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Dihydrogen monosulfide

Note: Multiple bonds are used ONLY when there are not enough lone pairs present for each atom to fulfill the octet rule. In the incorrect Lewis structure above, we know a multiple bond is needed because the correct total valence electrons are present but the octet rule is not fulfilled. 9 ©Hands-On Labs, Inc.

Experiment Molecular Modeling and Lewis Structures

Step 4) Check your work: ensure the duet and octet rules are satisfied and count the total number of valence electrons.

A review of the correct molecule shows that all atoms are surrounded by 8 valence electrons, satisfying the octet rule. The number of electrons drawn in the Lewis structure (in the lone pairs and the double bond) totals 12, which matches the calculation in step 1.

Example 3: Cyanide (CN-)

Step 1) Calculate the total number of valence electrons in the molecule.

As indicated in the molecular formula, this molecule has a negative charge. Therefore, an additional electron must be accounted for. Carbon contributes 4 valence electrons, nitrogen contributes 5 valence electrons, and the negative charge in the chemical formula indicates 1 additional valence electron.

There are 10 valence electrons total in CN-.

Note: In this example, a “–“charge was shown. If a “+” charge were present, then valence electron(s) would be subtracted rather than added.

Step 2) Arrange the atoms and create single bonds.

The two atoms are placed in a line and linked with a single bond.

Step 3) Calculate the number of remaining valence electrons; then distribute the electrons with the goal of satisfying the duet rule or octet rule.

Two electrons were added to the Lewis structure through the addition of the single bond; 8 more electrons were required. Through trial-and-error, it can be determined that the two atoms share 6 electrons in a triple bond. The C and N atoms each have one set of lone pairs, satisfying the octet rule. 10 ©Hands-On Labs, Inc.

Experiment Molecular Modeling and Lewis Structures