Question

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 more than and sometimes equal to its atomic number

Answer 1

Hint: You should know that atoms are the fundamental building blocks of all matter and are composed of protons, neutrons and electrons. Atoms are electrically neutral.

Complete step by step answer:
As we already know atoms are composed of particles called protons, electrons and neutrons. Protons carry a positive electrical charge, electrons carry a negative electrical charge and neutrons carry no electrical charge at all.

Atomic number
In simple words, Atomic numbers are the total number of protons present in the nucleus of an atom. Suppose if in an atom's nucleus one proton is present then its atomic number is 1 which is hydrogen. If in an atom's nucleus 8 protons are present then its atomic number will be the same and it is oxygen.

Mass number
Mass number is the total no of protons and neutrons within the nucleus of an atom and it is symbolically represented as ‘A’. For e.g. atomic mass number of hydrogen is 1 but we also have its isotopes i.e. deuterium and tritium whose atomic mass number is 2 and 3 respectively.

A Hydrogen atom has one proton and an electron and no neutrons is the most abundant isotope. It has an atomic number Z=1 and mass number A=1. Only in this case A=Z since, as we know a nucleus with more protons cannot be stable without neutrons.

Therefore, the mass number of a nucleus is more than atomic number. But hydrogen is an exceptional case. Therefore, the mass number of a nucleus is more than and equal to its atomic number. Hence, option D is the required answer.

Note: As we know hydrogen is the most abundant element in the universe and there are about 10 million known compounds that can be made with carbon.