, Cl = 17]
Magnesium (2,8,2) needs to lose 2 electrons to achieve an octet. Chlorine (2,8,7) only needs to gain 1 electron. Therefore, one Magnesium atom transfers its two electrons to two separate Chlorine atoms (one electron each).

10. 'Formation of hydrogen molecule takes place by sharing of electrons'. Give a reason why the molecule of hydrogen is not formed by – transfer of electrons.
Both hydrogen atoms have 1 valence electron and each requires 1 more to reach a stable duplet. Neither can completely transfer its electron, as it would be left empty. Therefore, they mutually share a pair.

11. Draw the atomic orbit structure diagram for formation of — (a) hydrogen molecule (b) chlorine molecule (c) oxygen molecule (d) nitrogen molecule [at. no.: H = 1, Cl = 17, O = 8, N = 7]
(Diagrams illustrate sharing of electron pairs: one pair for single bond in H₂ and Cl₂, two pairs for double bond in O₂, and three pairs for triple bond in N₂).

12. Give reasons for – formation of a single covalent bond between two chlorine atoms & a double covalent bond between two oxygen atoms – during formation of respective covalent molecules.
Each Chlorine atom (2,8,7) needs just 1 electron to complete its octet, thus they share one pair of electrons, forming a single bond. Each Oxygen atom (2,6) needs 2 electrons to complete its octet, thus they mutually share two pairs, forming a double bond.

13. Draw the atomic orbit structure diagram for formation of a — (a) water molecule (b) ammonia molecule [at. no.: H = 1, O = 8, N = 7]
(Diagrams depict Oxygen sharing 1 electron with each of two Hydrogens in water, leaving two lone pairs. In ammonia, Nitrogen shares 1 electron with each of three Hydrogens, leaving one lone pair).

14. With the help of an atomic orbit structure diagram – explain the formation of — (a) carbon tetrachloride (b) methane. [at. no.: C = 6, Cl = 17, H = 1]
(Diagrams show a central Carbon atom (2,4) sharing its 4 valence electrons. In CCl₄, it shares with four Chlorine atoms. In methane, it shares with four Hydrogen atoms, creating four single covalent bonds in both cases).

15. Explain – the formation of a radical from a covalent bond with an example.
When a covalent bond undergoes symmetrical breaking (bond fission), each separated atom retains one of the shared electrons. This results in the formation of two highly reactive radicals. For example, a Cl-Cl bond can break under light energy to form two chlorine radicals (Cl•).

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Multiple Choice Questions [MCQs] - Select the correct answer

1. Assertion (A): Noble gases have stable electronic configuration.
Reason (R): Atoms of elements other than noble gases have incomplete valence shell.
(a) Both A & R are true – and R is the correct explanation of A.
(b) Both A & R are true – but R is not the correct explanation of A.
(c) A is true – but R is false
(d) A is false – but R is true.
(b) Both A & R are true – but R is not the correct explanation of A.

2. An element 'X' has same no. of electrons in its first & in its outermost shell. The electronic configuration of X²⁺ is: (a) 2, 8, 1 (b) 2, 8 (c) 2, 8, 2 (d) 2, 8, 8
(b) 2, 8. (The neutral atom is Mg (2,8,2). The ion X²⁺ has lost 2 electrons).

3. An element 'A' needs 4 electrons to attain stable electronic configuration of the nearest noble gas – Neon. An element 'B' needs 1 electron to attain stable electronic configuration of the nearest noble gas – Argon. The covalent compound formed between 'A' & 'B' is: (a) AH₃ (b) AH₄ (c) A₂H₄ (d) ACl₄
(d) ACl₄. (A is Carbon, B is Chlorine).

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High Order Thinking Questions

1. State why – water has two lone pairs of electrons in its covalent molecule, ammonia has one lone pair & methane has no lone pairs in its molecule.
In water, oxygen (6 valence electrons) shares 2 with hydrogen, leaving 4 electrons (two lone pairs). In ammonia, nitrogen (5 valence electrons) shares 3 with hydrogen, leaving 2 electrons (one lone pair). In methane, carbon (4 valence electrons) shares all 4 with hydrogen, leaving no unshared electrons.

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3. TEST – HIGH ORDER & CRITICAL THINKING QUESTIONS
UNIT TEST PAPER – 4 (Page 78)

Q.1 Name the following:
1. An electrovalent compound formed by transfer of one electron from a metallic atom to a non-metallic atom.
Sodium chloride (NaCl).

