Question

The mass number of a nucleus is (1) sometimes less than and sometimes more than its atomic number (2) always less than its atomic number (3) always more than its atomic number (4) sometimes equal to its atomic number

Answer 1

Explanation:

THE MASS NUMBER OF AN ATOM IS ALWAYS MORE THAN ITS ATOMIC NUMBER BECAUSE THE MASS OF NEUTRONS IS ALSO CALCULATED AND THE MASS OF A NEUTRON IS EQUAL TO THE MASS OF A PROTON

(THOUGH THE MASS OF THE NEUTRON IS LITTLE MORE THAN THAT OF A PROTON BUT THAT IS MOSTLY NOT COUNTED)

FOR EXAMPLE,

TAKE HELIUM:

ATOMIC NUMBER (OR NUMBER OF PROTONS) =2

NUMBER OF NEUTRONS =2

SO THE THE MASS NUMBER WILL NOT BE 2 INSTEAD IT WILL BE 4.

NOW YOU MUST BE THINKING THAT HYDROGEN HAS ONLY ONE PROTON SO ITS ATOMIC NUMBER WILL BE EQUAL TO ITS MASS NUMBER.

BUT IF YOU THOROUGHLY CHECK THE PERIODIC TABLE YOU WILL SEE THAT THE MASS NUMBER IS LITTLE MORE.

THIS IS BECAUSE THE AVERAGE MASS OF ALL THE ISOTOPES OF HYDROGEN IS TAKEN AND SOMETIMES THE MASS OF ELECTRONS IS ALSO CALCULATED (WHICH IS VERY SMALL).

SO THE MASS NUMBER IS ALWAYS MORE THAN THE ATOMIC NUMBER.