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Lesson · Grade 11 · Chemistry

Drawing Lewis structures step by step

  • Grade 11
  • Chemistry
  • 2 min 3 s
  • British Columbia
  • BC11-SC-05

A labelled diagram of carbon dioxide is built up through four steps listed on the right, with a running count of electrons left to place. It goes from counting valence electrons (4 + 6 + 6 = 16), through single bonds and outer octets, to two double bonds giving O=C=O with two lone pairs on each oxygen (8 bonding + 8 lone pair = 16). The same steps then draw ammonia (5 + 1 + 1 + 1 = 8, three N–H bonds and one lone pair on nitrogen), showing that hydrogen is full with 2 electrons, not 8, before a one-line recap of the four steps.

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In one sentence

The question this lesson answers

A Lewis structure shows where the outer electrons in a molecule are. How do you draw one?

Let's draw one for carbon dioxide, in four steps.

Transcript

What the lesson says

A Lewis structure shows where the outer electrons in a molecule are. How do you draw one?

Let's draw one for carbon dioxide, in four steps.

Step one: count the valence electrons, the electrons in each atom's outer shell.

Carbon has 4 and each oxygen has 6.

So 4 plus 6 plus 6 makes 16 valence electrons to place.

Step two: put carbon in the centre and join each oxygen to it with a single bond.

A single bond is one shared pair of electrons, so two bonds use 4, leaving 12.

Step three: give each outer atom an octet, which means 8 electrons around it.

Each oxygen needs 6 more, so it gets 3 lone pairs, pairs that are not shared, using the last 12.

Step four: check the central atom. Carbon has only 4 electrons, not an octet.

So one lone pair on each oxygen becomes a second shared pair, making two double bonds.

Now carbon has 8, and each oxygen has 4 in bonds plus 4 in lone pairs, which is 8.

Check: 8 bonding plus 8 lone pair electrons makes 16, as we counted.

For ammonia, nitrogen has 5 valence electrons and each of the 3 hydrogens has 1, making 8.

Three single bonds to hydrogen use 6 electrons, leaving 2.

Hydrogen is full with just 2 electrons, not 8, so it gets no lone pairs.

The last 2 make a lone pair on nitrogen, giving it 6 plus 2, which is 8.

So: count the valence electrons, join with single bonds, complete the outer octets, then add double bonds if needed.

Curriculum

Where this fits

British Columbia, Grade 11

Chemistry 11, Bonding: valence electrons and Lewis structuresBC11-SC-05

Previous: Electron Pairs Push Apart: VSEPR Molecule ShapesNext: Ionic and Covalent Bonds: Transfer or Share?

The outcomes and strands follow the British Columbia curriculum.

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