ChemistryICSE Class 8 · Chapter 4

Atomic Structure ICSE Class 8: Subatomic Particles, Atomic and Mass Numbers, Valency

Updated 15 October 2026 · Fact-checked

An atom has a tiny central nucleus containing protons and neutrons, with electrons moving around it in the extra-nuclear part. In ICSE Class 8, atomic number is the number of protons, mass number is protons plus neutrons, and valency is the number of hydrogen atoms that combine with one atom of an element.

What Atomic Structure is about

An atom is the smallest particle of an element that can take part in a chemical reaction. Atoms are made of even smaller subatomic particles.

The CISCE Class VIII curriculum asks you to know:

the fundamental subatomic particles: electrons, protons and neutrons;
the nucleus and the extra-nuclear part of an atom;
atomic number and mass number;
valency as the number of hydrogen atoms that combine with one atom of the element.

Particle	Symbol	Charge	Relative mass	Where
Proton	p	+1	1	nucleus
Neutron	n	0	1	nucleus
Electron	e	−1	about 1/1836 of a proton	outside the nucleus

A neutral atom has equal numbers of protons and electrons, so its charges cancel.

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Chemistry: your school sets and marks the exam. Atomic Structure sits in the Theme: Atomic Structure theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

Subatomic particles

Protons: positively charged ( $+ 1$ ), found in the nucleus, relative mass 1.
Neutrons: no charge (neutral), found in the nucleus, relative mass about 1. (Ordinary hydrogen has no neutron.)
Electrons: negatively charged ( $- 1$ ), move around the nucleus, with a mass about $\frac{1}{1836}$ of a proton, so tiny that it is usually ignored.

In a neutral atom, number of electrons = number of protons.

Worked example

A neutral atom has 17 protons and 18 neutrons. How many electrons does it have, and what is its overall charge?

Try it first, then show the solutionSolution

Neutral atom: electrons = protons = 17.
Charges: $+ 17$ (protons) and $- 17$ (electrons) cancel; neutrons have no charge.

Answer: 17 electrons; no overall charge

Nucleus and extra-nuclear part

The nucleus is the small, dense centre of the atom. It contains the protons and neutrons (together called nucleons), so almost all the mass of the atom is in the nucleus. The nucleus is positively charged.
The extra-nuclear part is the space around the nucleus where the electrons move in definite paths called shells or orbits. It makes up almost all the volume of the atom.

The atom is mostly empty space: if the nucleus were the size of a marble, the atom would be the size of a large stadium.

Worked example

Why is almost all the mass of an atom in its nucleus?

Try it first, then show the solutionSolution

Protons and neutrons, which are in the nucleus, each have a relative mass of about 1.
Electrons, outside the nucleus, are about 1836 times lighter, so they add almost nothing to the mass.

Answer: Because the heavy particles (protons and neutrons) are in the nucleus.

Atomic number

The atomic number ( $Z$ ) of an element is the number of protons in the nucleus of one of its atoms. In a neutral atom it also equals the number of electrons.

$Z = number of protons$

Every element has its own atomic number: hydrogen 1, carbon 6, oxygen 8, sodium 11, chlorine 17, calcium 20. Atoms with a different number of protons are atoms of a different element.

Worked example

The atomic number of sodium is 11. How many protons and electrons does a neutral sodium atom have?

Try it first, then show the solutionSolution

Protons $= Z = 11$ .
Neutral atom: electrons $=$ protons $= 11$ .

Answer: 11 protons and 11 electrons

Mass number and neutrons

The mass number ( $A$ ) is the total number of protons and neutrons in the nucleus:
$A = protons + neutrons$ , so $neutrons = A - Z$ .

An atom is written as $_{Z}^{A} X$ : for example $_{11}^{23} Na$ has $Z = 11$ and $A = 23$ , so it has $23 - 11 = 12$ neutrons.

Electrons are not counted in the mass number because their mass is so small.

Worked example

An atom of aluminium is $_{13}^{27} Al$ . Find its numbers of protons, neutrons and electrons.

Try it first, then show the solutionSolution

Protons $= Z = 13$ ; electrons $= 13$ (neutral).
Neutrons $= A - Z = 27 - 13 = 14$ .

