The simplest atom is Hydrogen consisting of a proton in the nucleus and an electron orbiting it. It is also the most abundant atom in the universe making up some 75% of ordinary matter.

Next is another form of Hydrogen called Deuterium. It has one proton and one neutron. It is still Hydrogen because it has one proton, the number of protons determine which element it is, but the extra neutron makes it heavier than normal Hydrogen. For this reason it is sometimes called Heavy Hydrogen. Note that the charges of the electron and the proton balance each other out, the neutrons have no effect so the whole atom has no charge.

Atoms with the same number of protons but a different number of neutrons are called isotopes. Deuterium then is an isotope of Hydrogen.

A third rare form of Hydrogen is Tritium one proton and two neutrons.

The next heaviest element Helium occurs naturally in two forms – Helium 3 with two protons and one neutron and Helium 4 with two protons and two neutrons. The two protons make it Helium and the different number of neutrons makes the different isotopes. Because Helium has two protons, that is two positive charges in the nucleus, it requires two electrons to balance out the positive charge so there are two electrons in orbit around the nucleus.

Other elements are made by adding more protons. Lithium has 3 protons, Berilium 4 protons, Boron 5 protons, Carbon 6 protons and so on. In each case for an uncharged atom, the number of electrons will equal the number of protons. The number of neutrons can vary giving different isotopes.

Under conditions that we have on Earth most atoms do not have a charge as the number of electrons equals the number of protons and the positive charges on the protons are balanced out by the negative charges on the electrons.