STRUCTURE OF THE ATOM - Q&A
Questions Page 39
1. What are canal rays?
Canal rays are positively charged radiations produced in a discharge tube containing gas at low pressure. They were discovered by E. Goldstein in 1886. These rays led to the discovery of the positively charged sub-atomic particle called the proton.
2. If an atom contains one electron and one proton, will it carry any charge or not?
It will not carry any charge; it will be neutral.
Explanation: An electron has a negative charge (-1) and a proton has a positive charge (+1). If an atom has one of each, the charges cancel each other out (+1 - 1 = 0), making the atom electrically neutral.
Questions Page 41
1. On the basis of Thomson's model of an atom, explain how the atom is neutral as a whole.
According to Thomson's model (often called the "plum pudding" or "watermelon" model):
1. An atom consists of a positively charged sphere.
2. Electrons are embedded in it like seeds in a watermelon.
3. The negative and positive charges are equal in magnitude.
Therefore, the positive charge of the sphere and the negative charge of the electrons cancel each other out, making the atom electrically neutral as a whole.
[Image of Thomson's model of an atom]
2. On the basis of Rutherford's model of an atom, which subatomic particle is present in the nucleus of an atom?
On the basis of Rutherford's model, the proton is the subatomic particle present in the nucleus of an atom. Rutherford's experiments showed that the positive charge and most of the mass of the atom are concentrated in a very small volume called the nucleus.
3. Draw a sketch of Bohr's model of an atom with three shells.
Explanation:
- The center is the Nucleus (containing protons and neutrons).
- The first circle around the nucleus is the K shell (n=1).
- The second circle is the L shell (n=2).
- The third circle is the M shell (n=3).
Electrons revolve in these discrete orbits without radiating energy.
4. What do you think would be the observation if the $\alpha$-particle scattering experiment is carried out using a foil of a metal other than gold?
If a foil of a heavy metal (like platinum) is used, the observations would be similar to the gold foil experiment because heavy atoms also have a massive nucleus that can deflect $\alpha$-particles.
However, if a light metal (like lithium) is used, the massive $\alpha$-particles might push the light nucleus aside and may not show significant deflection or rebounding (scattering). Gold was chosen specifically because it can be hammered into extremely thin layers (1000 atoms thick) and has a heavy nucleus to scatter the particles.
Questions Page 42
1. Name the three sub-atomic particles of an atom.
The three sub-atomic particles are:
1. Proton (Positively charged)
2. Electron (Negatively charged)
3. Neutron (Neutral/No charge)
2. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Given:
Atomic Mass = 4 u
Number of Protons = 2
Formula:
Mass Number = Number of Protons + Number of Neutrons
Step-by-step Solution:
4 = 2 + Number of Neutrons
Number of Neutrons = 4 - 2
Number of Neutrons = 2
Questions Page 44
1. Write the distribution of electrons in carbon and sodium atoms.
Carbon (Atomic Number = 6):
- Total electrons = 6
- K shell (first shell) takes max 2 electrons.
- L shell (second shell) takes remaining 4 electrons.
Distribution: 2, 4
Sodium (Atomic Number = 11):
- Total electrons = 11
- K shell takes max 2 electrons.
- L shell takes max 8 electrons.
- M shell takes remaining 1 electron (11 - 2 - 8 = 1).
Distribution: 2, 8, 1
2. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Step-by-step:
1. The maximum capacity of the K shell is 2 electrons.
2. The maximum capacity of the L shell is 8 electrons.
3. If both are full, total electrons = 2 + 8 = 10.
Answer: The total number of electrons is 10.
Questions Page 45
1. How will you find the valency of chlorine, sulphur and magnesium?
Valency is determined by the number of electrons in the outermost shell (valence electrons).
Chlorine (Atomic Number 17):
- Electronic Configuration: 2, 8, 7
- Valence electrons = 7
- Since it is close to 8, it is easier to gain 1 electron.
- Valency = 8 - 7 = 1
Sulphur (Atomic Number 16):
- Electronic Configuration: 2, 8, 6
- Valence electrons = 6
- Needs 2 electrons to complete octet.
