ElementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of Click on Term to Read More occurring in the right-most column of the periodic table; also called “inert” gases. In these gases, the outer electron shell is completely filled, making them very unreactive.