ATOMIC STRUCTURE - Questions & Answers

EXERCISE

Atomic structure

The atom – ancient views

1. State the main postulates of - Dalton's atomic theory. Explain how the modern atomic theory contradicted Dalton's atomic theory.
The main postulates of Dalton's atomic theory are:
1. Matter consists of very small, indivisible particles called atoms.
2. Atoms can neither be created nor destroyed.
3. Atoms of the same element are alike in all respects (size, shape, mass) but differ from atoms of other elements.
4. Atoms of different elements combine in simple whole-number ratios to form compounds.
5. An atom is the smallest unit of matter that takes part in a chemical reaction.

Modern atomic theory contradicted Dalton's theory in the following ways:
1. Dalton said atoms are indivisible; Modern theory proves they are divisible into sub-atomic particles (protons, electrons, and neutrons).
2. Dalton said atoms are indestructible; Modern theory shows atoms can be destroyed during nuclear fission.
3. Dalton said atoms of the same element are alike in all respects; Modern theory shows that isotopes of the same element have different mass numbers.

Sub-atomic particles

2. With reference to the discovery of the structure of an atom, explain in brief - William Crookes experiment for the discovery of cathode rays, followed by - J.J. Thomsons experiment pertaining to the constituents of the cathode rays. State which sub-atomic particle was discovered from his experiment.
William Crookes' Experiment: He passed an electric discharge through a discharge tube containing a gas at very low pressure. He observed blue rays emitted from the cathode (negative plate), which he called cathode rays.
J.J. Thomson's Experiment: He studied the characteristics of these cathode rays and found they cast shadows and were deflected by a positively charged plate. He concluded that cathode rays consist of negatively charged particles.

The sub-atomic particle discovered was the electron.

3. Explain in brief - Goldstein's experiment which led to the discovery of the proton and - Lord Rutherford's experiment which led to the discovery of the atomic nucleus.
Goldstein's Experiment: He used a modified cathode ray tube with a perforated cathode. He observed new rays produced from the anode passing through the holes of the cathode in a direction opposite to cathode rays. These were called anode rays or positive rays, leading to the discovery of the proton.
Rutherford's Experiment: He projected alpha particles towards a thin gold foil. He observed that most particles went straight through, but some were deflected by small and large angles. He concluded that the atom has a concentrated positive mass in the center, which he called the atomic nucleus.

4. 'Electrons revolve around the nucleus in fixed orbits or shells called energy levels'. State how these energy levels are represented.
These energy levels are represented by integers 'n' (1, 2, 3, 4...) or by the letters K, L, M, N...

5. Draw a neat labeled diagram representing an atom. Name the three sub-atomic particles in the atom & represent them symbolically showing the mass & charge of each. State where the sub-atomic particles are present in the atom.
[Note: A typical diagram would show a central nucleus with protons and neutrons, and electrons in surrounding orbits.]

Sub-atomic Particles:
1. Proton (p): Positive charge [+1], Unit mass [1]. Present in the nucleus.
2. Neutron (n): No charge [0], Unit mass [1]. Present in the nucleus.
3. Electron (e): Negative charge [-1], Negligible mass [0]. Present in the orbits/shells around the nucleus.

Terms

6. Define the term – 'atomic number' of an atom. If an atom 'A' has an atomic number of - eleven, state the number of protons & electrons it contains.
Atomic Number (Z): It is the number of protons present in the nucleus of an atom. In a neutral atom, it is also equal to the number of electrons.

If an atom 'A' has an atomic number of 11:
Number of protons = 11
Number of electrons = 11

7. Define the term – 'mass number' of an atom. If an atom 'B' has mass number 35 & atomic number 17, state the number of protons, electrons & neutrons it contains.
Mass Number (A): It is the total number of protons and neutrons present in the nucleus of an atom.

For atom 'B' (A=35, Z=17):
Number of protons = 17
Number of electrons = 17
Number of neutrons = Mass Number - Atomic Number = 35 - 17 = 18

8. State why the atomic weight of an element is also termed – relative atomic mass.
The actual mass of an atom is extremely small and difficult to measure. Therefore, it is determined by comparing it with a standard (like 1/12th the mass of a carbon-12 atom). Because it is a comparative mass, it is termed relative atomic mass.

9. State how electrons are distributed in an atom. Explain in brief the rules which govern their distribution.
Electrons are distributed in various shells (K, L, M, N) according to the Bohr-Bury scheme.

Rules:
1. The maximum number of electrons in a shell is given by the formula 2n², where 'n' is the shell number.
2. The outermost shell cannot accommodate more than 8 electrons, even if it has the capacity for more.
3. Electrons are not filled in a new shell unless the inner shells are completely filled.

10. If an atom 'A' has atomic number 19 & mass number 39, state – i] Its electronic configuration. ii] The number of valence electrons it possesses.
i] Electronic Configuration: With 19 electrons, the distribution is 2, 8, 8, 1.
ii] Valence Electrons: The number of electrons in the outermost shell is 1.

Arrangement of electrons

11. Draw the atomic diagrams of the following elements showing the distribution of – protons, neutrons & the electrons in the various shells of the atoms.
a] Carbon - ¹²₆C, b] Oxygen - ¹⁶₈O, c] Phosphorus - ³¹₁₅P, d] Argon - ⁴⁰₁₈Ar, e] Calcium - ⁴⁰₂₀Ca
The distributions are as follows:
a] Carbon: Protons=6, Neutrons=6, Electrons=2, 4
b] Oxygen: Protons=8, Neutrons=8, Electrons=2, 6
c] Phosphorus: Protons=15, Neutrons=16, Electrons=2, 8, 5
d] Argon: Protons=18, Neutrons=22, Electrons=2, 8, 8
e] Calcium: Protons=20, Neutrons=20, Electrons=2, 8, 8, 2

Valency

12. 'Valency is the number of hydrogen atoms which can combine with [or displace] one atom of the element [or radical] forming a compound'. With reference to the above definition of valency, state the valency of chlorine in hydrogen chloride, giving reasons.
In Hydrogen Chloride (HCl), one atom of chlorine combines with one atom of hydrogen. According to the definition, the valency of chlorine is 1.

