Question

Atoms contain protons, neutrons and electrons. The mass of a neutron is assumed to be half of its original value while that of an electron is assumed to be twice of the original value. Calculate the change in atomic mass of carbon ($C^{12}$).

• A. 25 % more
• B. 75 % less
• C. 25 % less
• D. 50 % more

Answer 1

The correct option is C 25 % less

Let mass of neutron = n
Given: new mass of neutron = $n_1$ = $\frac{n}{2}$
mass of electron (originally) = e
mass of electron (new) = 2e
For carbon
mass number of carbon (originally) = A = 12
Atomic mass of C-12 atom = mass of neutron + mass of proton
mass of protons = 6 unit
mass of neutron = $6\times \frac{1}{2}$= 3 unit
New mass number of C-12 atom = 9
% Decrease in atomic mass = $\frac{A_1-A}{A}\times 100$ = $\frac{12-9}{12}\times 100$ = 25 %