- Valency = 8 - 6 = 2
Magnesium (Atomic Number 12):
- Electronic Configuration: 2, 8, 2
- Valence electrons = 2
- It is easier to lose 2 electrons.
- Valency = 2
Exercises
1. Compare the properties of electrons, protons and neutrons.
| Property | Electron | Proton | Neutron |
|---|---|---|---|
| Symbol | e- | p+ | n |
| Charge | Negative (-1) | Positive (+1) | Neutral (0) |
| Mass | Negligible (approx. 1/2000 u) | approx. 1 u | approx. 1 u |
| Location | Outside nucleus (in shells) | Inside nucleus | Inside nucleus |
2. What are the limitations of J.J. Thomson's model of the atom?
The limitations are:
1. It failed to explain the results of Rutherford's alpha-particle scattering experiment (why most particles passed through while some rebounded).
2. It could not explain the stability of an atom (how positive and negative charges were held together).
3. It did not mention the presence of a nucleus.
3. What are the limitations of Rutherford's model of the atom?
The main limitation was the stability of the atom.
According to electromagnetic theory, any charged particle (electron) undergoing acceleration (circular motion) should radiate energy. If this happened, the electron would lose energy and spiral into the nucleus, causing the atom to collapse. Rutherford's model could not explain why this doesn't happen.
4. Describe Bohr's model of the atom.
Neil Bohr proposed the following postulates to overcome Rutherford's limitations:
1. Electrons revolve around the nucleus in certain special discrete orbits called shells or energy levels.
2. While revolving in these discrete orbits, the electrons do not radiate energy.
3. These orbits are represented by letters K, L, M, N... or numbers n=1, 2, 3, 4...
This model successfully explained the stability of the atom.
5. Compare all the proposed models of an atom given in this chapter.
| Model | Key Feature | Correct Aspects | Incorrect Aspects |
|---|---|---|---|
| Thomson | Sphere of positive charge with electrons embedded (Plum Pudding). | Atom is electrically neutral. | No nucleus; electrons static. |
| Rutherford | Small, heavy, positive nucleus; electrons revolve around it. | Discovery of nucleus; mostly empty space. | Unstable; electrons would spiral into nucleus. |
| Bohr | Electrons revolve in discrete, fixed energy orbits. | Explains stability; distinct energy levels. | (Limitations not discussed in standard 9 scope). |
6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.
The rules (Bohr-Bury scheme) are:
1. The maximum number of electrons in a shell is given by the formula 2n2, where 'n' is the shell number.
- K shell (n=1): 2(1)2 = 2
- L shell (n=2): 2(2)2 = 8
- M shell (n=3): 2(3)2 = 18
2. The outermost shell cannot hold more than 8 electrons.
3. Electrons are not accommodated in a given shell unless the inner shells are filled (shells are filled stepwise).
7. Define valency by taking examples of silicon and oxygen.
Definition: Valency is the combining capacity of an atom, determined by the number of electrons it needs to lose, gain, or share to achieve a stable octet (8 electrons) in its outermost shell.
Example 1: Silicon (Atomic No. 14)
- Configuration: 2, 8, 4
- Valence electrons: 4
- It shares 4 electrons to complete its octet.
- Valency = 4
Example 2: Oxygen (Atomic No. 8)
- Configuration: 2, 6
- Valence electrons: 6
- It needs to gain 2 electrons to reach 8.
- Valency = 8 - 6 = 2
8. Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.
(i) Atomic Number (Z): The total number of protons in the nucleus of an atom.
Example: Carbon has 6 protons, so Z=6.
(ii) Mass Number (A): The sum of the total number of protons and neutrons in an atom.
Example: Carbon-12 has 6 protons and 6 neutrons, so A = 6+6 = 12.
(iii) Isotopes: Atoms of the same element having the same atomic number but different mass numbers.
Example: Hydrogen has three isotopes: Protium (1H1), Deuterium (1H2), Tritium (1H3).
(iv) Isobars: Atoms of different elements with different atomic numbers but the same mass number.
Example: Calcium (Atomic No. 20) and Argon (Atomic No. 18) both have Mass No. 40.
