ChemistryICSE Class 8 · Atomic Structure

Protons, Neutrons, Electrons and the Nucleus: Structure of the Atom (ICSE Class 8 Chemistry)

Updated 17 October 2026 · Fact-checked

An atom has a small, dense, positively charged nucleus holding protons (+1, relative mass 1) and neutrons (0, relative mass 1), and electrons (−1, about 1/1836 of a proton) moving around it in shells. In a neutral atom the numbers of protons and electrons are equal, and almost all the mass is in the nucleus.

Protons and neutrons sit together in the tiny nucleus while electrons move in shells around it; in a neutral atom, protons equal electrons.

Illustration drawn for QuestionBuddy.

Understand Subatomic Particles and the Structure of the Atom

An atom is the smallest particle of an element that can take part in a chemical reaction. It is made of three fundamental (subatomic) particles.

Particle	Symbol	Charge	Relative mass	Where found
Proton	p	+1	1	nucleus
Neutron	n	0	1	nucleus
Electron	e	−1	about $\frac{1}{1836}$	outside the nucleus

Protons are positively charged. Neutrons have no charge. (Ordinary hydrogen has no neutron.)
Electrons are negatively charged and so light that their mass is usually ignored in the mass of an atom.

The nucleus. The small, dense centre of the atom. It contains the protons and neutrons (together called nucleons), so it carries almost all the mass of the atom and has a positive charge.

The extra-nuclear part. The space around the nucleus in which the electrons move in definite paths called shells (or orbits). It makes up almost all the volume of the atom, so the atom is mostly empty space.

Neutral atom. In a neutral atom the number of electrons equals the number of protons, so the positive and negative charges cancel.

$number of electrons = number of protons$

Drawing an atom. A small circle for the nucleus with the numbers of protons and neutrons written in it; concentric circles for the shells; electrons shown as dots or crosses. For example, carbon-12: nucleus 6 p and 6 n; 6 electrons with 2 in the first shell and 4 in the second.

Why the mass is in the nucleus. A proton or neutron has a relative mass of 1, whereas 1836 electrons would be needed to match the mass of one proton.

Subatomic Particles and the Structure of the Atom: formulas and key facts

Particles of the atom: Proton +1, mass 1, nucleus
Neutron 0, mass 1, nucleus
Electron −1, mass about 1/1836, shells; Nucleons = protons + neutrons.
Neutral atom: $electrons = protons$; Charges cancel.
Mass of electrons: $1836 electrons \approx 1 proton$; Mass of an atom is in the nucleus.

Proton	+1, mass 1, nucleus
Neutron	0, mass 1, nucleus
Electron	−1, mass about 1/1836, shells

How to solve Subatomic Particles and the Structure of the Atom questions

Write the three particles, count them for the atom given, and use charge and mass to answer.

1Use the table of charge and relative mass.
2Nucleus charge = number of protons (positive); neutral atom has the same number of electrons.
3Relative mass of the atom ≈ protons + neutrons (electrons ignored).
4For mass comparison with electrons, multiply by 1836.

Subatomic Particles and the Structure of the Atom in real life

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.

The empty-space atom

Suppose a model of an atom is built as a marble in the middle of a huge stadium, with a few tiny dots moving in the stands.

What does the marble represent, and what does the stadium space represent?

Try it first, then show the solutionSolution

The marble is the nucleus (protons and neutrons), holding almost all the mass.
The large space with the moving dots is the extra-nuclear part, where the electrons move.

Answer: Marble: nucleus. Stadium: extra-nuclear part (electrons). The atom is mostly empty space.

Memory hook: Heavy small centre, light large cloud.

Why a neutral atom does not shock

Imagine a piece of iron does not give a shock when touched, although it has billions of charged particles.

Explain why.

Try it first, then show the solutionSolution

In each neutral atom the number of protons equals the number of electrons.
The positive and negative charges cancel.

Answer: The charges cancel, so the atom (and the iron) is neutral.

Memory hook: Equal and opposite.

Tricks to remember Subatomic Particles and the Structure of the Atom

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

PEN
Proton +, Electron −, Neutron neutral.
Use it when: Charges.
Nucleons at the centre
Protons and neutrons sit in the nucleus; electrons orbit outside.
Use it when: Location.
1836 electrons
It takes 1836 electrons to weigh one proton: ignore electrons in mass.
Use it when: Mass.

Subatomic Particles and the Structure of the Atom: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Particle with negligible mass · 1 mark · MCQBoard level

The subatomic particle with the smallest mass is the:
(a) proton
(b) neutron
(c) electron
(d) nucleon

Try it first, then show the solutionSolution

An electron has about $\frac{1}{1836}$ of the mass of a proton.

Answer: (c) electron

Example 2: Charge and place · 2 marks · Short answerBoard level

Name the particles found in the nucleus and state the charge on the nucleus as a whole. Why is a neutral atom electrically neutral?

Try it first, then show the solutionSolution

The nucleus has protons (+1) and neutrons (0), so it is positively charged. (1)
In a neutral atom the number of electrons equals the number of protons, so the charges cancel. (1)

Answer: Protons and neutrons; positive; equal numbers of p and e.

Example 3: An atom of chlorine · 4 marks · Long answerHarder board level

A neutral atom has 17 protons and 18 neutrons. (a) How many electrons does it have? (b) What is the charge on its nucleus? (c) How many nucleons? (d) What fraction of the total relative mass is due to the electrons? (Take electron mass $= \frac{1}{1836}$ .)

