The periodic table arranges elements in order of increasing atomic number and groups elements with similar chemical properties together. Elements in the same group have the same number of electrons in their outer shell, giving them similar chemical properties. Atoms are made up of protons and neutrons in the nucleus and electrons in energy levels outside the nucleus. Atoms are neutral when they have an equal number of protons and electrons. Isotopes are atoms of the same element that have different numbers of neutrons, giving them different atomic masses. An element's relative atomic mass takes into account the abundance of each isotope.