Answer: 13 protons, 14 neutrons, 13 electrons

Valency from hydrogen

Valency is the combining capacity of an element. In this chapter it is defined as the number of hydrogen atoms that combine with one atom of the element (hydrogen's valency is taken as 1).

Compound	Hydrogen atoms per atom	Valency
HCl	1 per Cl	Cl = 1
H₂O	2 per O	O = 2
NH₃	3 per N	N = 3
CH₄	4 per C	C = 4

The valency tells us the formula of simple hydrogen compounds, and later helps write formulae of other compounds.

Worked example

Sulphur forms hydrogen sulphide, $H_{2} S$ , and phosphorus forms phosphine, $P H_{3}$ . Find the valency of sulphur and phosphorus.

Try it first, then show the solutionSolution

Two hydrogen atoms combine with one sulphur atom: valency of S $= 2$ .
Three hydrogen atoms combine with one phosphorus atom: valency of P $= 3$ .

Answer: S = 2, P = 3

Drawing the structure of an atom

A simple diagram shows the nucleus and the electrons around it.

Draw-and-label checklist:

a small circle in the centre for the nucleus, with the numbers of protons (p) and neutrons (n) written inside;
concentric circles around it for the shells (extra-nuclear part);
electrons shown as dots or crosses on the shells, with the total equal to the number of protons;
the element's symbol, atomic number and mass number written beside it.

For example, carbon $_{6}^{12} C$ : nucleus 6p, 6n; 6 electrons around it (2 in the first shell, 4 in the second).

Worked example

What should be written inside the nucleus and how many electrons should be drawn for oxygen, $_{8}^{16} O$ ?

Try it first, then show the solutionSolution

Protons $= 8$ , neutrons $= 16 - 8 = 8$ : write '8p, 8n' in the nucleus.
Draw 8 electrons around the nucleus (2 in the first shell, 6 in the second).

Answer: 8p, 8n in the nucleus; 8 electrons outside

Chemistry in real life: Atomic Structure

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Mass in the nucleus

Imagine an atom were enlarged until its nucleus was the size of a cricket ball at the centre of a stadium.

Where would almost all the mass be, and what would fill the rest of the stadium?

Try it first, then show the solutionSolution

Protons and neutrons, the heavy particles, are in the nucleus: almost all the mass is in the 'cricket ball'.
The rest is mostly empty space, with tiny electrons moving far out around the nucleus.

Answer: In the nucleus; mostly empty space with electrons

Memory hook: Heavy centre, empty atom.

Counting particles

Suppose you are given a label on a periodic table tile reading 'Ar, atomic number 18, mass number 40'.

How many protons, electrons and neutrons does a neutral argon atom have?

Try it first, then show the solutionSolution

Protons $= 18$ and electrons $= 18$ .
Neutrons $= 40 - 18 = 22$ .

Answer: 18 p, 18 e, 22 n

Memory hook: n = A − Z.

Valency

Suppose a cooking gas such as methane has the formula $C H_{4}$ .

What is the valency of carbon in it?

Try it first, then show the solutionSolution

One carbon atom holds four hydrogen atoms.
So the valency of carbon is 4.

Answer: 4

Memory hook: Count the hydrogens on one atom.

Topics in this chapter

Atomic Structure formula sheet

Atomic number: $Z = protons$; = electrons in a neutral atom.
Mass number: $A = protons + neutrons$; Electrons not counted.
Neutrons: $n = A - Z$; From the symbol ᴬ_Z X.
Valency (from hydrogen): $valency = H atoms per atom$; HCl 1, H₂O 2, NH₃ 3, CH₄ 4.

Atomic Structure: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Structure of an atom

Draw the structure of an atom of carbon (or sodium), showing the nucleus and electrons.

Draw

A small central circle (nucleus) with the numbers of protons and neutrons written inside.
Two or three concentric circles around it (shells).
Electrons as dots on the shells, totalling the atomic number.

Label

Nucleus
Protons (p)
Neutrons (n)
Electrons (e)
Shells (extra-nuclear part)

Watch out: Carbon: 6p, 6n; electrons 2, 4. Sodium: 11p, 12n; electrons 2, 8, 1.

