NDA Chemistry · Teaching notes

Atomic Structure and Periodic Classification — NDA Chemistry

Atomic structure is the foundation of all of chemistry — 35 PYQs across 2017–2026, mostly EASY or MODERATE, and a mix of pure recall (who discovered the neutron, the maximum electrons in a shell) and short calculations (average atomic mass, isotopic abundance). The chapter teaches in five movements, building from how we learned the atom has a structure up to the periodic patterns that follow from it: (1) Atomic models — the Dalton, Thomson, Rutherford and Bohr pictures, and which discovery belongs to whom; (2) Atomic number, mass number and the three subatomic particles — the definitions and the simple counting that flows from them (formula mass, mass number = protons + neutrons); (3) Isotopes and isoelectronic species — same element different mass vs different species same electron count, plus the weighted-average-mass calculation; (4) Electron configuration and valence shells — the 2n² shell-filling rule and how the valence count decides bonding; (5) Periodic trends, valency and atomicity — group valencies, atomicity of the elements, halogen reactivity, noble gases, and the most-fundamental property of an element. Most concepts are reference tables: memorise the table, win the marks. The few calculation concepts (average atomic mass, abundance, electron counting) carry worked examples.

NDA Chemistry strategy NDA home

Subtopic notes

PYQ weightage by concept

16 concepts · 35 PYQs — where the marks actually sit, so you know what to drill first

Atomic Models — Dalton, Thomson, Rutherford, Bohr6 PYQs · 17%

Concept	PYQs	Share
Who discovered each subatomic particle and model	3	9%
The four atomic models and what each could not explain	2	6%
Dalton's pictorial element symbols	1	3%

Atomic Number, Mass Number and Subatomic Particles7 PYQs · 20%

Concept	PYQs	Share
The three subatomic particles — charge, mass and location	2	6%
Atomic number, mass number and counting nucleons	2	6%
Formula mass and reading valency from the atom	2	6%
Average atomic mass from isotope proportions	1	3%

Isotopes and Isoelectronic Species6 PYQs · 17%

Concept	PYQs	Share
Isotopes, isobars and isoelectronic species — the definitions	3	9%
Average atomic mass and finding isotope abundance	2	6%
Useful radioactive isotopes	1	3%

Electron Configuration and Valence Shells4 PYQs · 11%

Concept	PYQs	Share
Maximum electrons in a shell (the 2n² rule)	2	6%
Valence-shell electrons and bonding from the configuration	2	6%

Periodic Trends, Valency and Atomicity12 PYQs · 34%

Concept	PYQs	Share
Valency, groups and the most fundamental property	5	14%
Atomicity — atoms per molecule of a free element	3	9%
Reactivity and oxidising-power trends	2	6%
Noble gases — inertness and uses	2	6%

Formula & revision sheet

7 formulas · 9 reference tables · 19 gotchas across all subtopics — the exam-eve cheat-sheet

Atomic Models — Dalton, Thomson, Rutherford, Bohr

Reference tables (3)

Who discovered each subatomic particle and model4 rows

Discovery	Scientist	Experiment / note
Electron	J. J. Thomson	Cathode rays; proposed the plum-pudding model
Proton	E. Goldstein	Canal rays (anode rays)
Neutron	James Chadwick	1932; neutral particle in the nucleus NDA 2025 + 2020 — the neutron was discovered by James Chadwick.
Nucleus	Ernest Rutherford	Alpha-particle (gold-foil) scattering experiment NDA 2017 — Rutherford's alpha-scattering experiment discovered the nucleus.

The four atomic models and what each could not explain4 rows

Model	Key claim	Could NOT explain
Dalton	Indivisible solid sphere; identical atoms	Subatomic particles (electrons, nucleus)
Thomson (plum pudding)	Positive sphere with electrons embedded	The dense nucleus / scattering of alpha particles
Rutherford (nuclear)	Tiny dense POSITIVE nucleus; mostly empty space	Why electrons don't spiral in; fixed energy orbits Rutherford's nucleus is POSITIVELY charged, not neutral. Fixed energy orbits are a BOHR idea, not Rutherford's.
Bohr	Electrons in fixed circular orbits of definite energy	Spectra of multi-electron atoms (refined later)

Fixed energy orbits = Bohr. Nucleus = Rutherford. Plum pudding = Thomson. Indivisible sphere = Dalton.

Dalton's pictorial element symbols4 rows

Element	Dalton's symbol
Phosphorus	Circle with a cross (+) inside (circled cross) NDA 2023 — Dalton's phosphorus is the circle with the + sign inside.
Oxygen	A plain open circle
Hydrogen	A circle with a central dot
Sulphur	A circle with a letter mark inside

Watch out for (3)

Chadwick = neutron, not Rutherford
→ Who discovered each subatomic particle and model
Rutherford's nucleus is positive, not neutral
→ The four atomic models and what each could not explain
Fixed-energy orbits belong to Bohr
→ The four atomic models and what each could not explain

Atomic Number, Mass Number and Subatomic Particles

Formulas (3)

Atomic number, mass number and counting nucleons · Mass number and neutron count $A = Z + N \qquad N = A - Z \qquad e^-_{\text{(ion)}} = Z - (\text{charge})$
Formula mass and reading valency from the atom · Formula mass $\text{Formula mass} = \sum (\text{atomic mass} \times \text{number of atoms})$
Average atomic mass from isotope proportions · Weighted average atomic mass $\bar{M} = \frac{m_1 f_1 + m_2 f_2}{f_1 + f_2}$

Reference tables (1)

The three subatomic particles — charge, mass and location3 rows

Particle	Charge	Relative mass	Location
Proton	+1	≈ 1 u	Nucleus
Neutron	0 (neutral)	≈ 1 u	Nucleus
Electron	−1	≈ 1/2000 of a proton	Shells outside the nucleus NDA 2025 — the mass of an electron is about 1/2000 (precisely 1/1836) that of a proton.

