4CH1

The Periodic Table

Principles of Chemistry

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Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 8% of your exam marks.

stable

Low

Stable8%

Group trends, periods and atomic radius/ionisation energy are regularly examined.

Writing the configuration

An atom's electron arrangement can be shown in two ways:
- As an electron shell diagram (nucleus drawn with concentric shells, electrons placed as dots)
- As an electronic configuration (also called electronic structure): the number of electrons in each shell, comma-separated, innermost shell first
Shell-filling rules for the first 20 elements:
- 1st shell holds up to 2 electrons
- 2nd shell holds up to 8 electrons
- 3rd shell holds up to 8 electrons (a simplification used at this level: once the 3rd shell reaches 8, the 4th shell begins to fill before the 3rd fills further)
- 4th shell starts to fill once the 3rd reaches 8
Once the 3rd shell has 8 electrons, the next two electrons (for potassium and calcium) go into the 4th shell first, before the 3rd shell continues filling for heavier elements

Electron shell diagram of first, second and third shells alongside their maximum capacities: first shell 2 electrons, second shell 8 electrons, third shell 8 electrons — Source: Electronic Configurations by Save My Exams

Electronic configurations of the first 20 elements

Element	Z	Configuration
Hydrogen	1	1
Helium	2	2
Lithium	3	2,1
Beryllium	4	2,2
Boron	5	2,3
Carbon	6	2,4
Nitrogen	7	2,5
Oxygen	8	2,6
Fluorine	9	2,7
Neon	10	2,8
Sodium	11	2,8,1
Magnesium	12	2,8,2
Aluminium	13	2,8,3
Silicon	14	2,8,4
Phosphorus	15	2,8,5
Sulfur	16	2,8,6
Chlorine	17	2,8,7
Argon	18	2,8,8
Potassium	19	2,8,8,1
Calcium	20	2,8,8,2

Configurations of ions

An ion's configuration is its atom's configuration adjusted for electrons gained or lost
Sodium atom (Na): 11 electrons, configuration 2,8,1
- Sodium ion (Na⁺): 1 electron lost → 10 electrons → 2,8 (same as neon)
Chlorine atom (Cl): 17 electrons, configuration 2,8,7
- Chloride ion (Cl⁻): 1 electron gained → 18 electrons → 2,8,8 (same as argon)
Many ions end up with the same configuration as the nearest noble gas, which is why those ions are stable

Reading position from the electronic configuration

Period number = number of separate notations in the configuration
Group number = the last notation (for Groups 1 to 7)
For example, chlorine has configuration 2,8,7:
- 3 notations → 3 occupied shells → 3
- Last notation is 7 → 7 outer electrons → 7

The configuration 2,8,7 labelled to show the number of notations gives the period and the last notation gives the group, shown next to a chlorine electron shell diagram — Source: Electronic Configurations by Save My Exams

Periodic table with chlorine highlighted, showing it sits in Period 3 and Group 7 — Source: Electronic Configurations by Save My Exams

Group 0 is the exception
- Group 0 elements have full outer shells, not a single matching outer-electron count
- Helium is in Group 0 even though its configuration is just 2 (only one full shell)

Exam tip

Identifying an element's group from its electronic configuration

What comes up: given an electronic configuration (or the atomic number), state the group and/or period, or explain which group the element belongs to.

Write (two marks): (1) State the group number (e.g. "Group 4"). (2) Explain: the element has that many electrons in its outer shell (e.g. "because there are 4 electrons in the outer shell"). For period: the number of occupied shells equals the period number.

Watch out: for a 1-mark "give the group" question, the number alone is enough — but for a 2-mark "explain" question, you must state the outer-electron count as the reason. Writing only the group number without the justification scores just 1 of 2.