Structure of the Atom
NCERT Solutions • Class 9 Science • Chapter 4Chapter 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 $\frac{1}{2000}$ u) | Approx $1 \text{ u}$ | Approx $1 \text{ u}$ |
| Location | Revolve around nucleus | Inside the nucleus | Inside the nucleus |
2. What are the limitations of J.J. Thomson’s model of the atom?
Limitations:
1. It could not explain the results of the alpha-particle scattering experiment carried out by Rutherford (e.g., deflection of alpha particles).
2. It could not explain the stability of the atom (how positive and negative charges were held together without neutralizing).
3. It was based more on imagination than on experimental evidence.
1. It could not explain the results of the alpha-particle scattering experiment carried out by Rutherford (e.g., deflection of alpha particles).
2. It could not explain the stability of the atom (how positive and negative charges were held together without neutralizing).
3. It was based more on imagination than on experimental evidence.
3. What are the limitations of Rutherford’s model of the atom?
Major Limitation (Instability):
According to electromagnetic theory, a charged particle (electron) undergoing acceleration (circular motion) should radiate energy. If this happened, the electron would lose energy and spiral into the nucleus, making the atom unstable. However, atoms are known to be stable.
According to electromagnetic theory, a charged particle (electron) undergoing acceleration (circular motion) should radiate energy. If this happened, the electron would lose energy and spiral into the nucleus, making the atom unstable. However, atoms are known to be stable.
[Image of Bohr model of atom]
4. Describe Bohr’s model of the atom.
Postulates of Bohr’s Model:
1. Electrons revolve around the nucleus in certain special orbits known as discrete orbits.
2. While revolving in these discrete orbits, the electrons do not radiate energy.
3. These orbits or shells are called energy levels, represented by letters K, L, M, N… or numbers n = 1, 2, 3, 4…
1. Electrons revolve around the nucleus in certain special orbits known as discrete orbits.
2. While revolving in these discrete orbits, the electrons do not radiate energy.
3. These orbits or shells are called energy levels, represented by letters K, L, M, N… or numbers n = 1, 2, 3, 4…
[Image of comparison of atomic models]
5. Compare all the proposed models of an atom given in this chapter.
| Feature | Thomson’s Model | Rutherford’s Model | Bohr’s Model |
|---|---|---|---|
| Structure | Positive sphere with electrons embedded (Plum pudding). | Small positive nucleus with electrons revolving around it. | Positive nucleus with electrons in discrete orbits. |
| Nucleus | Not discovered yet. | Discovered nucleus. | Accepted nucleus concept. |
| Stability | Could not explain. | Unstable (electrons would spiral in). | Stable (no energy radiation in discrete orbits). |
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 present in a shell is given by the formula $2n^2$, where ‘$n$’ is the orbit number ($2, 8, 18, 32…$).
2. The maximum number of electrons that can be accommodated in the outermost orbit is 8.
3. Electrons are not accommodated in a given shell unless the inner shells are filled (step-wise filling).
1. The maximum number of electrons present in a shell is given by the formula $2n^2$, where ‘$n$’ is the orbit number ($2, 8, 18, 32…$).
2. The maximum number of electrons that can be accommodated in the outermost orbit is 8.
3. Electrons are not accommodated in a given shell unless the inner shells are filled (step-wise filling).
7. Define valency by taking examples of silicon and oxygen.
Definition: Valency is the combining capacity of an atom, determined by the number of valence electrons.
Example 1: Silicon (Atomic No. 14)
Configuration: $2, 8, 4$. Valence electrons = 4.
Since it has 4 electrons, it shares/loses 4 to become stable. Valency = 4.
Example 2: Oxygen (Atomic No. 8)
Configuration: $2, 6$. Valence electrons = 6.
It needs 2 more electrons to complete octet ($8-6=2$). Valency = 2.
Example 1: Silicon (Atomic No. 14)
Configuration: $2, 8, 4$. Valence electrons = 4.
Since it has 4 electrons, it shares/loses 4 to become stable. Valency = 4.
Example 2: Oxygen (Atomic No. 8)
Configuration: $2, 6$. Valence electrons = 6.
It needs 2 more electrons to complete octet ($8-6=2$). Valency = 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) Total number of protons in the nucleus. Ex: Carbon (Z=6).
(ii) Mass Number (A) Sum of protons and neutrons in the nucleus. Ex: Carbon-12 (A=12).
(iii) Isotopes Atoms of same element with same Atomic Number but different Mass Numbers. Ex: $_{1}^{1}H$ (Protium), $_{1}^{2}H$ (Deuterium).
(iv) Isobars Atoms of different elements with different Atomic Numbers but same Mass Numbers. Ex: $^{40}_{20}Ca$ and $^{40}_{18}Ar$.
Uses of Isotopes:
1. Uranium isotope is used as fuel in nuclear reactors.
2. Iodine isotope is used in the treatment of goitre.
(ii) Mass Number (A) Sum of protons and neutrons in the nucleus. Ex: Carbon-12 (A=12).
(iii) Isotopes Atoms of same element with same Atomic Number but different Mass Numbers. Ex: $_{1}^{1}H$ (Protium), $_{1}^{2}H$ (Deuterium).
