When a metallic substance shares electrons with a non-metal substance it forms covalent bonds, which make molecules. The metal becomes positively charged atoms, which means that the number of electrons is never less than the number of protons. Non-metals become negatively charged atoms, and now the number of electrons is more than the number of protons. When atoms share electrons of nonmetals then a covalent bond is formed inside the molecule. Monatomic elements are elements that only contain one type of atom. An example of this would be the noble gases, such as Helium or Neon. Diatomic elements are molecules that contain two atoms. An example of this would be H2, F2, or Cl2. Diatomic elements do not have to both be the same element. Polyatomic elements are molecules that contain many types of atoms. An example of a polyatomic element is ammonium, or NH4. A molecular element is a molecule with only one type of element, such as Oxygen, or O2. A molecular compound is made up of molecules that have two or more types of elements. For example, CO2 is made up of Carbon and Oxygen. Molecular, or covalent, compounds are gases, liquids and solids. Positive and negative ions are formed as a result of transferring electrons between atoms of metal and atoms of non-metals. A positive ion is formed by removing electrons from an atom or group of atoms until there are fewer electrons than protons. A negative ion is formed when an atom or group of atoms gain electrons until there are more electrons than protons. Both positive and negative ions are formed by the transfer of electrons, however only non-metals can form negative ions and only metals can form positive ions. A chemical bond in which one atom gives up an electron to another, forming two ions of opposite charge, generates an electrical force that holds the atoms together. The atoms are thus held together by the attractive force between a positively and a negatively charged ion.