Lesson 2: Sharing is bonding

Displayed formulas

Section 4 of 4

A formula like CH₄ tells you which atoms are in a molecule and how many. It does not tell you which atom is joined to which.

A drawing that shows every atom and every bond is called a displayed formula; it is one kind of structural formula. The ethane, butane, ethene, ethyne and carbon dioxide drawings in this lesson are all displayed formulas.

To draw one, you need to know how many bonds each kind of atom usually makes. It comes from counting outer electrons. Hydrogen has 1 and needs 2, so it makes 1 bond. Carbon has 4 and needs 8, so it makes 4. Nitrogen has 5, so it makes 3. Oxygen has 6, so it makes 2. Some students remember this as "HONC 1234": H 1, O 2, N 3, C 4.

What about oxygen's other outer electrons? Oxygen uses 2 of its 6 outer electrons in bonds. The other 4 stay on oxygen as 2 pairs that are not shared, called lone pairs. Nitrogen has one lone pair. You will see in lesson 3 that lone pairs matter for a molecule's shape.

Water, H₂OOne oxygen atom joined by single bonds to two hydrogen atoms, in a bent shape.OHH
Water, H₂O: oxygen makes 2 bonds. Drawn flat; a displayed formula only promises which atoms are joined.
Ammonia, NH₃One nitrogen atom joined by single bonds to three hydrogen atoms.HNHH
Ammonia, NH₃: nitrogen makes 3 bonds. Drawn flat.
Methane, CH₄One carbon atom joined by single bonds to four hydrogen atoms.HHCHH
Methane, CH₄: carbon makes 4 bonds.

Displayed formulas matter because the same atoms can be joined in different ways. Butane and the molecule below both have the formula C₄H₁₀, but their atoms are joined differently, so they are different substances. More on this in lesson 4.

Butane, C₄H₁₀Four carbon atoms in a straight chain, joined by single bonds, with ten hydrogen atoms around them.HHHHHCCCCHHHHH
Butane, C₄H₁₀: four carbons in one chain.
Isobutane (2-methylpropane), C₄H₁₀Three carbon atoms in a chain with a fourth carbon branching off the middle one, and ten hydrogen atoms around them.HHHHCCCHHHHCHH
Also C₄H₁₀, but with a branch: three carbons in a chain and one attached to the middle.

Quick check

How many bonds does an oxygen atom usually make?

That's the end of lesson 2

In this lesson you found out:

  • What a covalent bond is
  • The difference between single, double and triple bonds
  • How to read and draw displayed formulas
Start Lesson 3: Molecules in 3D All lessons