Question

The number of electrons and neutrons of an element is $18$ and $20$ respectively. Its mass number is:
(A) 2
(B) 17
(C) 37
(D) 38

Answer 1

Hint: Mass number is also known as atomic weight or nucleon number.
Mass number is the total number of protons and neutrons present in the nucleus.

Complete step by step answer:
Let us discuss Atomic number and mass number of an atom of element.
Atomic number $[Z]$ is the number of protons or electrons present in a nucleus.
Mass number $[A]$ is the number of protons and neutrons present in the nucleus.
Since in a given problem number of electrons $ = 18$ and number of neutrons $ = 20$ are given.
And we have to calculate the mass number.
But we should know the number of protons to calculate mass number.
As we know an atom of electrons [negatively charged] is equal in number.
Since, Number of electrons $ = 18$
Number of protons $ = 18$
Number of neutron $ = 20$
$\therefore $mass number$ = $Number of proton$ + $Number of neutron
Mass number$ = 18 + 20$
$ = 38$
$\therefore $mass number of atom$ = 38$

So, the correct answer is “Option D”.

Additional Information:
Protons, electrons and neutrons are subatomic particles.
Protons are positively charged particles. It forms a part of the nucleus of an atom. It weights $1$amu charge on proton is $( + 1)$electrons are negatively charged particles. Electrons are revolving around the nucleus. It weight is negligible and taken as zero charge on electron is $( - 1).$
Neutrons have no charge and they weigh $1$amu.

Note:
Protons and neutrons have almost same mass i.e., $1.67 \times {10^{ - 24}}gm.$
Electrons weigh only $9.11 \times {10^{ - 28}}gm.$They do not contribute to the element's atomic mass.
Since Isotopes have slightly different mass numbers, mass number is calculated by means of mass number for its isotopes.