Teaching tips

To help students gain a good picture of the physical trends I suggest you get them to use databases and simulations and try to first explain the trends themselves.

It is not immediately obvious that as atoms of elements gain in mass across a period they get smaller in size. However if students understand that electrons are being placed in the same energy level which is being attracted closer to the nucleus as the nucleus gains more protons it then become obvious.

It is worth including a brief discussion about what is meant by atomic radius and how it can be measured - after all you cannot precisely locate any electron so it cannot simply be the distance from the nucleus to the outermost electron!

Ionic radii are slightly harder. Firstly because sometimes ionic radius refers to positive ions and sometimes to negative ions. Also the charge on the ion changes depending upon the group the element is in. Get them to deduce the number of protons and the electron configuration. The relevant ions will all have the electron configuration of a rare gas and when the number of protons is taken into account it should be obvious how the sizes change both down a group and across a period. Although the word as such is not specified on the syllabus it can be useful to introduce the students to the word 'isoelectronic' here.

Melting points are harder to explain and in fact an explanation is not required. It depends both on the attractive forces between the atoms (for metals and non-metallic macromolecules such as diamond, graphite and silicon) or molecules (for other non-metals) and the type of packing.

I do not cover electronegativities at this point. They do not make sense until you are covering bonding as they imply knowledge of covalent bonding. I include them when I am discussing whether compounds are likely to be bonded ionically or covalently when I teach Topic 4 : Chemical bonding & structure. When you do cover electronegativity stress that there are no units and Pauling's scale is only one way of giving them values.

For the chemical properties it is well worth letting them loose on as many of the period 3 elements and their oxides as you can safely find. If you can demonstrate some of the more potentially dangerous reactions of the alkali metals and the halogens to them so much the better but if not show the videos below in 'other resources'.

The redox reactions of the halogens with halide ions should be done practically by students but you may wish to leave these until you cover  Topics 9: Redox processes so that they can fully understand them and write the half-equations.

Make sure students can explain the trends of the oxides across the period. Stress that the nature of their oxides is one of the best chemical differences between metals and non-metals. You will need to give them the equations for the reactions of Na₂O, MgO, P₄O₁₀ and the oxides of sulfur and nitrogen with water.

Discuss the international problems caused by the release of oxides of sulfur and nitrogen - acid deposition is no respecter of national boundaries.

Study guide

Pages 17 - 19

Questions

For ten 'quiz' multiple choice questions with the answers explained see MC test: Periodic trends.

For short-answer questions which can be set as an assignment for a test, homework or given for self study together with model answers see Periodic trends questions.

Vocabulary list

First ionization energy
Electronegativity
atomic radius
ionic radius
isoelectronic
Pauling's scale of electronegativities
Amphoteric

IM, TOK, Utilization etc.

See separate page which covers all of Topic 3

Practical work

Elements & oxides of the third period

The halogens

Periodicity. A database for graphing properties of the elements from the Royal Society of Chemistry