In covalent bonding, atoms share electrons to satisfy the octet rule or the duet rule for hydrogen. We can use this pattern to predict the Lewis structure for a compound formed between two elements (for example, hydrogen and oxygen). Hydrogen follows the duet rule, so it wants one additional electron, whereas oxygen wants two additional electrons to follow the octet rule. As a result, one oxygen atom can share a pair of electrons with two different hydrogen atoms. When one atom shares two electrons with another atom, a single bond results. When electrons are shared in a Lewis structure, they count as being on both atoms. In this Lewis structure for water, we have two types of electrons: bonding pairs of electrons are shared between two atoms, and lone pairs of electrons are considered on a single atom. A bonding pair of electrons can also be shown as a line to represent a single bond. We can also predict how two oxygen atoms will bond using Lewis structures. As we said previously, oxygen wants two additional electrons to satisfy the octet rule. So two oxygen atoms can share two pairs of electrons, and both will have a complete octet. When two atoms share four electrons, a double bond forms. Double bonds can be represented as two lines. We can also predict how two nitrogen atoms will bond using Lewis structures. Nitrogen wants three additional electrons to satisfy its octet because it starts with five valence electrons. So two nitrogen atoms can share three pairs of electrons, or six electrons total, to form a triple bond. Triple bonds are represented using three lines. Lewis structures can help us understand why some molecules form and others do not. For example, we already saw that H2O forms to satisfy the octet and duet rules for oxygen and hydrogen. If we tried to create H3O by adding another hydrogen atom, oxygen would have nine electrons, which would not be stable. So H3O does not form. However, if we remove one of the electrons from H3O, we'll form H3O+. This ion does form; it is called the hydronium ion. We'll learn more about the hydronium ion when we discuss acids and bases.