Try it first, then show the solutionSolution

(a) 17 electrons. (1)
(b) +17. (1)
(c) $17 + 18 = 35$ nucleons. (1)
(d) Electron mass $= \frac{17}{1836} = 0.00926$ ; total $= 35.00926$ ; fraction $= \frac{0.00926}{35.00926} = 0.000264$ , about 0.026%. (1)

Answer: (a) 17 (b) +17 (c) 35 (d) about 0.026%

Example 4: Heavy protons · 3 marks · Short answerHarder board level

How many electrons have the same total mass as 5 protons? Hence explain why the mass of electrons is ignored when finding the mass of an atom.

Try it first, then show the solutionSolution

One proton equals 1836 electrons. (1)
5 protons $= 5 \times 1836 = 9180$ electrons. (1)
An atom has only as many electrons as protons, so their mass is a tiny fraction of the mass of the nucleus and is ignored. (1)

Answer: 9180 electrons

Example 5: Carbon-12 · 4 marks · Case-basedHOTS / competency

Carbon-12 has 6 protons and 6 neutrons. (a) State the composition of its nucleus and the number and arrangement of electrons. (b) Describe how you would draw it. (c) State where almost all its mass is. (d) A student says "most of the atom is the nucleus". Correct this statement.

Try it first, then show the solutionSolution

(a) Nucleus: 6 p and 6 n; 6 electrons, 2 in the first shell and 4 in the second. (1)
(b) A small circle labelled 6p, 6n; two concentric circles; 2 dots on the first and 4 on the second. (1)
(c) In the nucleus. (1)
(d) The nucleus has almost all the mass but occupies only a tiny part of the volume; the atom is mostly empty space. (1)

Answer: (a) 6p 6n; 2,4 (c) nucleus (d) mass yes, volume no

Where students lose marks in Subatomic Particles and the Structure of the Atom

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

Mistake: Placing electrons in the nucleus
The nucleus has protons and neutrons only.
Wrong working
"The nucleus contains protons and electrons."
Right working
"The nucleus contains protons and neutrons; electrons are in shells."
Do this: Use the table.
Mistake: Counting electrons in the mass
Electron mass is negligible.
Wrong working
"Mass number = p + n + e."
Right working
"Mass number = p + n."
Do this: Ignore electrons in mass.
Mistake: Saying the nucleus is negative
Protons are positive.
Wrong working
"The nucleus is neutral."
Right working
"The nucleus is positive."
Do this: Neutrons are neutral, protons positive.

Exam tips

Give charge and relative mass in a table of three particles.
Draw the atom with nucleus, shells and electrons, and label them.
Say "equal numbers of protons and electrons" for neutrality.
Use relative mass 1 for p and n and ignore electrons.

Practice questions on Subatomic Particles and the Structure of the Atom

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 · Very short answerBoard level
What is the charge on a neutron?
Hint
Neutral.
Answer
Zero.
Q2 · 2 marks · Short answerHarder board level
An atom has 12 protons, 12 neutrons and 12 electrons. Find the number of nucleons, the charge on the nucleus and the total charge on the atom.
Hint
Add protons and neutrons.
Answer
Nucleons $= 12 + 12 = 24$ . Nuclear charge $= + 12$ . Total charge $= + 12 - 12 = 0$ .
Q3 · 3 marks · Short answerHarder board level
Why is almost all the mass of an atom in the nucleus but almost all its volume outside it?
Hint
Where are the heavy particles?
Answer
The nucleus contains the protons and neutrons, each of relative mass 1, while electrons have negligible mass, so the mass is in the nucleus. The electrons move in shells over a large region around the tiny nucleus, so that region makes up nearly all the volume.
Q4 · 3 marks · Short answerHarder board level
An atom has 9 protons, 10 neutrons and 9 electrons. Write its relative mass (ignoring electrons) and the number of particles in the nucleus. If one more proton were present in the nucleus, would the atom be of the same element? Why?
Hint
Atomic number is the number of protons.
Answer
Relative mass $\approx 9 + 10 = 19$ ; 19 particles are in the nucleus. No: an atom with a different number of protons is an atom of a different element.
Q5 · 2 marks · Assertion-reasonHOTS / competency
Assertion (A): An atom is electrically neutral.
Reason (R): It contains equal numbers of protons and neutrons.
(a) A and R true, R explains A
(b) A and R true, R does not explain A
(c) A true, R false
(d) A false, R true
Hint
Which particles must be equal?
Answer
A is true. R is false: the number of protons equals the number of electrons (not neutrons). So (c).

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.Charge on a proton?Show answer
+1.
2.Where are electrons found?Show answer
In shells outside the nucleus.
3.Which particles are in the nucleus?Show answer
Protons and neutrons.
4.Mass of an electron compared with a proton?Show answer
About 1/1836.
5.Why is an atom neutral?Show answer
Equal numbers of protons and electrons.

Subatomic Particles and the Structure of the Atom: frequently asked questions

What are the three subatomic particles?

Protons (+1, in the nucleus), neutrons (no charge, in the nucleus) and electrons (−1, moving in shells outside the nucleus).

Where is the mass of an atom concentrated?

In the nucleus, because protons and neutrons are about 1836 times heavier than electrons.

What is the extra-nuclear part of an atom?

It is the space around the nucleus where the electrons move in shells; it makes up almost all the volume of the atom.