When do two atoms become isotopes?

The correct answer is answering : The atoms have the same atomic number, but a different number of neutrons. This is key. The atomic number signifies how many protons an atom has. If it has three protons, the atomic number is three. When the electrons are greater or less than the protons, the atom has an electrical charge. If the protons and electrons are equal, then the atom has no charge.

Neutrons are special because they are not electrically charged. The only difference to anything that they can make is a difference in how heavy something is. So, one atom of uranium can have a certain amount of neurons and be radioactive, while another atom may have fewer and be completely stable.