How the school exam asks Atomic Structure

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
Particles	Name the subatomic particles and give their charges, masses and locations.	Use the table: p +1, n 0, e −1.
Atomic and mass number	Define atomic number and mass number.	Z = protons; A = protons + neutrons.
Count particles	Find p, n and e for ²⁷₁₃Al or ³⁹₁₉K.	p = e = Z; n = A − Z.
Valency	Find the valency of N in NH₃, or of C in CH₄.	Count hydrogen atoms per atom.
Structure diagram	Draw an atom showing the nucleus and electrons.	Nucleus with p and n; electrons on shells.

Tricks to remember Atomic Structure

Short memory aids. Say each one aloud once, then use it on a question the same day.

PEN in the middle? No: PN in, E out
Protons and Neutrons are in the nucleus; Electrons are outside.
Use it when: Locations.
Z for protons
Atomic number Z = number of protons; it is the element's identity card.
Use it when: Atomic number questions.
A minus Z
Neutrons = A − Z (mass number minus atomic number).
Use it when: Counting neutrons.
Count the H's
Valency = how many hydrogen atoms one atom holds: Cl 1, O 2, N 3, C 4.
Use it when: Valency from hydrides.

Atomic Structure: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Particle with no charge · 1 mark · MCQExam level

The subatomic particle with no charge is the:
(a) proton
(b) electron
(c) neutron
(d) nucleus

Try it first, then show the solutionSolution

Protons are positive and electrons negative.
Neutrons are neutral; the nucleus is not a single particle.

Answer: (c)

Example 2: Two definitions · 2 marks · Short answerExam level

Define atomic number and mass number.

Try it first, then show the solutionSolution

Atomic number: the number of protons in the nucleus of an atom.
Mass number: the total number of protons and neutrons in the nucleus.

Answer: As above

Example 3: Counting in sodium · 3 marks · Short answerExam level

For $_{11}^{23} Na$ , find the numbers of protons, electrons and neutrons.

Try it first, then show the solutionSolution

Protons $= 11$ .
Electrons $= 11$ (neutral atom).
Neutrons $= 23 - 11 = 12$ .

Answer: 11 p, 11 e, 12 n

Example 4: Finding A and Z · 3 marks · Short answerHarder exam level

An atom has 17 protons and 18 neutrons. Find its atomic number and mass number, and write its symbol in the form $_{Z}^{A} X$ (the element is chlorine, Cl).

Try it first, then show the solutionSolution

$Z = 17$ .
$A = 17 + 18 = 35$ .
Symbol: $_{17}^{35} Cl$ .

Answer: Z = 17, A = 35; ³⁵₁₇Cl

Example 5: Valencies from formulae · 3 marks · Short answerHarder exam level

Find the valency of chlorine, oxygen, nitrogen and carbon from the formulae HCl, H₂O, NH₃ and CH₄.

Try it first, then show the solutionSolution

Valency = number of hydrogen atoms combined with one atom of the element.
Cl: 1; O: 2.
N: 3; C: 4.

Answer: Cl 1, O 2, N 3, C 4

Example 6: Comparing particles · 4 marks · Long answerHarder exam level

Compare protons, neutrons and electrons in terms of charge, relative mass and location. Why is the mass number not affected by electrons?

Try it first, then show the solutionSolution

Proton: charge $+ 1$ , relative mass 1, in the nucleus.
Neutron: no charge, relative mass about 1, in the nucleus.
Electron: charge $- 1$ , mass about $\frac{1}{1836}$ of a proton, outside the nucleus.
Electrons are so light that their mass is negligible, so the mass number counts only protons and neutrons.

Answer: As above

Example 7: Same mass number, different elements · 4 marks · Long answerHOTS / competency

Atom X has 18 protons and 22 neutrons. Atom Y has 20 protons and 20 neutrons. (i) Find the mass number of each. (ii) Are they the same element? Explain.

Try it first, then show the solutionSolution

(i) X: $A = 18 + 22 = 40$ ; Y: $A = 20 + 20 = 40$ .
Both have mass number 40.
(ii) X has atomic number 18 and Y has 20.
The element is decided by the number of protons, so they are different elements, even though their mass numbers are equal.

