Ch 02: Atomic Structure | Exam Fusion Prep

⚛️ Atom — At a Glance

📘 Atom — Origin

Greek word for indivisible. Smallest particle of matter that participates in chemical reactions. Discoverer: Democritus. Called indestructible by Indian philosopher Maharishi Kanada.

Democritus

Discoverer of Atom concept

1808

Dalton's Atomic Theory

2n²

e⁻ in nth Orbit

1/1836

Electron mass vs Proton

📊 Key Discoverers — Quick Reference

Discovery / Law	Person	Year
Atom concept	Democritus	Ancient Greece
Law of Conservation of Mass	Antoine Lavoisier	1789
Law of Definite Proportion	Joseph Proust	1799
Law of Multiple Proportion + Atomic Theory	Dalton	1803 / 1808
Electron	J.J. Thomson	1897
Thomson's Atomic Model	J.J. Thomson	1898
Rutherford's Nuclear Model	Rutherford	1911
Proton	Rutherford	1919
Bohr's Model	Niels Bohr	1913
Neutron	James Chadwick	1932
Millikan Oil Drop (charge of e⁻)	R.A. Millikan	1909

📜 Laws of Chemical Combination

Law 1 — Antoine Lavoisier (1789)

Law of Conservation of Mass

"Matter can neither be created nor be destroyed." Total mass of reactants = total mass of products.

O (16u) + H₂ (2u) → H₂O (18u)Mass is conserved in reactions

Law 2 — Joseph Proust (1799)

Law of Definite Proportion (Law of Constant Proportion)

A compound always contains elements in the exact same proportion by weight, regardless of source.

H : O in water = 2:16 = 1:8 (by mass)Same everywhere — Ganges, ocean, lab

Law 3 — Dalton (1803)

Law of Multiple Proportion

When two elements form more than one compound, masses of one element combining with a fixed mass of the other are in ratio of small whole numbers.

📘 Example

CO and CO₂ — Oxygen combining with Carbon: 16:32 = 1:2 (small whole numbers)

⚠️ Exam Trap — Laws

Law of Conservation of Mass = Lavoisier (1789)
Law of Definite Proportion = Proust (1799) — also called Law of Constant Proportion
Law of Multiple Proportion = Dalton (1803)
These three are distinct laws — propounders are frequently swapped in MCQs!

🔬 Atomic Models — Timeline

1808

Dalton's Atomic Theory

John Dalton

Atoms are indivisible, smallest particles of matter. Atoms of same element have same mass & properties. Mass is conserved in reactions.

1898

Thomson's Atomic Model (Watermelon / Plum Pudding Model)

J.J. Thomson

Atom = spherical, electrically neutral (radius 10⁻¹⁰ m). Electrons embedded in positively charged matter — like seeds in a watermelon.

1911

Rutherford's Nuclear Model (α-Particle Scattering)

Ernest Rutherford

Most of atom is empty space (α-particles pass through)
Positive charge & most mass concentrated at nucleus (centre)
Electrons and nucleus bound by electrostatic force
α-particles that hit nucleus → repelled (positive charge)

1913

Bohr's Atomic Model

Niels Bohr (based on Planck's Quantum Theory)

Electrons revolve in fixed circular orbits (K, L, M, N shells)
Energy emitted in the form of Quanta when electron jumps from high to low energy level
Fixed energy for each orbit — no radiation while in orbit

⚠️ Exam Trap — Atomic Models

Thomson model = Watermelon model / Plum Pudding model
Rutherford model = Nuclear model (discovered nucleus)
Bohr model: electrons in K, L, M, N shells (fixed orbits)
Bohr's model based on Planck's quantum theory
Rutherford used α-particle scattering experiment on gold foil

⚡ Stable Elementary Particles

e⁻

Electron

J.J. Thomson — 1897

Charge−1.6022×10⁻¹⁹ C

Relative−1

Mass9.1095×10⁻³¹ kg

Mass (u)0.00054 u

LocationOutside Nucleus

p⁺

Proton

Rutherford — 1919

Charge+1.6022×10⁻¹⁹ C

Relative+1

Mass1.672×10⁻²⁷ kg

Mass (u)1.00727 u

LocationInside Nucleus

n⁰

Neutron

James Chadwick — 1932

Charge0 (Neutral)

