# An atom of a sodium has 11 proton, 11 electrons, and 12 neutrons, what is its mass number?

Nov 11, 2015

23 u

#### Explanation:

Here are some of the rules:

(1) the atomic number, $Z$, is always equal to the number of protons.

$Z$ = number of protons

(2) for ground state elements (no charge) the number of protons is equal to the number of electrons

$Z$ = $\text{number of protons}$ = $\text{number of electrons}$ (no charge)

otherwise, the number of electrons is greater than the number of protons if it's an anion (negative charge), and the number of electrons is less than the number of protons if it's a cation (positive charge).

$Z$ = $\text{number of protons}$ < $\text{number of electrons}$ ($\textcolor{red}{\text{negative}} , {X}^{-}$)

$Z$ = $\text{number of protons}$ > $\text{number of electrons}$ ($\textcolor{b l u e}{\text{positive}} , {X}^{+}$)

(3) the atomic mass, $A$, is always equal to the sum of the number of protons and neutrons.

$A$ = $\text{number of protons}$ + $\text{number of neutrons}$

or

$A$ = $Z$ + $\text{number of neutrons}$

$A$ = $Z$ + $\text{number of neutrons}$
$A$ = 11 protons + 12 neutrons
$A$ = 23 u