Answer: (i) Both 40 (ii) No: different atomic numbers

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 3 + 3 marks.

Atom cards

Suppose a teacher hands out cards: Card P: 12 protons, 12 neutrons. Card Q: atomic number 8, mass number 16. Card R: 13 electrons and 14 neutrons in a neutral atom.

(i) · 2 marks
Find the atomic number and mass number for cards P and R.
Answer
Answer: P: Z 12, A 24; R: Z 13, A 27
P: $Z = 12$ , $A = 12 + 12 = 24$ . R: neutral, so protons $= 13$ ; $A = 13 + 14 = 27$ .
(ii) · 3 marks
How many protons, neutrons and electrons are in the atom on card Q? Draw its structure in words.
Answer
Answer: 8 p, 8 n, 8 e
Protons $= 8$ , neutrons $= 16 - 8 = 8$ , electrons $= 8$ . Nucleus with 8p, 8n; 8 electrons around it (2 in the first shell, 6 in the second).
(iii) · 3 marks
The element on card Q forms the compound H₂O. What is its valency? Which element is it?
Answer
Answer: Valency 2; oxygen
One atom of the element holds two hydrogen atoms, so its valency is 2. An atomic number of 8 means 8 protons: the element is oxygen.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

State the charge and location of a proton and an electron. (2 marks)

Step	Marks
Proton: +1, in the nucleus	1
Electron: −1, outside the nucleus (in shells)	1

Find the numbers of protons, neutrons and electrons in $_{20}^{40} Ca$ . (3 marks)

Step	Marks
Protons $= 20$	1
Electrons $= 20$	1
Neutrons $= 40 - 20 = 20$	1

Draw a labelled diagram of an atom of carbon ( $_{6}^{12}$ C) and state its valency, given that it forms $C H_{4}$ . (4 marks)

Step	Marks
Nucleus shown with 6p and 6n	1
Six electrons on two shells (2 and 4)	1
Labels: nucleus, proton, neutron, electron, shell	1
Valency $= 4$ (four H atoms per C atom)	1

Where students lose marks in Atomic Structure

The slips examiners see most often, and what to write instead.

Mistake: Taking mass number as the number of neutrons
Mass number = protons + neutrons.
Wrong working
$_{11}^{23}$ Na has 23 neutrons
Right working
12 neutrons
Do this: Neutrons = A − Z.
Usually costs: 1 mark.
Mistake: Putting electrons in the nucleus
Only protons and neutrons are in the nucleus.
Do this: Electrons are in the extra-nuclear part (shells).
Usually costs: 1 mark.
Mistake: Mixing up A and Z in the symbol
The upper number is A (mass number), the lower is Z (atomic number).
Do this: Bigger number on top.
Usually costs: 1 mark.
Mistake: Counting the hydrogen's valency instead of the element's
Valency of N in NH₃ is 3 (three H per N), not 1.
Do this: Count H atoms attached to one atom of the element.
Usually costs: 1 mark.
Mistake: Wrong number of electrons in the diagram
A neutral atom has as many electrons as protons.
Do this: Count the dots before you finish.
Usually costs: 1 mark.

