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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 notesNDA 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

Intuition

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.

ParticleChargeRelative massLocation
Proton+1≈ 1 uNucleus
Neutron0 (neutral)≈ 1 uNucleus
Electron−1≈ 1/2000 of a protonShells 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

Try it yourself

True or false: a neutron is formed by combining a proton and an electron, which is why it is neutral. Justify in one line.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Which subatomic particle has a charge of +1 and sits in the nucleus?
  2. 2.
    About how many times heavier than an electron is a proton?
  3. 3.
    Where in the atom are electrons located?
  4. 4.
    What is the collective name for protons and neutrons together?

From the bank · past-year question

Example 1Atomic Structure and Periodic ClassificationMODERATE
Which one of the following statements is correct?

[Q72 · Apr · 2025]

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

Intuition

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+NN=A−Ze(ion)−=Z−(charge)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 1632S2−^{32}_{16}\text{S}^{2-}, find the number of electrons and the number of nucleons.
  1. The atomic number Z = 16, so a neutral sulphur atom has 16 protons and 16 electrons.
  2. The 2− charge means it has gained 2 electrons: electrons = 16 + 2 = 18.
  3. Nucleons = mass number A = 32 (protons + neutrons), unaffected by charge.
Answer:18 electrons and 32 nucleons.
Practice this conceptself-check · 4 quick reps

Try it yourself

An atom has atomic number 20 and mass number 40. How many neutrons does it contain?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    The atomic mass of an element equals the sum of the numbers of which particles?
  2. 2.
    What is the most fundamental characteristic of an element?
  3. 3.
    An atom of argon has mass number 40 and atomic number 18. How many electrons does the neutral atom have?
  4. 4.
    How many neutrons are in an atom with mass number 27 and atomic number 13?

From the bank · past-year question

Example 2Atomic Structure and Periodic ClassificationEASY
If xx is the number of electrons and yy is the number of nucleons in 1632S2−^{32}_{16}S^{2-}, then which one is correct?

[Q76 · Apr · 2026]

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

Intuition

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

Formula mass=∑(atomic mass×number of atoms)\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.)
  1. Calcium: 1 atom × 40 = 40 u.
  2. Carbon: 1 atom × 12 = 12 u.
  3. Oxygen: 3 atoms × 16 = 48 u.
  4. Add them: 40 + 12 + 48 = 100 u.
Answer:100 u.
Practice this conceptself-check · 3 quick reps

Try it yourself

Aluminium has atomic number 13 and mass number 27. Which is the FALSE statement: (a) it has 13 protons, (b) it has 14 neutrons, (c) its valency is 2, (d) it has 13 electrons when neutral?

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Find the formula mass of water H₂O (H = 1 u, O = 16 u).
  2. 2.
    What is the valency of aluminium (atomic number 13)?
  3. 3.
    Formula mass of carbon dioxide CO₂ (C = 12, O = 16)?

From the bank · past-year question

Example 3Atomic Structure and Periodic ClassificationEASY
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)

[Q65 · Sep · 2018]

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

Intuition

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

Mˉ=m1f1+m2f2f1+f2\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.
  1. The ratio 3 : 1 has 4 total parts.
  2. Weighted sum = 3 × 35 + 1 × 37 = 105 + 37 = 142.
  3. Divide by total parts: 142 / 4 = 35.5 u.
Answer:35.5 u.
Practice this conceptself-check · 2 quick reps

Try it yourself

An element has two isotopes of masses 10 u and 11 u present in equal amounts (1 : 1). What is its average atomic mass?

Practice — Level 1 (2 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Two isotopes of mass 16 u and 18 u occur in the ratio 3 : 1. What is the average atomic mass?
  2. 2.
    Two isotopes of mass 12 u and 14 u occur in the ratio 1 : 1. What is the average atomic mass?

From the bank · past-year question

Example 4Atomic Structure and Periodic ClassificationMODERATE
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?

[Q63 · Sep · 2018]

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)

Reference tables (1)

The three subatomic particles — charge, mass and location3 rows
ParticleChargeRelative massLocation
Proton+1≈ 1 uNucleus
Neutron0 (neutral)≈ 1 uNucleus
Electron−1≈ 1/2000 of a protonShells 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)

Mastery check — 3 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Atomic Structure and Periodic ClassificationEASY
The mass number of argon is 40. Which one of the following statements is correct?

[Q62 · Apr · 2022]

Example 2Atomic Structure and Periodic ClassificationEASY
Atomic mass of an element is equal to the sum of number of

[Q96 · Apr · 2020]

Example 3Atomic Structure and Periodic ClassificationEASY
For Aluminium (Al) (atomic number : 13, mass number : 27), which one among the following statements is NOT correct ?

[Q118 · Sep · 2024]

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