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NCERT Solutions for Class 9th Science Chapter 8 .6 Mass Number — Pause and Ponder

Book page 1508 Updated on2026-09-08

Q10.
An atom with an atomic number of 26 has 56 nucleons. Find out its number of electrons, protons and neutrons.
Answer

Electrons 26, protons 26, neutrons 30. The element is iron, 5626Fe.

Formulae: Z = number of protons = number of electrons (neutral atom); nucleons = A = p+ + n0, so n0 = A – Z.

Given: Z = 26, A = 56 nucleons

Number of protons = Z = 26
Number of electrons = Z = 26 (the atom is neutral)

Number of neutrons = A – Z
= 56 – 26
= 30
Why it happens: “nucleons” means protons and neutrons together, so 56 is the mass number, not the neutron count. Once you know Z you know both the protons and the electrons; subtracting Z from A leaves the neutrons.
Check it yourself: 26 + 30 = 56 ✓, and this is the commonest isotope of iron — the metal in every railway line and every haemoglobin molecule in your blood.
Q11.
The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it.
Answer

21 neutrons.

Formula: Mass number A = number of protons + number of neutrons, so n0 = A – p+.

Given: p+ = 20, A = 41

n0 = A – p+
= 41 – 20
= 21 neutrons

With Z = 20 the element is calcium, so this atom is 4120Ca. It also has 20 electrons, arranged 2, 8, 8, 2.

Did you know? Ordinary calcium is 4020Ca with 20 neutrons. 4120Ca is one of its isotopes — same Z, one extra neutron, so the same chemistry but a slightly greater mass.
Q12.
An atom has 18 neutrons and an atomic number of 17. What is its mass number?
Answer

Mass number = 35.

Formula: A = number of protons + number of neutrons, and number of protons = Z.

Given: Z = 17, so p+ = 17; n0 = 18

A = p+ + n0
= 17 + 18
= 35

Z = 17 makes the element chlorine, so the atom is 3517Cl — the more abundant of chlorine's two natural isotopes, making up about 75% of all chlorine atoms.

Why it happens: the mass number counts nucleons only. The 17 electrons are left out because an electron's mass is roughly 1/1836 of a proton's — negligible in this bookkeeping.
Q13.
An atom ²³A has 11 electrons. Find the number of neutrons in it.
Answer

12 neutrons.

Reading the symbol: in AZX the upper number is the mass number. Here the upper number is 23, so A = 23. The atom is neutral, so the number of protons equals the number of electrons.

Number of electrons = 11
⇒ number of protons, Z = 11 (neutral atom)
Mass number, A = 23

Number of neutrons = A – Z
= 23 – 11
= 12 neutrons

Z = 11 identifies the element as sodium, so ²³A is 2311Na. Its electronic configuration is 2, 8, 1 — one valence electron, which is why sodium is such a reactive metal.

Tip: “A” here is just a stand-in letter for an unknown element, not the symbol of a real one. The question is testing whether you can get Z from the electron count.
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