Covalent Bonds, the Octet Rule, and the Periodic Table
Dr. Walt Volland revised July 31, 2012 all rights reserved ©
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Number of Bonds Formed: Nonmetal atoms form one bond for every electron needed to complete the octet on the atom. Molecular compounds have definite formulas because nonmetal atoms have a definite number of valence electrons and a limited capacity to accept electrons. Remember covalent bonding occurs most often between nonmetals and nonmetals. Predicting numbers of bonds formed: Here are a few examples that show how formulas can be predicted. What happens to elements at the top of a group will usually happen to the other elements in the group. Here are the combinations of hydrogen with boron, carbon, nitrogen, oxygen and fluorine. Generally to complete an octet Group 4A will form four bonds, Group 5A will form 3 bonds, Group 6A will form 2 bonds, etc. |
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Three bonds: Boron is in group 3A. It has three valence electrons. Technically boron has space for five electrons according to the octet rule, but boron is a very small atom and five nonmetal atoms cannot pack around the boron nucleus. Boron commonly forms three bonds, BH3. This results in strange properties for boron compounds because they are electron "short". They still don't have an octet.
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Four bonds: Carbon, C, is in group 4A. Carbon forms four covalent bonds. It has four (4) valence electrons. There is space for four more electrons around the carbon atom. Carbon needs four electrons to complete the octet. Each hydrogen atom can provide only one electron. The formula for the combination of carbon and hydrogen is CH4. The other elements in group 4A behave the same way when they bond with hydrogen. SiH4. GeH4.
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| Three bonds: Nitrogen, N, is in group 5A. Nitrogen has five (5) valence electrons. Nitrogen needs 3 electrons to complete an octet. Nitrogen typically forms three bonds. Each hydrogen atom can provide only one electron. The formula for the combination of nitrogen and hydrogen is NH3. The other elements in group 5A behave the same way when they bond with hydrogen. PH3, AsH3. |
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Two bonds: Oxygen, O, is in group 6A. Oxygen has six (6) valence electrons. There is space for two more electrons around the oxygen atom. In other words oxygen needs two electrons to complete its octet. Oxygen forms two bonds. The formula for the combination of oxygen and hydrogen is H2O. The other elements in group 6A behave the same way when they bond with hydrogen. H2S, H2Se
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One bond: Fluorine, F, is in group 7A. Fluorine has seven (7) valence electrons. There is space for one more electron around the fluorine atom. In other words fluorine needs one electron to complete its octet. Fluorine forms one bond. The formula for the combination of fluorine and hydrogen is HF. The other elements in group 7A behave the same way when they bond with hydrogen. HCl, HBr, HI |
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