(iv) Isobars Atoms of different elements with different Atomic Numbers but same Mass Numbers. Ex: $^{40}_{20}Ca$ and $^{40}_{18}Ar$.
Uses of Isotopes:
1. Uranium isotope is used as fuel in nuclear reactors.
2. Iodine isotope is used in the treatment of goitre.
9. $Na^+$ has completely filled K and L shells. Explain.
Explanation:
Sodium atom ($Na$) has Atomic Number 11.
Electronic configuration: $K=2, L=8, M=1$.
To form the sodium ion ($Na^+$), it loses the 1 electron from the M shell.
New configuration: $K=2, L=8$.
Since K shell max is 2 and L shell max is 8, both shells are now completely filled in $Na^+$.
Sodium atom ($Na$) has Atomic Number 11.
Electronic configuration: $K=2, L=8, M=1$.
To form the sodium ion ($Na^+$), it loses the 1 electron from the M shell.
New configuration: $K=2, L=8$.
Since K shell max is 2 and L shell max is 8, both shells are now completely filled in $Na^+$.
10. If bromine atom is available in the form of, say, two isotopes $^{79}_{35}Br$ (49.7%) and $^{81}_{35}Br$ (50.3%), calculate the average atomic mass of bromine atom.
Average Mass Formula
$= (79 \times \frac{49.7}{100}) + (81 \times \frac{50.3}{100})$
$= (39.263) + (40.743)$
$= \mathbf{80.006 \text{ u}}$
$= (79 \times \frac{49.7}{100}) + (81 \times \frac{50.3}{100})$
$= (39.263) + (40.743)$
$= \mathbf{80.006 \text{ u}}$
11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes $^{16}_{8}X$ and $^{18}_{8}X$ in the sample?
Let percentage of $^{16}X$ be $x$. Then percentage of $^{18}X$ is $(100 – x)$.
$16.2 = (16 \times \frac{x}{100}) + (18 \times \frac{100-x}{100})$
$1620 = 16x + 1800 – 18x$
$1620 = 1800 – 2x$
$2x = 180$ $\Rightarrow$ $x = 90$
Answer:
Isotope $^{16}X$: 90%
Isotope $^{18}X$: 10%
$16.2 = (16 \times \frac{x}{100}) + (18 \times \frac{100-x}{100})$
$1620 = 16x + 1800 – 18x$
$1620 = 1800 – 2x$
$2x = 180$ $\Rightarrow$ $x = 90$
Answer:
Isotope $^{16}X$: 90%
Isotope $^{18}X$: 10%
12. If Z = 3, what would be the valency of the element? Also, name the element.
Z = 3 means Atomic Number is 3.
Electronic Configuration: $K=2, L=1$.
Valence electrons = 1.
Valency = 1 (It loses 1 electron).
Element Name: Lithium (Li).
Electronic Configuration: $K=2, L=1$.
Valence electrons = 1.
Valency = 1 (It loses 1 electron).
Element Name: Lithium (Li).
13. Composition of the nuclei of two atomic species X and Y are given as under: X (p=6, n=6), Y (p=6, n=8). Give mass numbers. Relation?
Mass Number ($A = p + n$):
For X: $6 + 6 =$ 12
For Y: $6 + 8 =$ 14
Relation: Since both have the same number of protons (Atomic No = 6) but different mass numbers, they are Isotopes (of Carbon).
For X: $6 + 6 =$ 12
For Y: $6 + 8 =$ 14
Relation: Since both have the same number of protons (Atomic No = 6) but different mass numbers, they are Isotopes (of Carbon).
14. For the following statements, write T for True and F for False.
(a) F (Thomson didn’t propose the nucleus; Rutherford did).
(b) F (Neutron is a subatomic particle, not electron+proton).
(c) T (Electron mass is approx 1/2000 of proton).
(d) F (Iodine isotope is for goitre; Tincture iodine uses ordinary iodine).
(b) F (Neutron is a subatomic particle, not electron+proton).
(c) T (Electron mass is approx 1/2000 of proton).
(d) F (Iodine isotope is for goitre; Tincture iodine uses ordinary iodine).
MCQs (15-18)
15. Discovery of: (a) Atomic Nucleus
16. Isotopes have: (c) different number of neutrons
17. Valence electrons in $Cl^-$: Chlorine (2,8,7) gains 1 $\rightarrow$ 2,8,8. Ans: (b) 8
18. Configuration of sodium: (d) 2,8,1
16. Isotopes have: (c) different number of neutrons
17. Valence electrons in $Cl^-$: Chlorine (2,8,7) gains 1 $\rightarrow$ 2,8,8. Ans: (b) 8
18. Configuration of sodium: (d) 2,8,1
19. Complete the following table.
*Note: The last row indicates 0 electrons for a neutral species of mass 1, usually Hydrogen ion ($H^+$), or simply Hydrogen atom has 1 electron.
| Atomic No | Mass No | Neutrons | Protons | Electrons | Atomic Species |
|---|---|---|---|---|---|
| 9 | 19 | 10 | 9 | 9 | Fluorine |
| 16 | 32 | 16 | 16 | 16 | Sulphur |
| 12 | 24 | 12 | 12 | 12 | Magnesium |
| 1 | 2 | 1 | 1 | 1 | Deuterium |
| 1 | 1 | 0 | 1 | 0* | Protium Ion ($H^+$) |