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Ions form when an atom or group of atoms gains or loses electrons to gain a full valence shell. They have a positive or negative charge because they do not have an equal number of electrons to protons and the overall charge is unbalanced.Metals tend to lose electrons to form positively-charged ions called cations. Non metals tend to gain electrons to form negatively-charged ions are called anions. For example, the oxygen atom has an atomic number of 8. This means that it has 8 protons and therefore 8 electrons because the charges in an atom are balanced. This gives it an electron configuration of 2,6. There are 6 electrons in the valence shell. For the atom to be stable and have a full valence shell, it must gain 2 electrons, becoming an ion. The oxide ion has an electron configuration of 2,8. Because it has gained two electrons and now has two more negatively-charged electrons to positively-charged protons, it has a negative charge of 2-. The oxide ion has the symbol O2-.On the other hand, atoms can also lose electrons to form positively-charged ions. For example, a lithium atom has 3 electrons, giving it an electron configuration of 2,1. To gain a full outer energy level, it must lose one electron so that its first shell is full. This is easier than gaining 7 electrons to fill up the second shell. The lithium ion therefore has one less negatively-charged electron to positively-charged protons and has a positive charge of 1+. The symbol for the lithium ion is Li+. .Ion Table: untitled.JPG