Two or more atoms that have the same number of protons, but have
different numbers of neutrons.
A & E are isotopes. They both have 6 protons (so they are Carbon), but they
have different numbers of neutrons. This gives them different mass
numbers.
Atomic mass is the average of the isotopes mass numbers. (12+14)/2 would
equal 13.
An ion is an atom that has an unbalanced electrical charge. The number of
protons is not equal to the number of electrons.
B, C, and D are ions. The number of protons and electron are not equal.
C and D are cations. Cations have a positive charge. C and D both have more
protons than electrons so they have a net positive charge.
B is an anions. Anions have a negative charge. B has more electrons than
protons so it has a net negative charge.
Atoms A, E, and F are neutral. The have the same number of protons and
electrons. They have no net electrical charge.