Question

The approximate total mass of a sodium atom is

• A. $23\text{a.m.u.}$
• B. $11\text{a.m.u.}$
• C. $12\text{a.m.u.}$

Answer 1

The correct option is A $23\text{a.m.u.}$

Sodium is represented as $2311\mathrm{Na}$ in the periodic table.
The atomic number ($\text{Z}$) $=11$
The mass number ($\text{A}$) $=23$
The atomic mass of an atom is approximately equal to the total mass of the nucleus, as the mass of an electron is negligible compared to that of a proton or a neutron.
So, the total mass of a sodium atom = mass number $\times$ mass of one proton (or neutron) $=23\times 1\text{a.m.u.}$
$=23\text{a.m.u.}$