Classification — AQA A-Level Chemistry
Test yourself on Classification with AQA A-Level practice questions.
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Classification explained
The Periodic Table is built in order of increasing proton number, and each element's position follows from its electron configuration.
Read the full explanation
The block is named after the sub-shell that receives the highest-energy electron: Groups 1 and 2 are filling an s sub-shell and form the s block; Groups 3 to 0 are filling a p sub-shell; the transition series are filling a d sub-shell; the lanthanides and actinides are filling an f sub-shell. The period number is the highest principal energy level occupied. In the s and p blocks, the group number (1 to 7) equals the number of outer-shell electrons, whereas Group 0 elements have a full outer shell. Helium is an exception: it is placed above the noble gases due to its inert chemical behaviour, but its 1s² configuration makes it an s-block element.
Your focus
- Deduce the block, period and group of an element from the configuration 1s² 2s² 2p⁶ 3s² 3p⁴.
- Explain why helium is classified as an s-block element despite being printed above the noble gases.
- Write the electron configuration of copper and identify the sub-shell that names its block.
Classification exam tips
Marking Points
- one mark for stating that the elements are arranged in order of increasing proton number
- one mark for identifying the block from the sub-shell holding the highest-energy electron
- one mark for deducing the period from the highest occupied principal energy level
- one mark for placing an element correctly from a given electron configuration
Examiner Tips
- 💡Write the full configuration before answering; the block, period and group all read straight off it.
- 💡Remember the two d-block exceptions, chromium and copper, which are examined regularly.
Common Mistakes
- ordering the table by relative atomic mass rather than by proton number
- placing helium in the p block because it sits above the noble gases, when 1s² makes it s block
- assuming Group 0 elements have zero outer electrons rather than a full outer shell