Relative0

Mass1.675×10⁻²⁷ kg

Mass (u)1.00868 u

LocationInside Nucleus

🧠 Memory Trick — NEP

Nacho → Neutron → Chadwick
Eat → Electron → Thomson
Pass → Proton → Rutherford

Electron mass vs Proton

1/1836 of proton

Proton mass vs Neutron

≈ Equal

Millikan Oil Drop

R.A. Millikan (1909)

Charge of e⁻

1.602×10⁻¹⁹ C

📘 Special Notes

Hydrogen nucleus = only proton (NO neutron) — only stable nucleus without neutron
Protons + Neutrons inside nucleus = Nucleons
De-Broglie: All particles have dual nature (wave + particle)

💥 Non-Stable (Unstable) Elementary Particles

Particle	Discoverer	Year	Sign	Mass
Positron	C.D. Anderson	1932	+1e⁰	= Electron mass
Meson	Yukawa	1935	π⁺, π⁻, π⁰	200× electron mass
Neutrino	Pauli	1927	ν	Indefinite
Anti-Proton	Segre	1955	p⁻	= Proton mass
Anti-Neutron	Cork	1956	−	= Neutron mass

🔢 Mass Number & Atomic Number

A = P + NMass Number = Protons + Neutrons

Z = P = e⁻ (neutral atom)Atomic Number = No. of Protons

A = e⁻ + N (neutral atom)Mass No. = Electrons + Neutrons

Symbol	Meaning
A	Mass Number
Z	Atomic Number
P	Number of Protons
N	Number of Neutrons
e⁻	Number of Electrons
ᴬzX	Standard element notation

⚠️ Exam Trap

Atomic Number (Z) = number of protons = number of electrons (neutral atom)
Mass Number (A) = protons + neutrons (NOT electrons!)
Carbon (C): Z = 6 → 6 protons, 6 electrons; A = 12 → 6 neutrons
Neutrons = A − Z

🧬 Isotopes, Isobars & Isotones

Type	Same	Different	Example
Isotopes	Atomic Number (Z)	Mass Number (A)	¹H, ²H (Deuterium), ³H (Tritium)
Isobars	Mass Number (A)	Atomic Number (Z)	¹⁸Ar⁴⁰, ¹⁹K⁴⁰, ²⁰Ca⁴⁰
Isotones	Number of Neutrons	Atomic Number & Mass No.	₁H³ and ₂He⁴ (both 2 neutrons)
Isoelectronic	Number of Electrons	Atomic Number	Ne, Na⁺, Mg²⁺, Al³⁺ (all 10 e⁻)

Special Isotope Facts

³₁H (Tritium) = Lightest isotope → Radioactive
Heaviest stable isotope = Lead (Pb)-208 (mass 207.974 u)
Element with most isotopes = Polonium (Po)
Hydrogen has 3 isotopes: Protium (¹H), Deuterium (²H), Tritium (³H)

Isoelectronic Examples

Ne, Na⁺, Mg²⁺, Al³⁺ — all have 10 electrons
Z determines basic properties of atom
In neutral atom: Z = number of electrons

⚠️ Exam Trap — Iso- Terms

Isotopes = same Z, different A (same element, different mass)
Isobars = same A, different Z (different elements, same mass)
Isotones = same neutron count (N = A − Z is same)
Most isotopes = Polonium (Po) (NOT Carbon or Hydrogen)
Tritium (³H) = radioactive isotope of hydrogen

🌀 Quantum Numbers

📘 Definition

Numbers indicating the position and energy of an electron in an atom. Four types required to uniquely identify any electron.

Principal Quantum Number

Indicates electron energy level and distance from nucleus. Values: 1, 2, 3, 4... (K, L, M, N shells)

Azimuthal Quantum Number

Shows the shape of the orbital. Values: 0 to (n−1). s(0), p(1), d(2), f(3)

Magnetic Quantum Number

Determines orientation of orbital in space. Values: −l to +l

Spin Quantum Number

Direction of spin of electron. Values: +½ (clockwise) or −½ (anti-clockwise)

📐 Electronic Configuration Rules

Max e⁻ in nth orbit = 2n²K(n=1)=2 | L(n=2)=8 | M(n=3)=18 | N(n=4)=32

Rule 1 — Aufbau Principle

Fill lowest energy orbitals first

Electrons are filled in increasing order of energy.

