Atoms are the basic building blocks of matter. When two or more atoms join together, they form molecules. An example would be two hydrogen atoms and an oxygen atom joining to form a water molecule.
Atoms are made up of three different particles. Protons and neutrons are found in the nucleus, which is the central core of the atom. Protons have a positive charge, while neutrons have no charge.
Electrons are found in energy shells outside of the nucleus. Each of these subatomic particles has a negative charge. A neutral atom contains the same number of protons and electrons.
Charged atoms, called ions, form when an atom gains or loses at least one electron. If an ion has more protons than electrons, it has a net positive charge. If the ion has more electrons than protons, it has a net negative charge.
The Periodic Table of the Elements organizes all of the elements according to their electron configurations and atomic properties. Using the Periodic Table, it is possible to determine how many protons and electrons are found in a neutral atom.
An element's atomic number corresponds to the number of protons found in one atom of that element. Helium has an atomic number of 2, so there are two protons in one atom of helium. In a neutral atom, the number of electrons is equal to the atomic number. Magnesium has an atomic number of 12. This means a neutral atom of helium has 12 protons and 12 electrons.