Practice questions on Atomic Structure

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQExam level
The mass number of an atom is the number of:
(a) protons only
(b) neutrons only
(c) protons and neutrons
(d) protons and electrons
Hint
Particles in the nucleus.
Answer
(c)
Q2 · 1 mark · Very short answerExam level
What is the charge on a neutron?
Hint
Neutral.
Answer
No charge (zero).
Q3 · 2 marks · Short answerExam level
What are the nucleus and the extra-nuclear part of an atom?
Hint
Centre and surroundings.
Answer
The nucleus is the small, dense, positively charged centre containing protons and neutrons. The extra-nuclear part is the space around it where electrons move in shells.
Q4 · 2 marks · Short answerHarder exam level
Find the numbers of protons, electrons and neutrons in $_{19}^{39} K$ .
Hint
n = A − Z.
Answer
19 protons, 19 electrons, 20 neutrons.
Q5 · 1 mark · Assertion-reasonHarder exam level
Assertion (A): An atom is electrically neutral.
Reason (R): An atom has equal numbers of protons and electrons.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
Hint
Charges cancel.
Answer
(c)
Q6 · 2 marks · Short answerHarder exam level
An atom has mass number 31 and 16 neutrons. Find its atomic number and number of electrons.
Hint
Z = A − n.
Answer
$Z = 31 - 16 = 15$ ; 15 electrons.
Q7 · 2 marks · Short answerHarder exam level
Find the valency of sulphur in $H_{2} S$ and of nitrogen in $N H_{3}$ .
Hint
Count hydrogens.
Answer
Sulphur 2; nitrogen 3.
Q8 · 2 marks · Short answerHarder exam level
Why is almost the whole mass of an atom concentrated in its nucleus?
Hint
Masses of particles.
Answer
The nucleus contains the protons and neutrons, which each have a relative mass of about 1; electrons are about 1836 times lighter.
Q9 · 3 marks · Short answerHarder exam level
Copy and complete for magnesium ( $Z = 12$ , $A = 24$ ): protons ___, neutrons ___, electrons ___.
Hint
Use Z and A.
Answer
Protons 12, neutrons 12, electrons 12.
Q10 · 3 marks · Short answerHarder exam level
Draw the structure of a fluorine atom ( $_{9}^{19}$ F), showing the nucleus and electrons.
Hint
9p, 10n.
Answer
Nucleus with 9p and 10n; 9 electrons around it (2 in the first shell, 7 in the second).
Q11 · 3 marks · Short answerHOTS / competency
Two atoms have 6 protons each; one has 6 neutrons and the other 8 neutrons. Are they the same element? Find their mass numbers.
Hint
Element is decided by protons.
Answer
Yes, both are carbon (6 protons). Mass numbers are 12 and 14.
Q12 · 3 marks · Short answerHOTS / competency
An element X forms a compound with hydrogen in which one atom of X holds 3 hydrogen atoms. Write the formula of the compound and the valency of X. If X has 7 protons and 7 neutrons, write its symbol with A and Z.
Hint
Like ammonia.
Answer
Formula $X H_{3}$ ; valency 3. With 7 protons and 7 neutrons it is $_{7}^{14}$ X (nitrogen).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Name the three subatomic particles.Show answer
Electrons, protons and neutrons.
2.Which particle has a negative charge?Show answer
Electron.
3.Where are protons found?Show answer
In the nucleus.
4.Define atomic number.Show answer
Number of protons in an atom.
5.Neutrons in ¹⁴₇N?Show answer
7.
6.Valency of oxygen in H₂O?Show answer
2.
7.Why is an atom neutral?Show answer
It has equal numbers of protons and electrons.
8.Which part of an atom holds almost all its mass?Show answer
The nucleus.

Best order to study it

Learn the three subatomic particles with charge, mass and location.
Learn the nucleus and the extra-nuclear part.
Define atomic number and mass number; practise counting particles.
Learn valency from hydrogen compounds.
Practise drawing simple atomic diagrams.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Can you give the charge and mass of each particle?
Do you know which particles are in the nucleus?
Can you find p, n and e from ᴬ_Z X?
Can you work out A from p and n?
Can you find valency from HCl, H₂O, NH₃, CH₄?
Does your atom diagram have the right number of electrons?

Atomic Structure: frequently asked questions

Do all atoms have neutrons?

Almost all do. The common form of hydrogen has one proton and no neutron.

Why don't electrons count in the mass number?

An electron's mass is about 1/1836 of a proton's mass, so it makes almost no difference.

Can two different elements have the same mass number?

Yes, but they will have different atomic numbers (different numbers of protons). The atomic number decides the element.

Is valency always the number of hydrogen atoms?

In this chapter it is defined that way. You will meet other ways of finding valency, using electrons, in higher classes.

Why is the nucleus positively charged?

It contains positive protons and neutral neutrons, so its overall charge is positive.

Based on: CISCE Curriculum for Upper Primary Classes (VI-VIII), Science: Chemistry (Class VIII); CISCE RDCD: Curriculum Pre-School to Class VIII