why do atoms have different amount of neutrons in them?

Question

elements have to have the same amount of protons and electrons in an atom, why can they have different amount of neutrons?

Answer 1

If the number of protons and neutrons in a given element is different, then it wouldn't be the same element.

Example: Oxygen, with 8 protons. If it had 9, it's Fluorine. If it had 7, then it's nitrogen. The atomic number is equal to the amount of protons in a given element.

The amount of neutrons change because an element can have isotopes.

Examples: Carbon-12, Carbon-13, and Carbon-14. Each have 6, 7, and 8 neutrons, respectively. The number of protons and neutrons remain the same (6)

That clear things up?

Answer 2

This is necessary to have the nuclei stick together rather than blowing apart. As the number of protons increases, there has to be an increasing number of neutrons to provide the binding energy to hold the nuclei together. Just as there are "magic" numbers of electrons in "shells" needed to produce most stable atoms, so in the nucleus there are two sets of "magic" numbers, one of protons, the other of neutrons, to keep the nucleus together. In two places of the periodic table, this doesn't work. For technetium, atomic number 43, and promethium, atomic number 61, the numbers of protons and neutrons cannot get close to magic numbers, so these elements have no stable isotopes. They are all radioactive.