Discovering chemistry
Discovering chemistry

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Discovering chemistry

1.1 Valency and the chemical bond

Up to this point you have simply treated valencies as just been numbers that are used to predict the formulae of compounds. But in the case of covalent substances they can tell us so much more.

In particular, they reveal how the atoms are linked together in a molecule. This information is obtained from a two-dimensional drawing of the structural formula of the molecule. (Note that structural formulae cannot be assumed to carry any implications about molecular shape.)

Consider, for example, the molecules H2, Cl2, NCl3 and CH4. Their structural formulas are shown here as Structures 4.1-4.4. They can be drawn correctly by ensuring that the number of lines or bonds emerging from any atom is equal to its valency.

H2 and Cl2 are simply represented as a single line between the two atoms, both hydrogen and chlorine have a valency of one.

Recalling that nitrogen has a valency three, and chlorine a valency of one; NCl3 is shown with three bonds emerging from the nitrogen atom and one from each chlorine atom.

  • Account for the structure of methane CH4 (4.4) in terms of the valency of its atoms.

  • Carbon has a valency of four, so will form four bonds to hydrogen, which has a valency of one.

At this point, check that you are comfortable with these bonding ideas by attempting the following.

  • Draw structural formulas for the following molecular substances: hydrogen chloride (HCl), ammonia (NH3), water (H2O), oxygen (O2), carbon dioxide (CO2), ethane (C2H4), hydrogen cyanide (HCN), ethyne (C2H2) and ethanal (CH3CHO).

  • The structural formulae are as shown below:


Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371