NDA Chemistry · Atomic Structure and Periodic Classification

Atomic Number, Mass Number and Subatomic Particles

An atom is built from protons, neutrons and electrons; the atomic number counts the protons, the mass number counts protons plus neutrons, and from these two numbers everything else follows.

All Atomic Structure and Periodic Classification notes NDA Chemistry strategy

Why this matters

7 PYQs. Half are pure definition recall (mass of an electron, atomic mass = protons + neutrons) and half are short counts (electrons in an ion, formula mass of a compound). Lock down the three particle properties and the two defining numbers, and the counting questions become one-line arithmetic.

Concept 1 of 4: The three subatomic particles — charge, mass and location

Every atom is made of three particles. Protons (positive) and neutrons (neutral) sit in the nucleus and carry essentially all the mass; electrons (negative) orbit outside and are almost massless. The one number the bank loves: an electron is about 1/1836 (roughly 1/2000) the mass of a proton.

Definition

The three particles and their properties:

Proton — charge +1, mass ≈ 1 u, located in the nucleus. Its count is the atomic number.
Neutron — charge 0 (neutral), mass ≈ 1 u, located in the nucleus. A neutron is its OWN particle — NOT a proton plus an electron stuck together.
Electron — charge −1, mass ≈ 1/1836 ≈ 1/2000 of a proton (≈ 9.1 × 10⁻³¹ kg), located in shells outside the nucleus.

Protons and neutrons together are called nucleons.

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.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2025 · Q72Moderate

Example 1 · Atomic Structure and Periodic Classification · Atomic Number, Mass Number and Subatomic Particles

Which one of the following statements is correct?

A neutron is not a proton + electron

'A neutron is formed by combination of an electron and a proton, therefore it is neutral' is false. The neutron is a distinct fundamental particle.

Drill 1 more on the three subatomic particles — charge, mass and location

Concept 2 of 4: Atomic number, mass number and counting nucleons

Two numbers define a nuclide. The atomic number Z is the number of protons — and it is the single most fundamental property of an element, because it never changes for that element. The mass number A is protons plus neutrons (nucleons). From these, you read off electrons, neutrons and even the charge.

Definition

The two defining counts and what flows from them:

Atomic number (Z) = number of protons. It is the most fundamental characteristic of an element — it identifies the element.
Mass number (A) = number of protons + neutrons (nucleons). The atomic mass is therefore the sum of protons and neutrons only (electrons are negligible).
Number of neutrons = A − Z.
In a neutral atom, electrons = protons = Z. For an ion, add electrons for a negative charge and subtract for a positive charge.
For ⁳²₁₆S²⁻: nucleons (y) = A = 32; electrons (x) = 16 + 2 = 18.

Mass number and neutron count

A = Z + N \qquad N = A - Z \qquad e^-_{\text{(ion)}} = Z - (\text{charge})

Amass number (nucleons)
Zatomic number (protons)
Nnumber of neutrons

Worked example

For the sulphide ion

^{32}_{16}\text{S}^{2-}

, find the number of electrons and the number of nucleons.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2026 · Q76Easy

Example 2 · Atomic Structure and Periodic Classification · Atomic Number, Mass Number and Subatomic Particles

x

is the number of electrons and

y

is the number of nucleons in

^{32}_{16}S^{2-}

, then which one is correct?

Atomic mass = protons + neutrons, not + electrons

The atomic mass is the sum of protons and neutrons only. Electrons are about 1/2000 the mass of a nucleon, so they contribute nothing — do not add electrons.

Charge changes electrons, not nucleons

A 2− or 3+ charge changes the electron count, never the proton or neutron count. The mass number (nucleons) of an ion equals that of its neutral atom.

Drill 1 more on atomic number, mass number and counting nucleons

Concept 3 of 4: Formula mass and reading valency from the atom

Once you know atomic masses, the formula mass of a compound is just the sum over every atom in the formula. And once you know the atomic number, you know the electron arrangement — which fixes the element's valency. The bank pairs these: add up a compound's mass, or spot the one wrong statement about an element from its atomic and mass numbers.

Definition

Two skills the bank tests:

Formula mass = sum of (atomic mass × number of atoms) for every element in the formula. Example — anhydrous sodium carbonate Na₂CO₃: 2(23) + 12 + 3(16) = 46 + 12 + 48 = 106 u.
Valency from the atom — work out the electron configuration from Z, then the valency is the electrons gained, lost or shared to reach an octet. Aluminium (Z = 13, config 2,8,3) loses 3 electrons → valency 3, NOT 2.

Formula mass

\text{Formula mass} = \sum (\text{atomic mass} \times \text{number of atoms})

Worked example

Find the formula mass of calcium carbonate, CaCO₃. (Atomic masses: Ca = 40 u, C = 12 u, O = 16 u.)

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Sep 2018 · Q65Easy

Example 3 · Atomic Structure and Periodic Classification · Atomic Number, Mass Number and Subatomic Particles

What is the formula mass of anhydrous sodium carbonate? (Given that the atomic masses of sodium, carbon and oxygen are 23 u, 12 u and 16 u respectively)

Aluminium's valency is 3, not 2

From the configuration 2,8,3, aluminium loses 3 electrons to reach an octet — valency 3. A statement claiming 'the valency of Al is 2' is the false one.

Drill 1 more on formula mass and reading valency from the atom

Concept 4 of 4: Average atomic mass from isotope proportions

Most elements are a mixture of isotopes — same protons, different neutrons, so different masses. The atomic mass on the periodic table is the WEIGHTED average of those isotope masses, weighted by how common each one is. The bank gives you the masses and the ratio and asks for the average.

Definition

The average atomic mass is the sum of (each isotope's mass × its fraction):

Convert the ratio to fractions that add to 1 (e.g. a 3 : 1 ratio → 3/4 and 1/4).
Multiply each isotope's mass by its fraction and add.
Example — oxygen with masses 16 u and 18 u in the ratio 3 : 1: average = (3 × 16 + 1 × 18)/4 = 66/4 = 16.5 u.

Weighted average atomic mass

\bar{M} = \frac{m_1 f_1 + m_2 f_2}{f_1 + f_2}

m_1, m_2isotope masses
f_1, f_2their proportions (parts of the ratio)

Worked example

Chlorine occurs as two isotopes of masses 35 u and 37 u in the proportion 3 : 1. Find its average atomic mass.

Practice this conceptself-check · 2 quick reps

The same idea in a real exam question:

NDA · Sep 2018 · Q63Moderate

Example 4 · Atomic Structure and Periodic Classification · Atomic Number, Mass Number and Subatomic Particles

A sample of oxygen contains two isotopes of oxygen with masses 16 u and 18 u respectively. The proportion of these isotopes in the sample is 3 : 1. What will be the average atomic mass of oxygen in this sample?

Weight by proportion, don't just average the masses

For a 3 : 1 mix of 16 u and 18 u the answer is 16.5 u, not the plain mean 17 u. Always multiply each mass by its fraction (3/4 and 1/4) before adding.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

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

Test yourself on Atomic Structure and Periodic Classification

15 past NDA questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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