Uses of Isotopes:
1. An isotope of Uranium is used as fuel in nuclear reactors.
2. An isotope of Cobalt is used in the treatment of cancer.
9. $Na^+$ has completely filled K and L shells. Explain.
Explanation:
- A neutral Sodium (Na) atom has Atomic Number 11.
- Electronic configuration of Na: 2, 8, 1 (K=2, L=8, M=1).
- To form the sodium ion ($Na^+$), the atom loses its one outermost electron from the M shell.
- Remaining configuration: 2, 8.
- Now, the K shell has 2 electrons (full) and the L shell has 8 electrons (full). Thus, $Na^+$ has completely filled K and L shells.
10. If bromine atom is available in the form of, say, two isotopes $_{35}^{79}Br$ (49.7%) and $_{35}^{81}Br$ (50.3%), calculate the average atomic mass of bromine atom.
Formula: Average Atomic Mass = (Mass A × %A) + (Mass B × %B)
Calculation:
Average Mass = (79 × $\frac{49.7}{100}$) + (81 × $\frac{50.3}{100}$)
= (79 × 0.497) + (81 × 0.503)
= 39.263 + 40.743
= 80.006 u
Answer: The average atomic mass of bromine is approx 80 u.
11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes $_{8}^{16}X$ and $_{8}^{18}X$ in the sample?
Let the percentage of $_{8}^{16}X$ be $x$.
Then the percentage of $_{8}^{18}X$ will be $(100 - x)$.
Equation:
$16.2 = (16 \times \frac{x}{100}) + (18 \times \frac{100 - x}{100})$
$16.2 = \frac{16x + 1800 - 18x}{100}$
$1620 = -2x + 1800$
$2x = 1800 - 1620$
$2x = 180$
$x = 90$
Answer:
Percentage of $_{8}^{16}X$ = 90%
Percentage of $_{8}^{18}X$ = 100 - 90 = 10%
12. If Z = 3, what would be the valency of the element? Also, name the element.
Given Z (Atomic Number) = 3
- Electronic Configuration: 2, 1 (K=2, L=1)
- Valence electrons = 1
- To become stable, it loses 1 electron.
- Valency = 1
Name of Element: Lithium (Li).
13. Composition of the nuclei of two atomic species X and Y are given as under
X Y
Protons = 6 6
Neutrons = 6 8
Give the mass numbers of X and Y. What is the relation between the two species?
Step 1: Calculate Mass Numbers (Protons + Neutrons)
- Mass Number of X = 6 + 6 = 12
- Mass Number of Y = 6 + 8 = 14
Step 2: Determine Relation
- Both X and Y have the same number of protons (6), meaning they have the same Atomic Number (Z=6).
- However, they have different Mass Numbers (12 and 14).
- Therefore, they are Isotopes (specifically, isotopes of Carbon).
14. For the following statements, write T for True and F for False.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons. F (Thomson did not propose a nucleus; Rutherford did).
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral. F (A neutron is a fundamental sub-atomic particle, not a combination).
(c) The mass of an electron is about $\frac{1}{2000}$ times that of proton. T
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. F (Tincture iodine uses normal iodine; the isotope is used for treating goitre).
Put tick ($\checkmark$) against correct choice and cross ($\times$) against wrong choice in questions 15, 16 and 17
15. Rutherford's alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic Nucleus
(b) Electron
(c) Proton
(d) Neutron
Answer: (a) Atomic Nucleus $\checkmark$
16. Isotopes of an element have
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers
Answer: (c) different number of neutrons $\checkmark$
17. Number of valence electrons in $Cl^-$ ion are:
(a) 16
(b) 8
(c) 17
(d) 18
Answer: (b) 8 $\checkmark$
Explanation: Chlorine (2,8,7) gains 1 electron to become $Cl^-$ (2,8,8). So valence electrons are 8.
18. Which one of the following is a correct electronic configuration of sodium?
(a) 2,8
(b) 8,2,1
(c) 2,1,8
(d) 2,8,1
Answer: (d) 2,8,1 $\checkmark$