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

2.4 Sketching the Lewis structures for Cl2 and NaCl

The description of the bonding in Cl2 molecules and the ionic solid NaCl, in the previous sections took a lot of words.

However there is a way of doing this, very concisely, using simple diagrams, the chemical symbols you met earlier and “dots and crosses” to represent valence electrons.

Here’s how it works.

Figure 3(a) below shows the Lewis structure for the Cl2 molecule. The valence electrons are grouped in pairs, to reflect the pairing of electrons you’ve previously seen for atomic orbitals.

Described image
Figure 3 Lewis structures for (a) Cl2 and (b) the ions in solid NaCl

The shared pair is shown as a dot and a cross in the centre. The remaining electrons in the valance shell of each atom are distributed in pairs, so taking the shared electrons into account there are eight electrons around each chlorine atom in the molecule. Note that the use of a dot or a cross is just to indicate where a particular electron originates, they are purely symbols. There’s no difference between an electron represented by a cross and one allocated a dot.

Likewise the ions in sodium chloride have also been represented in this way in Figure 3(b).

The chloride anion has eight outer electrons; recall the chlorine atom has gained an electron to complete its octet. Similarly sodium loses its outer electron to form a cation, so the Lewis structure of the cation has no electrons shown.

In both structures, the formation of a chemical bond involves the production of a new electron pair in the outer shell of chlorine. However, in Cl2, because the two atoms are identical, the electron pair must be equally shared between the two atoms. In contrast you can view the electron pair as being located on the chloride anion in NaCl.

You will return to this point later, but first a look at a molecule which does not follow the octet rule.


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