Class 9 Journey Inside the Atom: cheat sheet

CBSE Science. Protons, neutrons and electrons, the models of the atom, electron shells and valency, isotopes and isobars.

Key ideas

Thomson: a positive sphere with electrons in it. Rutherford: a tiny dense positive nucleus with electrons around it. Bohr: electrons in fixed shells.
Protons (+) and neutrons (no charge) are in the nucleus; electrons (−) move in shells around it.
The atomic number Z is the number of protons; the mass number A is protons + neutrons.
Shells K, L, M, N hold at most 2, 8, 18, 32 electrons (2n²); the outer shell holds at most 8.
Valency is the number of electrons an atom gains, loses or shares to complete its outer shell.
Isotopes have the same Z but different A (¹²C, ¹⁴C); isobars have the same A but different Z (⁴⁰Ar, ⁴⁰Ca).

Neutrons	A − Z
Shell capacity	2n² (n = 1, 2, 3, …)
Examples	Na 2, 8, 1 (valency 1); Cl 2, 8, 7 (valency 1); O 2, 6 (valency 2)
Average atomic mass	Σ(mass × fraction), e.g. Cl: 35 × 0.75 + 37 × 0.25 = 35.5 u

Taking the mass number as the number of neutrons.
Filling the third shell to 18 before starting the fourth in the first 20 elements (it stops at 8).
Mixing up isotopes (same element) and isobars (different elements).

Are these pairs isotopes, isobars or neither? (a) ¹⁵₇N and ¹⁴₇N (b) ²⁰₁₀Ne and ²²₁₀Ne

Working: isotopes: same Z; isobars: same A, different Z

Answer: (a) isotopes; (b) isotopes

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