9.3 Chemical periodicity of other elements
- Syllabus
- 9701–2028–2029
- Topic
- 9.3
- Level
- AS
Learning objectives
9.3.1The characteristic properties of an element• Predict the characteristic properties of an element in a given group by using knowledge of chemical periodicity9.3.2The nature, possible position in the Periodic• Deduce the nature, possible position in the Periodic Table and identity of unknown elements from given information about physical/chemical properties
Predict an element by transferring group patterns and periodic trends
Elements in the same group have the same number of outer-shell electrons, so they tend to form ions with the same charge and compounds with analogous formulae. Moving down the group adds an occupied shell: atomic radius and shielding increase, while the attraction between the nucleus and an outer electron generally weakens.
| Given clue | Prediction it can support | Required explanation |
|---|---|---|
| group number | outer-electron count, common ion charge, analogous oxide/chloride/hydride formulae | same outer-shell pattern |
| position lower in a group | larger atomic radius; generally lower first ionisation energy and electronegativity | extra shell and greater shielding outweigh increased nuclear charge |
| molecular size increases down a molecular group | stronger instantaneous dipole–induced dipole attractions; often higher melting/boiling points | more electrons and greater polarisability |
| known reaction trend within that group | likely relative reactivity of the new element | apply the group's established causal trend, not a universal down-group rule |
Example: let M be the element immediately below Mg in Group 2. Predict a +2 ion because both atoms have two outer-shell electrons; predict formulae MO and MCl₂ by charge balance. M should have a larger radius and lower first ionisation energy than Mg because its outer electrons occupy an additional, more shielded shell.
Write each prediction as evidence → trend → property. A shared group does not make numerical values identical, and reactivity does not always change in the same direction down every group; use the reaction mechanism or a stated group trend before predicting it.
Identify an unknown by intersecting independent clues
| Evidence supplied | What it can locate or classify |
|---|---|
| proton/atomic number | exact element identity |
| electron configuration or a large jump in successive ionisation energies | period from occupied shells; group from outer electrons or the number removed before the jump |
| common ion charge and compound formulae | likely group and oxidation states |
| conductivity, melting/boiling point and physical state | metal/non-metal character and possible metallic, ionic, giant covalent or molecular structure |
| oxide acidity/basicity and reactions with water, acid or base | approximate left-to-right position and metallic/non-metallic character |
| comparison with known neighbouring elements | relative position within a group or period |
- Translate every observation into a constraint. 2. Use the most diagnostic clues to propose a group, period and whether the element is metallic or non-metallic. 3. Intersect the candidates rather than allowing one clue to decide. 4. Test the proposed identity against every remaining physical and chemical observation; reject it if any reliable clue conflicts.
| Given information for E | Deduction |
|---|---|
| E is a Period 3 solid that conducts electricity | metallic candidate in the left/central part of Period 3 |
| a very large successive-ionisation-energy jump occurs after the third electron | three outer-shell electrons; Group 13 |
| E forms E₂O₃ and ECl₃ | oxidation state +3, consistent with the same group clue |
| E₂O₃ is amphoteric | matches the ionic–covalent transition in Period 3 |
| intersection | E is aluminium |
A property such as high melting point or electrical conductivity is not usually unique. State what each clue supports, distinguish a possible position from a confirmed identity, and reserve an exact identity for a unique identifier or a mutually consistent set of independent clues.