What does it mean to say that the atoms in a sample of an element are
isotopes of each other?

They have the same number of protons, but a different number of neutrons.

Explanation:

All atoms of the same element have the same number of protons, however, they don't necessarily have the same number of neutrons. If atoms of the same element have different numbers of neutrons, they are isotopes of one another. An example is carbon-12 and carbon-13. Carbon-12 has 6 protons and 6 neutrons. Carbon-13 has 6 protons and 7 neutrons. They are isotopes.

c, liquids evaporate, creating a gas