Radioactive isotopes are forms of an element that have an unstable ratio of neutrons and protons and hence undergo radioactive decay.
Protons are positive and in the nucleus push away from each other.
Hydrogen with only one proton does not need neutrons to make it stable.
Neutrons (which are neutral) are attracted to the positively-charged protons by a short-range strong nuclear force. If two (or more) protons are attracted to the same neutron, this attractive force (which is not related to charge) helps to bind the nucleus together.
Helium with two protons joined to two neutrons is very stable.
Carbon with six protons joined to six neutrons is very stable. However, Carbon with six protons joined to eight neutrons is not stable. There is a significant excess of neutrons, so one of the neutrons emits an electron turning it into a proton (
#beta^(-)#decay), creating nitrogen with a stable seven protons and seven neutrons:
#""_6^(14) "C" -> ""_7^(14) "N" + ""_(-1)^(0) e#
An atom with too few or too many neutrons may be an unstable isotope and undergo some form of radioactive decay.