🔬 Structure of Atom – Full Breakdown

AMU Class 11 Science Chemistry June 2026

        This chapter covers the history of the atom — from Thomson to Rutherford to Bohr — plus subatomic particles, atomic number, mass number, isotopes, isobars, and electronic configuration. AMU asks direct questions about who discovered what, differences between isotopes/isobars, and calculating protons/neutrons/electrons. The electronic configuration part (2,8,8,18...) is pure gold for AMU. Let's break it all down.
    

1) Subatomic Particles — The Basics

Particle	Symbol	Charge	Mass (kg)	Mass (u)	Location
Electron	e⁻	–1	9.1 × 10⁻³¹	1/1836 u (≈0)	Outside nucleus (shells)
Proton	p⁺	+1	1.67 × 10⁻²⁷	≈1 u	Nucleus
Neutron	n⁰	0	1.67 × 10⁻²⁷	≈1 u	Nucleus

⚠️ AMU Direct Questions:
• "Which particle is not present in hydrogen atom?" → Neutron (H-1 has only 1 proton, 0 neutrons)
• "Who discovered neutron?" → Chadwick (1932)
• "Who discovered electron?" → J.J. Thomson (1897)
• "Who discovered proton?" → Goldstein / Rutherford

2) Atomic Number (Z) & Mass Number (A)

Atomic Number (Z) = Number of protons = Number of electrons (in neutral atom)
Mass Number (A) = Number of protons + Number of neutrons

Number of neutrons = A – Z

Example: Carbon-12: Z = 6, A = 12 → 6 protons, 6 neutrons, 6 electrons.

Notation: ^A_ZX → ¹²₆C, ²³₁₁Na, ⁴⁰₂₀Ca

3) Models of Atom — The Big Three for AMU

Thomson's Model (Plum Pudding Model) — 1898

Atom = sphere of positive charge with electrons embedded in it (like raisins in a pudding)
Explained: Atom is neutral overall
Failed: Could not explain the results of Rutherford's gold foil experiment

Rutherford's Model (Nuclear Model) — 1911

The Gold Foil Experiment:

α-particles (He²⁺) fired at thin gold foil
Most α-particles passed straight through → most of atom is empty space
A few were deflected by small angles → positive charge is spread throughout
Very few (~1 in 12,000) bounced back → tiny, dense, positively charged nucleus at centre

Conclusions:

Atom has a tiny, dense, positively charged nucleus at the centre
Nucleus contains protons + neutrons (most of the mass)
Electrons revolve around nucleus in circular orbits
Most of the atom is empty space

⚠️ AMU Classic: "Why were some α-particles deflected back in Rutherford's experiment?" Answer: They collided with the tiny, dense, positively charged nucleus and were repelled back. "Why gold foil?" → Gold is highly malleable and can be beaten into extremely thin sheets.

Drawbacks of Rutherford's Model:

According to Maxwell's electromagnetic theory, an accelerating charged particle (electron) should radiate energy → lose energy → spiral into nucleus → atom should collapse.
But atoms are stable → Rutherford couldn't explain this.

Bohr's Model — 1913

Three Postulates (AMU loves these):

Electrons revolve around nucleus in fixed circular paths called shells/orbits.
Each shell has a fixed energy (quantised). Shells are labelled K, L, M, N or n = 1, 2, 3, 4.
An electron can jump from one shell to another by absorbing or emitting energy. Energy absorbed/emitted = E₂ – E₁ = hν.

Limitations of Bohr's Model:

Works only for hydrogen-like atoms (single electron).
Couldn't explain fine spectrum or Zeeman effect.

4) Electronic Configuration — Shell Distribution

Maximum electrons in a shell = 2n²
K shell (n=1): 2 × 1² = 2 electrons
L shell (n=2): 2 × 2² = 8 electrons
M shell (n=3): 2 × 3² = 18 electrons
N shell (n=4): 2 × 4² = 32 electrons

Rules: Fill from innermost shell outward. The outermost shell can have a maximum of 8 electrons (octet rule).

Element	Z	Electronic Configuration	Valence Electrons
H	1	1	1
He	2	2	2 (duplet — stable)
C	6	2, 4	4
N	7	2, 5	5
O	8	2, 6	6
Na	11	2, 8, 1	1
Cl	17	2, 8, 7	7
Ar	18	2, 8, 8	8 (octet — stable)
K	19	2, 8, 8, 1	1
Ca	20	2, 8, 8, 2	2

🧠 Trick — Electronic Configuration:
For elements up to Z=20, just remember: 2, 8, 8, 2 pattern.
K shell fills first (2), then L (8), then M (8), then N (2 for Ca).
BUT remember: M shell can hold 18, but it fills only 8 before N starts filling (that's why K is 2,8,8,1 not 2,8,9).

5) Valence Electrons & Valency

Valence electrons: Electrons in the outermost shell.

Valency: Combining capacity of an element.

Elements with 1-3 valence electrons → positive valency (tend to lose electrons). E.g., Na = +1, Mg = +2, Al = +3
Elements with 5-7 valence electrons → negative valency (tend to gain electrons). E.g., Cl = –1, O = –2, N = –3
Elements with 4 valence electrons → can share (covalent). C = 4 (shares 4)
Elements with 8 valence electrons → zero valency (noble gases). He, Ne, Ar

⚠️ AMU Question: "Why does helium have zero valency?" Answer: It has a duplet (2 electrons in K shell) — completely filled outermost shell → stable → no tendency to gain/lose electrons → valency = 0.

