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THINK ABOUT IT

The HONC 1234 rule is a great trick to help you figure out the structures of molecules. But why does it work? Why does carbon connect with four atoms, while hydrogen can connect with only one atom? To answer these questions, let’s take a closer look at bonds.

How does one atom bond to another in a molecule?

To answer this question, you will explore

EXPLORING THE TOPIC

The Covalent Bond

All of the smelly compounds we have studied so far have been molecules. In fact, most of the substances that smell are made up of molecules, not ionic, metallic, or network covalent compounds.

The bonds that are found in molecules are called covalent bonds. Covalent bonds are formed between the atoms of nonmetallic elements. There are only about 15 nonmetallic elements in the periodic table. That is not many compared to the number of metallic elements.

The atoms involved in a covalent bond share a pair of valence electrons between them. The drawing below shows the difference between covalent bonds and ionic bonds.

In covalent bonds, the nonmetal atoms are sharing electrons. As a result, the nonmetal atoms are tightly bound together, but the atoms do not become ions with charges.

Lewis Dot Symbols and Structures

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Structural formulas are one way to show the structure of a molecule on paper. Each line in a structural formula can be replaced with a pair of dots to represent the electrons that are being shared.

A drawing of a molecule that uses dots to represent the valence electrons is called a Lewis dot structure. Above is a molecule of dimethyl ether, C₂H₆O. Both the structural formula and the Lewis dot structure represent the same molecule.

Lewis dot structures keep track of every valence electron in every atom of a molecule. If you break the molecule apart into its individual atoms, you can see where each valence electron came from.

A Lewis dot symbol is the symbol of an element with dots to show the number of valence electrons that a single atom of that element has. Examine these Lewis dot symbols for hydrogen, oxygen, nitrogen, and carbon. Notice that some electrons are not paired up. These electrons are referred to as unpaired electrons and are available for bonding. The unpaired electrons form covalent bonds with other atoms. Lewis dot symbols can help to explain the HONC 1234 rule and the bonding of nonmetal atoms.

Recall that you can determine the number of valence electrons in an atom by locating its element on the periodic table. Hydrogen is in Group 1A and has one valence electron. Carbon is in Group 4A and has four valence electrons. Once you know the number of valence electrons an atom has, it is easy to draw a Lewis dot symbol for it.

The Lewis dot symbol is named after Gilbert Newton Lewis, a chemist who was a professor at the University of California at Berkeley. Lewis first introduced the idea of shared electrons and covalent bonds in 1916. He was the first scientist to use a system of dots to represent valence electrons in atoms.

Bonded Pairs and Lone Pairs

Lewis dot symbols can be used to draw Lewis dot structures and structural formulas for molecules. Imagine bringing together the Lewis dot symbols for hydrogen and carbon to form methane, CH₄.

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Because carbon bonds four times, it will bond with four hydrogen atoms. Once they are bonded, every valence electron in all of the atoms is paired up. The pairs of electrons between atoms are called bonded pairs. There are four bonded pairs in the methane molecule shown.

A pair of electrons in a molecule that are not shared between atoms is called a lone pair of electrons. In a molecule of ammonia, the nitrogen atom has one lone pair of electrons.

LESSON SUMMARY

How does one atom bond to another in a molecule?

Molecules are made of nonmetal atoms that are covalently bonded. A covalent bond is a bond in which a pair of electrons is shared by two atoms. Lewis dot symbols keep track of the number of valence electrons in each atom. They can help you to predict covalent bonding between atoms. A Lewis dot structure shows how the atoms in an entire molecule are bonded together. The valence electrons that are involved in bonding are called a bonded pair. The valence electrons that are paired up in a molecule, but do not take part in bonding, are called lone pairs.

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