what is the mass number of this atom?

Explanation:

Step1: Recall mass - number formula

The mass number ($A$) of an atom is the sum of the number of protons and neutrons in the nucleus of the atom, i.e., $A = p + n$, where $p$ is the number of protons and $n$ is the number of neutrons.

Step2: Identify protons and neutrons in lithium atom

For a lithium atom, from the diagram, the number of protons ($p$) is 3 (usually the atomic number of lithium) and the number of neutrons ($n$) is 4.

Step3: Calculate mass number

$A=3 + 4=7$

Answer:

7