ChemistryICSE Class 8 · Atomic Structure

Atomic Number, Mass Number, Neutrons and Valency: Z, A and n = A − Z (ICSE Class 8 Chemistry)

Updated 17 October 2026 · Fact-checked

Atomic number Z is the number of protons in the nucleus (equal to electrons in a neutral atom); mass number A is protons plus neutrons, so neutrons = A − Z. Valency, in this chapter, is the number of hydrogen atoms that combine with one atom of the element: Cl 1, O 2, N 3, C 4.

Understand Atomic Number, Mass Number and Valency

Atomic number (Z). The number of protons in the nucleus of an atom of the element. In a neutral atom it also equals the number of electrons. Atoms with a different number of protons are atoms of a different element.

Examples: H 1, C 6, O 8, Na 11, Cl 17, Ca 20.

Mass number (A). The total number of protons and neutrons in the nucleus:

$A = protons + neutrons$ and $neutrons = A - Z$

Electrons are not counted because their mass is negligible.

Notation. An atom is written $_{Z}^{A} X$ . For $_{11}^{23} Na$ : $Z = 11$ protons, $A = 23$ , so $23 - 11 = 12$ neutrons, and 11 electrons.

Different elements, same mass number is possible: the identity of the element is fixed by $Z$ , not $A$ . Two atoms with the same $A$ but different $Z$ have different numbers of neutrons.

Valency. The combining capacity of an element. Here it is defined as the number of hydrogen atoms that combine with one atom of the element (the valency of hydrogen is 1).

Compound	H 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

So an element X with a hydride XH₃ has valency 3. Valency tells us the formula of simple compounds and helps in writing other formulae.

Atomic Number, Mass Number and Valency: formulas and key facts

Atomic number: $Z = number of protons$; = 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: $V a l e n cy = n u mb er o f H a t o m sco mbinin g w i t h o n e a t o m ∣ H C l 1∣ H_{2} O 2∣ N H_{3} 3∣ C H_{4} 4$; Hydrogen is 1.

How to solve Atomic Number, Mass Number and Valency questions

Read Z and A from the symbol, then get everything else from them.

1Protons = Z; electrons = Z in a neutral atom.
2Neutrons = A − Z.
3To compare atoms, tabulate Z, A, protons, neutrons, electrons.
4For valency, count the hydrogen atoms joined to one atom of the element in its hydride.

Atomic Number, Mass Number and Valency 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.

Identity card of an atom

Suppose a label on a bottle says the atoms inside are written ^{35}_{17}X.

List the composition of one atom.

Try it first, then show the solutionSolution

Protons = Z = 17; electrons = 17.
Neutrons = 35 − 17 = 18.

Answer: 17 protons, 17 electrons, 18 neutrons.

Memory hook: Z is the head count of protons.

Hydrogen as a ruler

Imagine you measure how many hydrogen atoms join one atom of element Y in the compound YH₂ and in YH₃.

What valencies would you assign?

Try it first, then show the solutionSolution

In YH₂, 2 hydrogen atoms join one atom of Y: valency 2.
In YH₃, valency 3 (if Y can form both, the question needs the data given).

Answer: 2 and 3 respectively.

Memory hook: Count the hydrogens.

Tricks to remember Atomic Number, Mass Number and Valency

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

Z is for Zero-in
Z zeroes in on the element: protons only.
Use it when: Atomic number.
A minus Z
Neutrons are what is left of the mass number after you take away the protons.
Use it when: Neutron count.
Hydrogen headcount
Valency = H atoms joined: HCl 1, H₂O 2, NH₃ 3, CH₄ 4.
Use it when: Valency.

Atomic Number, Mass Number and Valency: solved examples

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

Example 1: Neutrons in sodium · 1 mark · MCQBoard level

The number of neutrons in $_{11}^{23} Na$ is:
(a) 11
(b) 12
(c) 23
(d) 34

Try it first, then show the solutionSolution

$n = A - Z = 23 - 11 = 12$ .

Answer: (b) 12

Example 2: Sulphur · 2 marks · Short answerBoard level

A neutral atom of sulphur has atomic number 16 and mass number 32. State the number of protons, electrons and neutrons it contains.

Try it first, then show the solutionSolution

Protons $= 16$ and electrons $= 16$ . (1)
Neutrons $= 32 - 16 = 16$ . (1)

Answer: 16 protons, 16 electrons, 16 neutrons

Example 3: Unknown element · 3 marks · Short answerHarder board level

A neutral atom of element X has 19 electrons and 20 neutrons. Find (a) the atomic number, (b) the mass number, (c) its representation $_{Z}^{A} X$ , (d) the charge on its nucleus.

