Lewis Symbols and Structures: Drawing Where a Molecule's Electrons Go

Chemical bonds come down to what atoms do with their valence electrons, the ones in the outermost shell: they share them (covalent bonds) or hand them over (ionic bonds). Drawing those electrons as dots is the quickest way to see how a molecule holds together.

Lewis symbols

A Lewis symbol is an element's symbol with one dot for each valence electron. Calcium, with two, is Ca with a pair of dots. Across the third period of the periodic table, sodium has one dot, magnesium two, and so on up to argon with eight.

Lewis symbols for the third period, sodium to argon. Image: OpenStax, CC BY 4.0.

The same symbols show ions forming. Sodium loses its one valence electron to become Na⁺ and calcium loses two to become Ca²⁺; chlorine gains one to become Cl⁻ and sulfur gains two to become S²⁻. In an ionic compound such as sodium chloride, the dots show the electron moving from metal to non-metal; the total number of electrons does not change.

Cations form when atoms lose electrons, anions when atoms gain them. Image: OpenStax, CC BY 4.0.

Lewis structures and the octet rule

A Lewis structure uses the same dots to show covalent bonds in molecules and polyatomic ions. When two chlorine atoms form Cl₂, each shares one electron, so the molecule has one shared pair between the atoms and three unshared lone pairs on each. A shared pair can also be drawn as a line: one shared pair is a single bond.

Two chlorine atoms share a pair of electrons. Image: OpenStax, CC BY 4.0.

Each chlorine atom now has eight valence electrons around it, the configuration of a noble gas. The tendency of main-group atoms to form enough bonds to reach eight is the octet rule, and it predicts how many bonds an atom forms, especially for carbon, nitrogen, oxygen and fluorine:

• Group 14 (carbon, silicon): four valence electrons, so four bonds, as in CCl₄ and SiH₄;
• Group 15 (nitrogen): five, so three bonds and one lone pair, as in ammonia, NH₃;
• Group 16 (oxygen): six, so two bonds and two lone pairs, as in water;
• Halogens (fluorine, chlorine and the rest): seven, so one bond and three lone pairs.

Ammonia, water and hydrogen fluoride. Image: OpenStax, CC BY 4.0.

Sometimes atoms must share more than one pair to complete their octets. Two shared pairs make a double bond, as in formaldehyde (CH₂O) and ethylene (C₂H₄); three make a triple bond, as in carbon monoxide (CO) and the cyanide ion (CN⁻).

Triple bonds in carbon monoxide and the cyanide ion. Image: OpenStax, CC BY 4.0.

Drawing a Lewis structure, step by step

1. Count the valence electrons. Add them up for every atom; for a negative ion add one electron per negative charge, for a positive ion take one away per positive charge. SiH₄ has 4 + (4 × 1) = 8; the formate ion CHO₂⁻ has 4 + 1 + 12 + 1 = 18; NO⁺ has 5 + 6 − 1 = 10; OF₂ has 6 + 14 = 20.
2. Draw a skeleton, with a central atom (usually the least electronegative; hydrogen never) joined to the others by single bonds.
3. Put the remaining electrons on the outer atoms as lone pairs until each (except hydrogen) has an octet.
4. Put anything left over on the central atom.
5. Make multiple bonds if needed, by turning outer lone pairs into shared pairs, until every atom has an octet where possible.

The four worked examples: SiH₄, CHO₂⁻, NO⁺ and OF₂. Image: OpenStax, CC BY 4.0.

The book tries this on molecules found on Titan, Saturn's moon, where NASA's Cassini-Huygens mission detected a large cloud of toxic hydrogen cyanide: HCN (10 valence electrons) ends up with a triple bond between carbon and nitrogen, acetylene (HCCH) with a triple bond between its carbons, and ethane and ammonia need no multiple bonds at all.

An aside: a new form of carbon

Carbon soot has been known since prehistoric times, yet a new form of carbon was discovered only recently: the C₆₀ molecule, buckminsterfullerene. Its discovery won Richard Smalley, Robert Curl and Harold Kroto the 1996 Nobel Prize in Chemistry. Smalley, of Rice University, became a leading advocate of fullerene chemistry, and on his death in 2005 the U.S. Senate honoured him as the "Father of Nanotechnology."

Richard Smalley (1943–2005). Image: United States Department of Energy.

When the octet rule fails

Many molecules have a central atom without eight electrons. They fall into three groups:

• Odd-electron molecules, or free radicals, have an odd number of valence electrons, so one is left unpaired. Nitric oxide, NO, made in car engines when nitrogen and oxygen react at high temperature, has 11: the best that can be drawn gives oxygen an octet and nitrogen seven electrons, joined by a double bond.
• Electron-deficient molecules have too few. Beryllium in BeH₂ has four electrons around it and boron in BF₃ six. Such atoms are very reactive: BF₃ readily takes a lone pair from ammonia, forming a bond between boron and nitrogen.
• Hypervalent molecules have more than eight. Second-period elements cannot exceed eight, having only four valence orbitals, but elements from the third period on can: phosphorus shares five pairs in PCl₅ and sulfur six in SF₆. In some, such as IF₅ and XeF₄, the central atom also carries lone pairs. Even the noble gas xenon forms stable compounds: XeF₂ (22 valence electrons) has two bonds and three lone pairs on xenon, and XeF₆ (50) has six bonds and one lone pair.

Nitric oxide, a free radical. Image: OpenStax, CC BY 4.0.

PCl₅ and SF₆: more than eight electrons around the central atom. Image: OpenStax, CC BY 4.0.

Sources

• Paul Flowers, William R. Robinson, Richard Langley and Klaus Theopold, Chemistry, "Lewis Symbols and Structures", OpenStax (Rice University), licensed under CC BY 4.0. Changed: rewritten in hubnx's own words and shortened, the exercises left out; the figures are the book's, and the photograph of Richard Smalley is the U.S. Department of Energy's. This page is shared under the same licence.