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Structure of the Atom

CBSE Class 9 Science • Chapter 4 • Detailed Master Notes

Chapter Overview:

We know atoms are indivisible (Dalton). But discovery of electrons and protons challenged this. Here we study Sub-atomic particles, various Atomic Models (Thomson, Rutherford, Bohr), distribution of electrons in shells (Valency), and concepts of Isotopes and Isobars.

1. Sub-atomic Particles

Particle	Charge	Mass	Discoverer	Location
Electron ($e^-$)	-1 (Negative)	Negligible ($1/2000$ of H atom)	J.J. Thomson	Outside nucleus (Shells)
Proton ($p^+$)	+1 (Positive)	1 unit	E. Goldstein	Inside nucleus
Neutron ($n$)	0 (Neutral)	1 unit	J. Chadwick	Inside nucleus

2. Structure of Atom

(a) Thomson’s Model of Atom (Plum Pudding Model)

He proposed that an atom is similar to a Christmas pudding or a Watermelon.

Atom consists of a positively charged sphere.
Electrons are embedded in it like seeds in red edible part of watermelon.
Negative and positive charges are equal in magnitude, so the atom is electrically neutral.

(b) Rutherford’s Model of Atom (Alpha-particle scattering exp.)

Fast moving alpha ($\alpha$) particles were made to fall on a thin Gold foil.

Observations:

Most $\alpha$-particles passed straight (Space is empty).
Some were deflected by small angles (Positive charge is occupied in small space).
1 out of 12,000 rebounded ($180^\circ$) (Mass is concentrated in center).

Conclusions:

Positive charge and mass are concentrated in a very small volume called Nucleus.
Electrons revolve around nucleus in circular paths.

Drawback: Accelerating charged particle radiates energy. So electron should fall into nucleus, making atom unstable.

(c) Bohr’s Model of Atom

To overcome Rutherford's drawback, Neils Bohr proposed:

Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
While revolving in discrete orbits, electrons do not radiate energy.
These orbits are called Energy Shells represented by Letters K, L, M, N... or numbers n=1, 2, 3, 4...

3. Distribution of Electrons (Electronic Config.)

Rules for filling electrons:

Max number of electrons in a shell is given by $2n^2$.
- K-shell (n=1): $2(1)^2 = 2$.
- L-shell (n=2): $2(2)^2 = 8$.
- M-shell (n=3): $2(3)^2 = 18$.
Max electrons in the outermost orbit is 8 (Octet Rule).
Electrons are not filled in a given shell unless inner shells are filled.

4. Valency

Combining capacity of an atom.

If valence electrons $\le 4$: Valency = Valence electrons. (e.g., $Na(2,8,1) \Rightarrow$ Valency 1).
If valence electrons $> 4$: Valency = 8 - Valence electrons. (e.g., $Cl(2,8,7) \Rightarrow 8-7 = 1$).
Noble gases have fully filled shells, so Valency = 0.

5. Atomic Number and Mass Number

Atomic Number (Z): Total number of protons in the nucleus. ($Z = p$). (Also equal to $e$ in neutral atom).

$$ Z = p $$

Mass Number (A): Sum of total number of protons and neutrons.

$$ A = p + n $$

Notation: $_Z^A X$ (e.g., $_7^{14}N$ means Nitrogen with Z=7, A=14).

6. Isotopes and Isobars

Isotopes: Atoms of the same element with same Atomic Number but different Mass Numbers.

Hydrogen: Protium ($^1_1H$), Deuterium ($^2_1H$), Tritium ($^3_1H$).
Carbon: $^{12}C, ^{14}C$.
Chlorine: $^{35}Cl, ^{37}Cl$ (occur in ratio 3:1). Average Atomic Mass = 35.5 u.
Applications: Uranium isotope used in nuclear reactors. Cobalt isotope in cancer treatment. Iodine isotope in goitre.

Isobars: Atoms of different elements with different Atomic Numbers but same Mass Numbers.

e.g., Calcium ($^{40}_{20}Ca$) and Argon ($^{40}_{18}Ar$). Both have mass 40.

Practice Zone

Q1: An atom has mass number 23 and atomic number 11. Find neutrons.

Ans: $n = A - Z = 23 - 11 = 12$ neutrons.

Q2: Write electronic configuration of Chlorine (Z=17).

Ans: K=2, L=8, M=7. ($2, 8, 7$). Valency = $8-7=1$.