The basic building block of all matter is the atom. Atoms are very, very small. Human beings are made up of billions and billions of atoms. The center of an atom is made up of a nucleus that consists of neutrons and protons .
Electrons orbit the nucleus much the same way the planets orbit the sun.
Atoms with the same number of protons are called elements . For example, all iodine atoms have 53 protons and all uranium atoms have 92 protons. The Periodic Table gives the abbreviation for each element and how many protons are found in nucleus of that element. Atoms with the same number of protons but different number of neutrons are called isotopes. Even though there are 103 known elements there are thousands of isotopes.