Question

What the difference between the mass number and the atomic number gives?
A. number of neutrons
B. number of electrons
C. charge of element
D. number of protons

Answer 1

Hint: While Atomic number gives the number of protons in an atom, Mass number gives the sum of protons and neutrons present in the atom. Thus, the difference of atomic number and mass number would give the number of neutrons.

Complete step by step answer: The atomic number of an atom represents the number of protons in an atom. It is equal to the number of electrons present in an electrically neutral atom. It is denoted by Z.
Whereas, the mass number of an atom is the sum of the number of protons and number of neutrons present in an atom. It is donated by A.
An element is represented as ${}_Z^AX$ .
Where, X is the element.
A is a mass number.
and z is an atomic number.
If we see both the definitions carefully, we will get to know that the only difference lies in the number of neutrons.
Therefore, if we subtract the atomic number of an atom from the mass number of the same atom, we would be subtracting the number of protons from the sum of neutrons and protons. Thus, we will get the number of neutrons as an answer.
 For example,
(Z) Atomic number of carbon is $6$
(A) Mass number of carbon is $12$
Therefore, Number of electrons in c $ = 6{e^ - }$
Number of proton in c $ = 6{p^ + }$
Number of neutrons in c $ = A - Z$
$ = 12 - 6$
$ = 6n$
So, the correct answer is “Option A”.

Note: The atomic number of an atom can be equal to the number of electrons present in it only when the atom is electrically neutral. That is, the atom has not gained or lost an electron.