Question

Why is the atomic mass of the sodium atom "23 amu"?

Answer 1

Atomic mass:

• The atomic mass is the mass of an atom.
• The SI unit of atomic mass is the unified atomic mass unit.
• One unified atomic mass unit is defined as $\frac{1}{12}\mathrm{th}$ part of the mass of a single carbon-$12$ atom which is assigned a mass of 12 a.m.u.

Atomic mass of the sodium:

• Sodium $\left(\mathrm{Na}\right)$ is an alkali metal that appears first in the periodic table's third period
• Atomic mass is the sum of the total number of protons and the total number of neutrons in an atom.
• The nucleus of a sodium atom has $11$ protons and $12$ neutrons.
• The atomic mass of sodium is as follows:

$\begin{array}{rcl}\mathrm{Atomic}\mathrm{mass}& =& \mathrm{Number}\mathrm{of}\mathrm{protons}+\mathrm{Number}\mathrm{of}\mathrm{neutrons}\\ & =& 11+23\\ & =& 23\end{array}$

• Since $\mathrm{Na}$ has an atomic mass of $23$, its atomic mass is $23\mathrm{amu}$.