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 (the simplified model — in fact it can hold up to 18, but the next 8 are stable enough to treat it as 8 at this level)
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
Electronic configurations of the first 20 elements
Element
Z
Configuration
Hydrogen
1
1
Helium
2
2
Lithium
3
2,1
Beryllium
4
2,2
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
number = number of separate notations in the configuration
number = the last notation (for Groups 1 to 7)
For example, chlorine has configuration 2,8,7:
3 notations → 3 occupied shells → Period 3
Last notation is 7 → 7 outer electrons → Group 7
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.