6) Isotopes, Isobars & Isotones

Concept	Definition	Example
Isotopes	Same Z, different A (Same protons, different neutrons)	¹²₆C, ¹³₆C, ¹⁴₆C
Isobars	Same A, different Z (Same mass number, different protons)	⁴⁰₁₈Ar & ⁴⁰₂₀Ca
Isotones	Same N, different Z & A (Same neutrons)	¹⁴₆C & ¹⁵₇N (both have 8 neutrons)

🧠 Memory Trick — "ISO" words:
Isotopes = Same Z (position in periodic table). "Topes" → same TOP (same atomic number).
Isobars = Same A (same mass). "Bars" = weight bar → same weight.
Isotones = Same N (same neutrons). "Tones" → n for neutrons.

Properties: Isotopes → same chemical properties, different physical properties. Isobars → different elements, different chemical properties.

Applications of Isotopes (AMU Loves This)

U-235: Nuclear fission (atomic bomb, nuclear power)
Co-60: Cancer treatment (radiotherapy)
I-131: Thyroid disorders diagnosis & treatment
C-14: Carbon dating (archeological fossils)
H-2 (Deuterium): Heavy water (moderator in nuclear reactors)

7) Common AMU-Style MCQs with Answers

Q1:

Who discovered the electron?

(a) Rutherford (b) J.J. Thomson (c) Chadwick (d) Bohr

Answer: (b) J.J. Thomson

Q2:

The atomic number of an element is 13 and mass number is 27. The number of neutrons is:

(a) 13 (b) 14 (c) 27 (d) 40

Answer: (b) 14 (n = A – Z = 27 – 13 = 14)

Q3:

Which of the following are isobars?

(a) ¹²C and ¹⁴C (b) ⁴⁰Ar and ⁴⁰Ca (c) ¹H and ²H (d) ¹⁶O and ¹⁸O

Answer: (b) ⁴⁰Ar and ⁴⁰Ca (same mass number 40, different atomic numbers)

Q4:

The maximum number of electrons in the M shell is:

(a) 8 (b) 18 (c) 32 (d) 2

Answer: (b) 18 (2n² = 2 × 3² = 18)

Q5:

Which particle has no charge?

(a) Electron (b) Proton (c) Neutron (d) α-particle

Answer: (c) Neutron

Q6:

In Rutherford's experiment, most α-particles passed straight through the gold foil because:

(a) Gold has low density (b) Most of the atom is empty space (c) α-particles are very fast (d) Nucleus repels them

Answer: (b) Most of the atom is empty space

Q7:

Electronic configuration of sodium (Z=11) is:

(a) 2, 8, 1 (b) 2, 9 (c) 8, 2, 1 (d) 2, 8

Answer: (a) 2, 8, 1

Q8:

Isotopes of an element have:

(a) Same chemical properties (b) Same physical properties (c) Different atomic number (d) Different number of protons

Answer: (a) Same chemical properties (same number of electrons → same chemical behavior)

Q9:

The valency of chlorine is:

(a) +1 (b) –1 (c) +7 (d) 0

Answer: (b) –1 (needs 1 electron to complete octet)

Q10:

Which of the following has the same number of electrons as Na⁺?

(a) Mg²⁺ (b) F⁻ (c) O²⁻ (d) All of these

Answer: (d) All of these (Na⁺ = 10 e⁻, Mg²⁺ = 10 e⁻, F⁻ = 10 e⁻, O²⁻ = 10 e⁻. These are isoelectronic species.)

8) Quick One-Liner Revision

Electron (−): J.J. Thomson (1897). Proton (+): Goldstein/Rutherford. Neutron (0): Chadwick (1932).
Atomic number Z = number of protons. Mass number A = protons + neutrons.
Rutherford's gold foil: most α passed through → empty space. Few bounced back → tiny dense + nucleus.
Bohr's postulates: fixed shells, quantised energy, electron jumps with hν.
Max electrons in shell = 2n². K(2), L(8), M(18), N(32).
Valence electrons = outermost shell electrons. Valency = combining capacity.
Isotopes: same Z, diff A. Isobars: same A, diff Z. Isotones: same N.
Isotopes: same chemical, diff physical properties.
Noble gases (He, Ne, Ar): zero valency (full octet/duplet).
Isoelectronic species: same number of electrons (Na⁺, Mg²⁺, F⁻, O²⁻ all have 10).

🧠 From a 2026 AMU topper — my final tips:
1. Subatomic particles table (electron, proton, neutron with discoverer, charge, mass) = 100% guaranteed 1-2 questions.
2. Rutherford's experiment is a favourite — know the three observations and their conclusions.
3. Isotopes vs Isobars difference is a classic AMU MCQ. Remember: Isotopes = same position (Z), Isobars = same weight bar (A).
4. Electronic configuration up to Ca (Z=20) — practice writing these fast.
5. Bohr's postulates (3) are asked directly. Memorise them word-for-word.