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Structure of Atom
Chemistry · Class 11NCERT pp. 29–682 sets × 25 questionsFree
Key facts and formulas for Structure of Atom, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.
Formula sheet for this chapter
| Idea | Rule or fact |
|---|
| Charge and mass of particles | q = n e with e = 1.602 × 10⁻¹⁹ C; neutrons = A − Z; electrons = Z − charge |
| Wavelength, frequency, wavenumber | c = νλ; wavenumber = 1/λ (in cm⁻¹ when λ is in cm); 1 nm = 10⁻⁹ m |
| Photon energy and photoelectric effect | E = hν = hc/λ; KE = h(ν − ν₀); threshold λ₀ = hc/W₀ (hc = 1240 eV nm) |
| Rydberg formula | 1/λ = R (1/n₁² − 1/n₂²) with R = 1.09677 × 10⁷ m⁻¹; lines from level n = n(n − 1)/2 |
| Bohr model for H-like species | rₙ = 52.9 pm × n²/Z; Eₙ = −2.18 × 10⁻¹⁸ J × Z²/n²; vₙ ∝ Z/n; m v r = n h/2π |
| de Broglie and uncertainty | λ = h/(mv) = h/p; λ = h/√(2meV); Δx·Δp ≥ h/4π; ΔE·Δt ≥ h/4π |
| Quantum numbers | Orbitals in shell n = n²; radial nodes = n − l − 1; angular nodes = l; L = √(l(l+1)) h/2π |
| Electronic configuration rules | Aufbau, Pauli, Hund; Cr 3d⁵4s¹, Cu 3d¹⁰4s¹; cations lose 4s electrons first |
| Magnetic moment and stability | μ = √(n(n+2)) BM; half-filled d⁵ and full d¹⁰ are extra stable; d⁵ has 10 exchanges |
Three sample questions
Q1. Which statement about cathode rays is NOT correct?
- They travel from cathode to anode
- Their properties change with the gas in the tube
- They are deflected by electric and magnetic fields
- They make zinc sulphide glow
Hint and solution
Hint: Recall Thomson's finding: do cathode rays depend on the gas or the electrode metal used in the tube?
- Asked: the statement that is NOT correct — We must find the false statement, so we check each option against known facts.
- Cathode rays = electron stream, same for all gases — Thomson showed the rays behave the same whatever gas or electrode metal is used.
- Options A, C, D are true — Rays go from cathode to anode, bend in electric and magnetic fields, and make zinc sulphide glow.
- Option B says properties change with gas — This is false, so it is the answer. Option B.
Answer: B. Cathode ray properties are the same for every gas and every electrode metal. That is why electrons are a part of all atoms.
Q2. A charged oil drop carries 4.8 × 10⁻¹⁹ C. The number of excess electrons on it is about (e = 1.602 × 10⁻¹⁹ C)
- 2
- 4
- 5
- 3
Hint and solution
Hint: Charge is quantised: total charge equals the number of electrons times the charge of one electron. Divide.
- q = n × e — Total charge is a whole number of electron charges.
- n = 4.8 × 10⁻¹⁹ / 1.602 × 10⁻¹⁹ — Divide the drop's charge by one electron's charge. The powers of ten cancel.
- n = 4.8 / 1.602 = 2.996 — Arithmetic gives 2.996.
- n ≈ 3 — Electrons are whole numbers, so round to 3. Option D.
Answer: D. q = n e, so n = 4.8 × 10⁻¹⁹ / 1.602 × 10⁻¹⁹ ≈ 3.
Q3. The numbers of protons, neutrons and electrons in ²⁷Al³⁺ (Z = 13) are respectively
- 13, 14, 10
- 13, 14, 13
- 10, 14, 13
- 13, 27, 10
Hint and solution
Hint: Protons equal Z. Neutrons equal mass number minus Z. A 3+ ion has lost three electrons from the neutral atom.
- Protons = Z = 13 — The atomic number gives the number of protons, and it does not change in an ion.
- Neutrons = A − Z = 27 − 13 = 14 — Subtract the atomic number from the mass number.
- Electrons = 13 − 3 = 10 — A 3+ ion has lost three electrons from the neutral atom.
- 13 protons, 14 neutrons, 10 electrons — So the set is 13, 14, 10. Option A.
Answer: A. Protons = 13, neutrons = 27 − 13 = 14, electrons = 13 − 3 = 10.
How to use this chapter
Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.