Ions, Atoms and Isotopes.

Sometimes, when an atom gains one or more electrons, e.g when a metal reacts with a non-metal, then it gains a negative charge because it has more electrons than protons, which means more negativity than positivity. You would say it has become a negative ion.

When it loses electrons, it would be a positive ion because it has more protons than electrons.

An ion is a charged atom.

The bottom number is the its atomic number giving you the protons.

The top number is the mass number.

At least 7 billion billion billion atoms in the human body:

7000000000000000000000000000

Atoms of the same element always have the same number of protons, however there can be different numbers of neurons.

Atoms of the same element which have different numbers of neutrons are called isotopes.

Sometimes extra neutrons make the nucleus unstable, so it is radioactive. 

However, most are simlply atoms of the same elements that has a different mass.

Samples of isotopes of an element have different physical properties.E.g different density, they may or may not be radioactive.