how many neutrons does the isotope of lithium have?

Explanation:

Step1: Recall atomic number and mass - number relationship

The atomic number of lithium ($Li$) is $3$, which is the number of protons. For a common isotope of lithium - 7 ($^{7}Li$), the mass number is 7.

Step2: Calculate the number of neutrons

The formula to calculate the number of neutrons ($n$) is $n = A - Z$, where $A$ is the mass number and $Z$ is the atomic number. Here, $A = 7$ and $Z = 3$. So, $n=7 - 3=4$.

Answer:

4