13. 'Valency is also the number of electrons - donated or accepted by an atom so as to achieve stable electronic configuration of the nearest noble gas'. With reference to this definition –
a] State what is meant by 'stable electronic configuration'.
b] State why the valency of – i] sodium, magnesium & aluminium is: +1, +2 & +3 respectively. ii] chlorine, oxygen & nitrogen is: -1, -2 & -3 respectively.
a] Stable Electronic Configuration: It refers to having a completely filled outermost shell (either a duplet of 2 electrons for the first shell or an octet of 8 electrons for other shells), similar to noble gases.

b] i] Sodium (2,8,1), Magnesium (2,8,2), and Aluminium (2,8,3) have 1, 2, and 3 electrons in their valence shells. They lose these electrons to achieve an octet, resulting in valencies of +1, +2, and +3.
ii] Chlorine (2,8,7), Oxygen (2,6), and Nitrogen (2,5) need 1, 2, and 3 electrons respectively to complete their octets. They accept these electrons, resulting in valencies of -1, -2, and -3.

Formation of compounds

14. With reference to formation of compounds from atoms by electron transfer – electrovalency, state the basic steps in the conversion of sodium & chlorine atoms to sodium & chloride ions leading to the formation of the compound – sodium chloride. [electronic configuration of: Na = 2,8,1 & Cl = 2,8,7]
The steps are:
1. Sodium atom (2,8,1) loses its single valence electron to achieve a stable octet (2,8), becoming a positively charged sodium ion (Na⁺).
2. Chlorine atom (2,8,7) accepts the electron lost by sodium to complete its octet (2,8,8), becoming a negatively charged chloride ion (Cl⁻).
3. The strong electrostatic force of attraction between the oppositely charged Na⁺ and Cl⁻ ions results in the formation of the compound Sodium Chloride (NaCl).

CRITICAL THINKING OBJECTIVE TYPE QUESTIONS

Atomic structure

Q.1 Match the statements in List I with the correct answer from List II.
List I
1. Mass number of an atom is the number of protons and E: Neutrons
2. The sub-atomic particle with a negligible mass. A: Electron
3. An atom having stable electronic configuration. B: Argon
4. A molecule formed by sharing of electrons [covalency]. C: Nitrogen
5. A metallic atom having unstable electronic configuration. D: Sodium

Q.2 Select the correct answer from the choice in bracket to complete each sentence:-
1. An element 'X' has six electrons in its outer or valence shell. Its valency is -2 [+2/-2/-1].
2. An element 'Y' has electronic configuration 2, 8, 6. The element 'Y' is a non-metal [metal/non-metal/noble gas].
3. A neutron [proton/neutron] is a sub-atomic particle with no charge and unit mass.
4. An element Z with zero valency is a noble gas [metal/noble gas/non-metal].
5. Magnesium atom with electronic configuration 2, 8, 2 achieves stable electronic configuration by losing two electrons, thereby achieving stable electronic configuration of the nearest noble gas neon [neon/argon].

Q.3 The diagram represents an isotope of hydrogen [H]. Answer the following:
1. Are isotopes atoms of the same element or different elements.
They are atoms of the same element.

2. Do isotopes have the same atomic number or the same mass number.
They have the same atomic number.

3. If an isotope of 'H' has mass no. = 2, how many electrons does it have.
It has 1 electron.

4. If an isotope of 'H' has mass no. = 3, how many neutrons does it have.
It has 2 neutrons (3 - 1 = 2).

5. Which sub-atomic particles in the 3 isotopes of 'H' are the same.
The protons and electrons are the same.

Q.4 State the electronic configuration for each of the following:
1. Hydrogen [p = 1]. 1
2. Boron [p = 5]. 2, 3
3. Nitrogen [p = 7]. 2, 5
4. Neon [p = 10]. 2, 8
5. Magnesium [p = 12]. 2, 8, 2
6. Aluminium [p = 13]. 2, 8, 3
7. Sulphur [p = 16]. 2, 8, 6
8. Argon [p = 18]. 2, 8, 8
9. Potassium [p = 19]. 2, 8, 8, 1
10. Calcium [p = 20]. 2, 8, 8, 2

Q.5 Draw the structure of the following atoms showing the nucleus containing – protons, neutrons and the orbits with the respective electrons:
1. Lithium [At. no. = 3, Mass no. = 7].
Nucleus: 3p, 4n; Shells: K=2, L=1

2. Carbon [At. no. = 6, Mass no. = 12].
Nucleus: 6p, 6n; Shells: K=2, L=4

3. Silicon [At. no. = 14, Mass no. = 28].
Nucleus: 14p, 14n; Shells: K=2, L=8, M=4

4. Sodium [At. no. = 11, Mass no. = 23].
Nucleus: 11p, 12n; Shells: K=2, L=8, M=1

5. Isotopes of hydrogen [¹₁H, ²₁H, ³₁H]
¹₁H (Protium): 1p, 0n; Shell: K=1
²₁H (Deuterium): 1p, 1n; Shell: K=1
³₁H (Tritium): 1p, 2n; Shell: K=1