# Question 94b00

May 1, 2017

The expected mass is 23.184 016 u.

#### Explanation:

A nucleus of $\text{_11^23"Na}$ contains 11 protons and 12 neutrons.

Its expected mass is the sum of the masses of its protons and neutrons.

Here's how to calculate the expected mass:

$\text{Mass of protons":color(white)(mll) "11 × 1.007 276 u" = "11.080 036 u}$
$\text{Mass of neutrons": color(white)(m)"12 × 1.008 665 u" = "12.103 980 u}$
color(white)(mmmmmmmmmmmml)"Total mass/u":color(white)(l)stackrel(—————)("23.184 016 u")#

∴ The expected mass is 23.184 016 u.

Note: This is different from the actual mass of a nucleus of $\text{_11^23"Na}$, which is 22.989 769 u.

The difference in mass is called the mass defect.