# The sodium atom contains 11 electrons, 11 protons, and 12 neutrons. What is the mass number of sodium?

May 11, 2017

#### Explanation:

The mass number is the sum of the neutrons and protons in an atom.
So 11 protons + 12 neutrons= 23

May 11, 2017

We have the ""^23Na isotope.........i.e. $\text{mass number} = 23$.

#### Explanation:

A sneakier question would have asked:

$\text{an atom contains 11 protons, and 12 neutrons; how may electrons}$ $\text{in the neutral atom; what is the atom?}$.

The sum of neutrons and protons, the massive nuclear particles, gives the mass number, with which we often label the elemental symbol as a left hand superscript. But the number of $\text{nucular protons}$, i.e. $Z$, UNEQUIVOCALLY identifies the atom; $Z = 11$ for sodium, $Z = 10$ for neon; $Z = 12$ for magnesium.

It is not too difficult to incorporate these ideas into your understanding, and if you have a Periodic Table in front of you (and you should have one NOW!) these problems are straightforward.