Atomic Structure
Key Concepts
| # | Concept | Explanation |
|---|---|---|
| 1 | Atomic Number (Z) | Number of protons in the nucleus; decides the element’s identity. |
| 2 | Mass Number (A) | Sum of protons + neutrons; written as superscript (e.g. 23Na). |
| 3 | Isotopes | Atoms of same element (same Z) but different A (different neutron count). |
| 4 | Isobars | Atoms of different elements (different Z) but same A. |
| 5 | Electron Distribution | Electrons occupy K,L,M,N… shells; 2n² rule for max electrons in a shell. |
| 6 | Valency | Combining capacity = electrons in outermost shell (≤4) OR 8 – outermost electrons (>4). |
| 7 | Cathode Rays | Stream of electrons; travel straight, cast shadow, rotate paddle wheel → prove electron is a particle. |
| 8 | Rutherford’s Findings | Most of atom is empty; entire positive charge & mass concentrated in tiny nucleus. |
15 Practice MCQs
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The number of neutrons in ³⁹₁₉K is A) 19 B) 20 C) 39 D) 58
Answer: B) 20
Solution: Neutrons = A – Z = 39 – 19 = 20
Shortcut: “A minus Z gives N”
Concept tag: Mass number & neutron calculation -
Which pair represents isobars? A) ¹H, ²H B) ¹⁴C, ¹⁴N C) ³⁵Cl, ³⁷Cl D) ¹⁶O, ¹⁶O²⁻
Answer: B) ¹⁴C, ¹⁴N
Solution: Same A (14), different Z → isobars
Shortcut: “Same top, different bottom”
Concept tag: Isobars -
Maximum electrons in the M-shell (n = 3) is A) 8 B) 10 C) 18 D) 32
Answer: C) 18
Solution: 2n² = 2 × 3² = 18
Shortcut: “Two n square”
Concept tag: Shell capacity -
The valency of silicon (atomic number 14) is A) 2 B) 3 C) 4 D) 1
Answer: C) 4
Solution: EC = 2,8,4 → outermost = 4 → valency = 4
Shortcut: “Outermost ≤4, valency = same”
Concept tag: Valency -
In Rutherford’s α-scattering, the false conclusion was A) Atom has a nucleus B) Electrons revolve in orbits C) Most space is empty D) Mass is in nucleus
Answer: B) Electrons revolve in orbits
Solution: Orbit idea came later from Bohr
Shortcut: “Rutherford → nucleus only”
Concept tag: Rutherford model -
The charge on cathode rays particles is A) +1 B) –1 C) 0 D) +2
Answer: B) –1
Solution: Cathode rays = electrons → –1e
Shortcut: “Cathode → electron → negative”
Concept tag: Cathode rays -
Which shell has the least energy? A) K B) L C) M D) N
Answer: A) K
Solution: Energy ∝ 1/n; n = 1 for K
Shortcut: “Near nucleus, low energy”
Concept tag: Shell energy -
An element with 17 protons and 18 neutrons is A) Cl-35 B) Cl-37 C) Ar-35 D) S-35
Answer: A) Cl-35
Solution: Z = 17 → Cl; A = 17 + 18 = 35
Shortcut: “Z gives name, A gives mass”
Concept tag: Nuclide symbol -
The number of electrons in Na⁺ (Z = 11) is A) 11 B) 12 C) 10 D) 22
Answer: C) 10
Solution: Na → 11e; Na⁺ → 11 – 1 = 10
Shortcut: “Cation → minus charge”
Concept tag: Ions -
Isotopes of an element have different A) Proton number B) Neutron number C) Electron number D) Valency
Answer: B) Neutron number
Solution: Isotopes = same Z, different N
Shortcut: “Isotopes → new-trons differ”
Concept tag: Isotopes -
The electronic configuration of Ar (Z = 18) is A) 2,8,8 B) 2,8,7 C) 2,8,9 D) 2,8,6,2
Answer: A) 2,8,8
Solution: Fills K,L,M completely
Shortcut: “Noble gas → 2,8,8 stable”
Concept tag: Electronic configuration -
The nucleus of an atom contains A) e⁻ & p⁺ B) p⁺ & n⁰ C) e⁻ only D) n⁰ only
Answer: B) p⁺ & n⁰
Solution: Nucleons = protons + neutrons
Shortcut: “Nucleus = P & N canteen”
Concept tag: Nucleus composition -
Which experiment led to the discovery of the nucleus? A) Cathode ray B) Oil drop C) α-scattering D) discharge tube
Answer: C) α-scattering
Solution: Rutherford’s gold-foil expt.
Shortcut: “Alpha → nucleus”
Concept tag: Experiments -
The valency of oxygen (Z = 8) is A) 1 B) 2 C) 6 D) 8
Answer: B) 2
Solution: EC = 2,6 → 8 – 6 = 2
Shortcut: “Need 2 to make octet”
Concept tag: Valency -
An isobar of ⁴⁰₂₀Ca is A) ⁴⁰₁₉K B) ³⁹₁₉K C) ⁴⁰₂₁Sc D) ⁴²₂₀Ca
Answer: A) ⁴⁰₁₉K
Solution: Same A = 40, different Z
Shortcut: “Top match, bottom mismatch”
Concept tag: Isobars
Speed Tricks
| Situation | Shortcut | Example |
|---|---|---|
| Neutron count | A – Z in head | ³⁷Cl → 37 – 17 = 20 n |
| Max electrons in shell | 2n² | n = 4 → 2 × 16 = 32 |
| Valency ≤4 | valency = outermost electrons | Al (13) → 2,8,3 → valency 3 |
| Valency >4 | valency = 8 – outermost | P (15) → 2,8,5 → 8 – 5 = 3 |
| Isobar vs Isotope | “Top same = isobar, bottom same = isotope” | ¹⁴C & ¹⁴N → isobars; ³⁵Cl & ³⁷Cl → isotopes |
Quick Revision
| Point | Detail |
|---|---|
| 1 | Proton charge = +1e, mass ≈ 1 u |
| 2 | Electron charge = –1e, mass ≈ 1/1836 u |
| 3 | Neutron mass ≈ 1 u, no charge |
| 4 | Atomic number = proton count = electron count in neutral atom |
| 5 | Mass number = protons + neutrons (whole number) |
| 6 | Isotopes have same chemical properties (same e⁻ arrangement) |
| 7 | Cathode rays produce green fluorescence on ZnS screen |
| 8 | Rutherford: Nucleus volume ~ 10⁻¹⁵ m, atom ~ 10⁻¹⁰ m |
| 9 | Octet rule: atom tends to have 8 e⁻ in outermost shell |
| 10 | Bohr’s orbits = stationary energy levels; no energy radiation while in orbit |
BETA
