Question

describing isotopes
what is the mass number of the isotope lithium - 7?
lithium has 3 protons. how many neutrons are in the isotope lithium - 8?

Explanation:

Step1: Recall mass - number definition

The mass - number of an isotope is given in its name. For lithium - 7, the mass - number is 7.

Step2: Recall neutron - proton relationship

The mass - number ($A$) is the sum of protons ($p$) and neutrons ($n$), i.e., $A=p + n$. Lithium has 3 protons ($p = 3$), and for lithium - 8 ($A = 8$). We solve for $n$: $n=A - p$.

Step3: Calculate number of neutrons

Substitute $A = 8$ and $p = 3$ into the formula $n=A - p$. So, $n=8 - 3=5$.

Answer:

7
5