2. A covalent hydrocarbon molecule, having four single covalent bonds.
Methane (CH₄).

3. A covalent molecule, having two lone pair of electrons.
Water (H₂O).

4. A covalent molecule having a triple covalent bond.
Nitrogen (N₂).

5. A covalent molecule having two shared pairs of electrons in its molecule.
Oxygen (O₂).

Q.2 Give reasons for the following: To attain stable electronic configuration of the nearest noble gas —
1. Hydrogen atom ¹₁H - needs one electron.
Because its K shell currently has 1 electron, and it needs 1 more to reach a stable duplet configuration like Helium.

2. Oxygen atom ¹⁶₈O - needs two electrons.
Because its configuration is 2, 6, requiring 2 more electrons to achieve a stable octet like Neon.

3. Nitrogen atom ¹⁴₇N - needs three electrons.
Because its configuration is 2, 5, requiring 3 more electrons to achieve a stable octet like Neon.

4. Carbon atom ¹²₆C - needs four electrons.
Because its configuration is 2, 4, requiring 4 more electrons to achieve a stable octet like Neon.

5. Chlorine atom ³⁵₁₇Cl - needs one electron.
Because its configuration is 2, 8, 7, requiring 1 more electron to achieve a stable octet like Argon.

Q.3 Complete the table given below, pertaining to formation of covalent compounds:
(a) Two atoms of 'A' (Atomic number 9) : No. of covalent bonds = 1
(b) Two atoms of 'B' (Atomic number 7) : No. of covalent bonds = 3
(c) Two atoms of 'C' (Atomic number 8) : No. of covalent bonds = 2
(d) Two atoms of 'D' (Atomic number 1) : No. of covalent bonds = 1
(e) Two atoms of 'E' (Atomic number 17) : No. of covalent bonds = 1

Q.4 Differentiate between the following with a suitable example:
1. Lone pair & shared pair of electrons.
A shared pair of electrons is mutually shared between atoms to form a bond (e.g., the bond in H₂). A lone pair is a pair of valence electrons not involved in bonding (e.g., the one pair on N in NH₃).

2. Duplet rule & octet rule.
The duplet rule governs atoms attaining 2 electrons in the first shell for stability (e.g., Hydrogen forming H₂). The octet rule governs atoms attaining 8 electrons in their valence shell for stability (e.g., Carbon in CH₄).

3. Stable electronic configuration & unstable configuration.
A stable configuration refers to a completely filled valence shell (e.g., Neon 2,8). An unstable configuration refers to an incomplete valence shell (e.g., Oxygen 2,6).

4. Electrovalent bond & covalent bond.
An electrovalent bond is formed by the complete transfer of electrons (e.g., NaCl). A covalent bond is formed by mutual sharing of electrons (e.g., Cl₂).

5. Single covalent bond & triple covalent bond.
A single covalent bond involves sharing one pair of electrons (e.g., H₂). A triple covalent bond involves sharing three pairs of electrons (e.g., N₂).

Q.5 Atomic numbers of the following elements are given below:
A = 8; B = 7; C = 17; D = 11; E = 20
1. State which of the above is a divalent metal.
E (Calcium, atomic no. 20, config 2,8,8,2).

2. State which of the above is a non-metal.
A (Oxygen), B (Nitrogen), and C (Chlorine) are non-metals.

3. State the type of bonding between two atoms of 'A'.
Covalent bond (specifically, a double covalent bond in O₂).

4. State the type of bonding between 'D' & 'C'.
Electrovalent (ionic) bond (forming NaCl).

5. State the number of covalent bonds formed in a molecule of 'B₂'.
Three (triple covalent bond in N₂).

Q.6 The representation below shows the outline formation of an electrovalent compound.
If the atomic number of element 'X' is 11 and of element 'Y' is 17 –
1. State why an electron is transferred from 'X' to 'Y' during the formation of 'XY'.
X (Sodium, 2,8,1) transfers 1 electron to lose its unstable valence shell and attain a stable octet. Y (Chlorine, 2,8,7) accepts this electron to complete its valence octet.

2. Give a reason why electrons are not shared between 'X' & 'Y' – during the formation of 'XY'.
X is a metallic element that readily loses an electron and cannot attain stability via sharing due to only having 1 valence electron, while Y is highly electronegative and readily accepts an electron.

3. State the difference between 'X' and 'X¹⁺'.
'X' is a neutral, unstable atom with 11 electrons. 'X¹⁺' is a stable, positively charged cation with 10 electrons (octet structure).

4. Does 'Y^1-' have a stable or an unstable electronic configuration.
It has a stable electronic configuration (octet).

5. If a compound is formed from atom A [at. no. 19] and an atom Y [at. no. 17], would the compound 'AY' be an electrovalent or covalent compound. Give reasons.
Electrovalent compound. Atom A (Potassium, 2,8,8,1) will completely transfer its 1 valence electron to atom Y (Chlorine, 2,8,7) so both achieve stable noble gas octets.