Protons + neutrons = nucleons, carrying nearly all the mass. Electrons are almost massless.

Watch out for (5)

A neutron is not a proton + electron
→ The three subatomic particles — charge, mass and location
Atomic mass = protons + neutrons, not + electrons
→ Atomic number, mass number and counting nucleons
Charge changes electrons, not nucleons
→ Atomic number, mass number and counting nucleons
Aluminium's valency is 3, not 2
→ Formula mass and reading valency from the atom
Weight by proportion, don't just average the masses
→ Average atomic mass from isotope proportions

Isotopes and Isoelectronic Species

Formulas (1)

Average atomic mass and finding isotope abundance · Weighted average and back-solving abundance $\bar{M} = \frac{m_1 x + m_2 (100 - x)}{100}$

Reference tables (2)

Isotopes, isobars and isoelectronic species — the definitions3 rows

Term	What is the same	What differs	Example
Isotopes	Protons (Z) — same element	Neutrons / mass number	³⁵Cl and ³⁷Cl
Isobars	Mass number (A)	Element / proton count	⁴⁰Ar and ⁴⁰Ca
Isoelectronic	Number of electrons	Element and charge	Na⁺, F⁻, O²⁻, Ne (all 10 e⁻) To test isoelectronic species, just count electrons: protons minus the charge.

Useful radioactive isotopes4 rows

Isotope	Main use
Cobalt-60	Cancer treatment (radiotherapy) NDA 2020 — the isotope used to treat cancer is cobalt-60. (Cobalt, not iodine.)
Iodine-131	Treating thyroid disorders
Carbon-14	Radiocarbon dating of fossils
Uranium-235	Nuclear fuel (fission)

Watch out for (4)

N⁻ is NOT isoelectronic with F⁻
→ Isotopes, isobars and isoelectronic species — the definitions
Isotopes vs isobars vs isoelectronic
→ Isotopes, isobars and isoelectronic species — the definitions
Read the order of the answer
→ Average atomic mass and finding isotope abundance
Cobalt-60 for cancer, iodine-131 for thyroid
→ Useful radioactive isotopes

Electron Configuration and Valence Shells

Formulas (2)

Maximum electrons in a shell (the 2n² rule) · Maximum electrons per shell $\text{Maximum electrons} = 2n^2$
Valence-shell electrons and bonding from the configuration · Electrons gained/lost to reach an octet $\text{Valency} = 8 - (\text{valence electrons}) \;\text{ or }\; (\text{valence electrons})$

Watch out for (2)

2n² is a CAP, not the actual filling
→ Maximum electrons in a shell (the 2n² rule)
Valence electrons live in the OUTERMOST shell only
→ Valence-shell electrons and bonding from the configuration

Periodic Trends, Valency and Atomicity

Formulas (1)

Valency, groups and the most fundamental property · Standard valencies by group $\text{Ne}=0 \quad \text{Mg}=2 \quad \text{N}=3 \quad \text{Si}=4$

Reference tables (3)

Atomicity — atoms per molecule of a free element6 rows

Element	Molecule	Atomicity
Neon	Ne	1 (monatomic)
Nitrogen	N₂	2 (diatomic)
Chlorine	Cl₂	2 (diatomic)
Iodine	I₂	2 (diatomic)
Phosphorus	P₄	4 (tetra-atomic, polyatomic) NDA 2024 — phosphorus is the polyatomic element (P₄), unlike diatomic Cl₂ or metallic Al.
Sulphur	S₈	8 (octa-atomic, polyatomic)

Noble gases = 1; common gases = 2; phosphorus = 4; sulphur = 8.

Reactivity and oxidising-power trends2 rows

Trend	Direction down the group	Extreme the bank asks for
Halogen oxidising power	Decreases (F strongest, I weakest)	Increasing order: I < Br < Cl < F NDA 2023 — increasing oxidising order of halogens is I, Br, Cl, F.
Alkali-metal reactivity with water	Increases (Li least, Cs most)	Lithium is least reactive with water NDA 2017 — among alkali metals, lithium is the least reactive with water.

Noble gases — inertness and uses5 rows

Noble gas	Signature use
Argon	Fills bulbs so the tungsten filament lasts longer
Neon	Advertising / neon signs
Krypton	Airport landing lights and lighthouses
Xenon	Photographer's flash gun
Radon	An inert (noble) gas; also radioactive NDA 2017 — radon is an inert gas (Group 18).

Watch out for (5)

Same group = same valency
→ Valency, groups and the most fundamental property
Phosphorus is P₄, sulphur is S₈
→ Atomicity — atoms per molecule of a free element
Iodine is diatomic, not monatomic
→ Atomicity — atoms per molecule of a free element
Oxidising power falls down the group, reactivity rises
→ Reactivity and oxidising-power trends
Argon protects the filament; neon makes the glow
→ Noble gases — inertness and uses