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5dAufbau Order | "Aufbau" = German for "construction/building up"

📘 Special Cases (Exceptions to Aufbau)

²⁴Cr: Expected 3d⁴4s² → Actual: 3d⁵4s¹ (half-filled 3d is more stable)
²⁹Cu: Expected 3d⁹4s² → Actual: 3d¹⁰4s¹ (fully-filled 3d is more stable)

Rule 2 — Pauli Exclusion Principle

No two electrons can have identical quantum numbers

If n, l, m are the same for two electrons → their spin (s) must be opposite (+½ and −½).

→ Maximum 2 electrons per orbital (opposite spins)

Rule 3 — Hund's Rule of Maximum Multiplicity

Fill each orbital singly before pairing

Electrons occupy orbitals with same energy singly first, then pair up.

p subshell filling pattern:

p¹:

↑

p²:

↑

p³:

↑

(half-filled = stable)

p⁴:

↑↓

↑

p⁵:

↑↓

↑

p⁶:

↑↓

(fully filled = stable)

⚠️ Exam Trap — Electronic Config

In Aufbau: 4s fills before 3d (4s has lower energy than 3d)
Cr (24) and Cu (29) are exceptions — half-filled & fully-filled d orbitals are more stable
Pauli Exclusion: max 2 electrons per orbital, opposite spins
Hund's Rule: all orbitals in subshell get one electron first, then pairing
Max electrons: K=2, L=8, M=18, N=32 (formula: 2n²)

🎯 High-Frequency BPSC/BSSC Exam Points

Atom concept: Democritus (Greek); Indestructible: Maharishi Kanada
Law of Conservation of Mass: Lavoisier (1789)
Law of Definite Proportion = Law of Constant Proportion: Proust (1799)
Law of Multiple Proportion: Dalton (1803)
Thomson model (1898) = Watermelon/Plum Pudding model
Rutherford model (1911) = Nuclear model (α-particle scattering on gold foil)
Bohr model (1913) = electrons in K, L, M, N fixed orbits; energy as Quanta
Electron: J.J. Thomson (1897) | Mass = 9.1095×10⁻³¹ kg | Charge = −1
Proton: Rutherford (1919) | Mass = 1.672×10⁻²⁷ kg | Charge = +1
Neutron: James Chadwick (1932) | Mass = 1.675×10⁻²⁷ kg | Charge = 0
Electron mass = 1/1836 of proton mass
Proton mass ≈ Neutron mass
Millikan Oil Drop = R.A. Millikan (1909) → charge of e⁻ = 1.602×10⁻¹⁹ C
Hydrogen = only stable nucleus with no neutron
Protons + Neutrons = Nucleons
Mass Number A = P + N | Atomic Number Z = P = e⁻ (neutral)
Isotopes = same Z, different A | Isobars = same A, different Z | Isotones = same N
Most isotopes: Polonium (Po)
Tritium (³H) = radioactive | Heaviest stable = Pb-208
Max e⁻ in nth orbit = 2n²
Aufbau: 4s fills before 3d | Exceptions: Cr (3d⁵4s¹), Cu (3d¹⁰4s¹)
Hund's Rule: each orbital gets one electron first, then pairing
Pauli: max 2 electrons per orbital (opposite spin)
Isoelectronic: Ne, Na⁺, Mg²⁺, Al³⁺ — all 10 electrons

🧠 Memory Tricks

Particle Discoverers — NEP

Nacho → Neutron → Chadwick
Eat → Electron → Thomson
Pass → Proton → Rutherford

Laws Order — LEP→D (Chronological)

Lavoisier → Conservation of Mass (1789)
Proust → Definite Proportion (1799)
Dalton → Multiple Proportion (1803)

⚠️ Most Common Exam Traps

Thomson = Watermelon model (NOT nuclear)
Rutherford = discovered nucleus & proton (two different discoveries)
Bohr = fixed orbits; energy as Quanta
Most isotopes = Polonium (NOT Tin or Carbon)
Heaviest stable isotope = Lead (Pb)
Aufbau: 4s before 3d (order matters!)
Cr & Cu: exceptions to Aufbau (half/fully filled d is stable)
Neutrons in H = 0 (only proton, no neutron)
Electron mass = 1/1836 × proton (not 1/2000)
Isobar = same A; Isotope = same Z; Isotone = same N

⚛️ Atomic Structure