Try it first, then show the solutionSolution

(a) $Z = 19$ . (1)
(b) $A = 19 + 20 = 39$ . (1)
(c) $_{19}^{39} X$ ; (d) the nuclear charge is +19. (1)

Answer: (a) 19 (b) 39 (c) ³⁹₁₉X (d) +19

Example 4: Valency · 3 marks · Short answerHarder board level

The hydrogen compounds of elements P, Q and R are PH₃, QH₂ and RH. Find the valency of each. Which could be nitrogen, oxygen and chlorine in that order?

Try it first, then show the solutionSolution

P: 3; Q: 2; R: 1. (1½)
P = nitrogen (NH₃), Q = oxygen (H₂O), R = chlorine (HCl). (1½)

Answer: P 3, Q 2, R 1; N, O, Cl

Example 5: Compare four atoms · 4 marks · Case-basedHOTS / competency

Atoms: P ( $Z = 11$ , $A = 23$ ), Q ( $Z = 12$ , $A = 24$ ), R ( $Z = 17$ , $A = 35$ ), S ( $Z = 18$ , $A = 40$ ). (a) Find the neutrons in each. (b) Which two have the same number of neutrons? Are they the same element? (c) Find the electrons in R.

Try it first, then show the solutionSolution

(a) P: 12; Q: 12; R: 18; S: 22. (2)
(b) P and Q have 12 neutrons each; they are different elements because their atomic numbers differ. (1)
(c) 17 electrons. (1)

Answer: (a) 12, 12, 18, 22 (b) P and Q; different elements (c) 17

Where students lose marks in Atomic Number, Mass Number and Valency

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

Mistake: Using A as the number of protons
Protons = Z, not A.
Wrong working
"Na-23 has 23 protons."
Right working
"Na-23 has 11 protons and 12 neutrons."
Do this: Read Z first.
Mistake: Adding electrons to the mass number
A counts protons and neutrons only.
Wrong working
"A = 11 + 12 + 11 = 34."
Right working
"A = 11 + 12 = 23."
Do this: Leave out electrons.
Mistake: Mixing valency with number of electrons
Here valency is defined by hydrogen atoms.
Wrong working
"The valency of oxygen is 8 because Z = 8."
Right working
"The valency of oxygen is 2 (H₂O)."
Do this: Use the hydride formulae.

Exam tips

Write Z and A at the start and derive protons, neutrons, electrons in a table.
Show n = A − Z with numbers.
For valency, name the hydride and count hydrogens.
Say "the number of protons fixes the element".

Practice questions on Atomic Number, Mass Number and Valency

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
Define atomic number.
Hint
Think of protons.
Answer
The number of protons in the nucleus of an atom of the element.
Q2 · 2 marks · Short answerHarder board level
A neutral atom has 13 electrons and a mass number of 27. Write its atomic number, the number of neutrons and its symbol form $_{Z}^{A} X$ .
Hint
Electrons equal protons.
Answer
$Z = 13$ ; neutrons $= 27 - 13 = 14$ ; the form is $_{13}^{27} X$ .
Q3 · 3 marks · Short answerHarder board level
Why are electrons not counted in the mass number? Find the mass number of an atom with 9 protons, 10 neutrons and 9 electrons.
Hint
Mass of an electron is tiny.
Answer
An electron has only about $\frac{1}{1836}$ of the mass of a proton or neutron, so its contribution is negligible. $A = 9 + 10 = 19$ .
Q4 · 3 marks · Short answerHarder board level
An element Z forms a hydride ZH₂. State its valency. Predict the formula of its compound with chlorine (valency 1), and give the reasoning.
Hint
Chlorine is like hydrogen: valency 1.
Answer
Valency of Z is 2. Each Z atom combines with 2 atoms of a valency-1 element, so with chlorine the formula is ZCl₂ (as 2 hydrogen atoms are replaced by 2 chlorine atoms).
Q5 · 4 marks · Long answerHOTS / competency
Atoms S ( $Z = 18$ , $A = 40$ ) and T ( $Z = 20$ , $A = 40$ ) both have mass number 40. (a) Find the neutrons in each. (b) Are they the same element? (c) What fixes the identity of an element?
Hint
Compare Z.
Answer
(a) S: $40 - 18 = 22$ ; T: $40 - 20 = 20$ . (b) No, their atomic numbers differ. (c) The atomic number, i.e. the number of protons.

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.Neutrons formula?Show answer
A − Z.
2.Mass number of an atom with 6p and 6n?Show answer
12.
3.Valency of carbon?Show answer
4 (CH₄).
4.Valency of nitrogen?Show answer
3 (NH₃).
5.Valency of oxygen?Show answer
2 (H₂O).

Atomic Number, Mass Number and Valency: frequently asked questions

What is the difference between atomic number and mass number?

Atomic number is the number of protons; mass number is the number of protons plus neutrons.

How do you find the number of neutrons?

Subtract the atomic number from the mass number: neutrons = A − Z.

What is valency?

Valency is the combining capacity of an element. At this level it is the number of hydrogen atoms that combine with one atom of the element.