Paper II · Part B — Chemistry

Atomic Structure

Exam-focused notes, visual explanation, traps, revision sheet and every mapped past NDA question.

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Core5 mapped NDA questions

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Build exam-ready understanding by connecting the core idea, formulas or facts, worked example and previous NDA questions. Focus on why each rule works, then test it under timed conditions.

Concept

An atom has a central nucleus of protons and neutrons, surrounded by electrons in shells. The atomic number is the number of protons; the mass number is protons plus neutrons.

Electrons fill shells in order, determining chemical behaviour (valency).

Key Points & Formulas

Proton: +1 charge; neutron: neutral; electron: −1 charge.
Atomic number (Z) = number of protons; mass number (A) = protons + neutrons.
Isotopes: same Z, different mass number.
Valency relates to electrons in the outermost shell.

Worked Example

Q. An atom has atomic number 11 and mass number 23. Find its protons, electrons and neutrons.

Protons = atomic number = 11.

Electrons (neutral atom) = 11.

Neutrons = mass − protons = 23 − 11 = 12.

Common traps

Reading the prompt too quickly and missing a qualifier.
Memorising the result without checking when it applies.
Skipping units, signs, chronology or the final reasonableness check.

NDA shortcuts

State the governing rule before substituting values.
Eliminate impossible options before doing full calculation.

One-screen revision sheet

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Core definition
Most-used rule
One representative example

Past-paper bank

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5 NDA questions

Choose an answer before the solution appears. Option order is mixed automatically.

Question 1

NDA I 2026

If $x$ is the number of electrons and $y$ is the number of nucleons present in $^{32}_{16}\text{S}^{2-}$ , then which one of the following is correct?

Question 2

NDA I 2025

Element X forms a chloride with the formula $XCl_2$ , which is a solid with high melting point. X would most likely be in the same group of periodic table as

Question 3

NDA I 2024

The relative atomic mass of boron (which exists in two isotopic forms 10B and 11B) is 10.81. What will be the abundance of 10B and 11B, respectively (consider a sample of 100 atoms)?

Question 4

NDA II 2024

For Aluminium (Al) (atomic number : 13, mass number : 27), which one among the following statements is NOT correct ?

Question 5

NDA II 2024

Which of the following anions has a valency of −3 ?

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