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Carbon tetrabromide

A Lewis structure shows how valence electrons are arranged among atoms in a molecule. In arranging valence electrons, the duet and octet rules are very important. The duet rule applies to molecules containing hydrogen, as hydrogen is most stable when sharing two valence electrons. The octet rule is based upon the observation that atoms (other than hydrogen) are most stable when surrounded by eight valence electrons. These 8 valence electrons can either be shared (bonds) or not shared (lone pairs).

Consider the bonding of hydrogen and fluorine. Hydrogen (Group IA) has 1 valence electron and fluorine (Group VIIA) has 7 valence electrons. See Figure 4. The atoms form the molecule hydrogen fluoride (HF), which has a total of 8 valence electrons in its electron cloud. The hydrogen follows the duet rule, and the fluorine follows the octet rule. Two electrons are shared between them. Six electrons surrounding fluorine are not shared, and are considered lone pairs. There are a total of three lone pairs around fluorine, which are shown as three pairs of dots.

Figure 4. Bonding of hydrogen fluoride (HF). Hydrogen has two valence electrons and obeys the duet rule; fluorine has eight valence electrons and obeys the octet rule.

The Lewis structure of the molecule can be drawn so that a single dash represents the shared electrons, as shown in Figure 5.

Figure 5. A. Hydrogen and fluorine share one pair of electrons (shown in the circle); there are also three lone pairs. B. The shared pair may be represented as a single dash that signifies a

bond, while the lone pairs are still drawn as dots. 6 ©Hands-On Labs, Inc.

Experiment Molecular Modeling and Lewis Structures