2003 AIPMT NucleiComposition of Nucleus Easy Question 2 of 2 in Composition of Nucleus

The mass number of a nucleus is

A always less than its atomic number
B always more than its atomic number
C sometimes equal to its atomic number
D sometimes less than and sometimes more than its atomic number

The answer is C.

Solution

Mass number A = number of protons + number of neutrons = Z + N; atomic number Z = number of protons.
Since $N \geq 0$, we always have $A \geq Z$; A can never be less than Z.
The equality A = Z holds when there are no neutrons. This happens only for the ordinary hydrogen nucleus $^{1}_{1}H$ (a single proton). For all other nuclei A is greater than Z.
So the mass number is sometimes equal to its atomic number.

Concept Mass number and atomic number

Composition of Nucleus in past papers

2 questions from this chapter have appeared across 2 exam years.