Proton and mass numbers describe a nucleus

1. The atomic number, also called the proton number, identifies the element because every atom of one element has the same number of protons.
2. A nucleus containing six protons is carbon, and its positive nuclear charge is $+6$ in relative units.

1. The mass number, also called the nucleon number, counts all nucleons in the nucleus.
2. The relationship is $A=Z+N$, where $A$ is mass number, $Z$ is atomic number and $N$ is number of neutrons.
3. Rearranging gives $N=A-Z$.

Isotopes share protons but differ in neutrons

1. Isotopes have the same atomic number and the same positive nuclear charge because they contain the same number of protons.
2. They have different mass numbers because their nuclei contain different numbers of neutrons.
3. Changing the neutron number makes a different isotope, while changing the proton number makes a different element.
4. Carbon-12 has six protons and six neutrons, while carbon-13 has six protons and seven neutrons.

Nuclear notation places both numbers on the left

1. An isotope is written as ${}_Z^A\mathrm{X}$, where $\mathrm{X}$ is the element symbol.
2. The mass number $A$ is the upper-left number and the atomic number $Z$ is the lower-left number.
3. For carbon-13, ${}_6^{13}\mathrm{C}$ shows thirteen nucleons in total and six protons.

Comparisons must name both similarities and differences

1. Two nuclei are isotopes when their lower-left numbers are equal and their upper-left numbers are different.
2. A full explanation states that they have the same number of protons but different numbers of neutrons.