Electronic Configuration and the Periodic Table
How electrons are arranged determines everything about an element’s chemistry — its group, its reactivity and the ions it forms.
1. Filling the shells
Electrons occupy shells around the nucleus, filling from the innermost outwards.
Maximum electrons: 1st shell 2, 2nd shell 8, 3rd shell 8 (for the first 20 elements).
Writing a configuration: separate the shells with commas, starting from the inside.
| Element | Proton number | Configuration |
|---|---|---|
| Hydrogen | 1 | 1 |
| Carbon | 6 | 2,4 |
| Oxygen | 8 | 2,6 |
| Sodium | 11 | 2,8,1 |
| Chlorine | 17 | 2,8,7 |
| Argon | 18 | 2,8,8 |
| Calcium | 20 | 2,8,8,2 |
The total must equal the PROTON NUMBER for a neutral atom — that is your check. If your digits don’t add up to the atomic number, something is wrong.
For IGCSE, only the first 20 elements are required. Tutors flagged this directly — beyond calcium the filling order changes, and you won’t be asked.
The 3rd shell takes 8 at this level, which is why potassium is 2,8,8,1 and not 2,8,9.
2. Finding the group and period
This is the most-asked question on the topic.
GROUP NUMBER = number of electrons in the OUTER shell PERIOD NUMBER = number of occupied SHELLS
Examples:
- 2,8,6 → 6 outer electrons, 3 shells → Group VI, Period 3
- 2,8,1 → Group I, Period 3 (sodium)
- 2,4 → Group IV, Period 2 (carbon)
Tutors noted questions often ask for group and period from a configuration — practise going both ways, including writing the configuration when given the position.
Special case: elements with a full outer shell (2 or 8) are in Group VIII (0) — the noble gases.
3. Why it matters
Elements in the same GROUP have the SAME number of outer electrons — which is why they have SIMILAR chemical properties.
It is the OUTER-SHELL electrons that determine chemical behaviour.
Predicting the ion formed:
| Outer electrons | Tendency | Ion |
|---|---|---|
| 1, 2 or 3 | LOSE them (metals) | 1+, 2+, 3+ |
| 5, 6 or 7 | GAIN to fill the shell (non-metals) | 3−, 2−, 1− |
| 4 | usually shares (covalent) | rarely simple ions |
| 8 (full) | neither — inert | none |
Atoms react to achieve a FULL OUTER SHELL, like a noble gas. Losing 1–3 electrons is easier than gaining 5–7, and vice versa.
4. Configurations of ions
Work out the ATOM’s configuration first, then ADD or REMOVE electrons.
| Ion | From | Configuration |
|---|---|---|
| Na⁺ | 2,8,1 | 2,8 (lost 1) |
| Mg²⁺ | 2,8,2 | 2,8 (lost 2) |
| Al³⁺ | 2,8,3 | 2,8 (lost 3) |
| Cl⁻ | 2,8,7 | 2,8,8 (gained 1) |
| O²⁻ | 2,6 | 2,8 (gained 2) |
Remove the electrons BEFORE writing the configuration. Writing Al³⁺ as 2,8,3 was a recorded error — the whole point is that the outer shell has gone.
Notice how many ions end up as 2,8 — the configuration of neon. That is the “full outer shell” they were aiming for.
The proton number does NOT change when an ion forms — only the electrons.
5. Isotopes and configuration
Isotopes of an element have the SAME electronic configuration, because they have the same number of protons and therefore the same number of electrons.
This is exactly why isotopes have identical CHEMICAL properties — chemistry depends on electrons, and isotopes differ only in neutrons.
6. Mistakes that cost marks
Electrons not adding up to the proton number.
Putting more than 8 in the second or third shell.
Writing an ion’s configuration without adjusting the electrons.
Confusing group with period.
Counting all electrons for the group number instead of just the outer shell.
Going beyond element 20 where the pattern changes.
Saying isotopes have different configurations.
Frequently asked questions
How many electrons fit in each shell? 2, then 8, then 8 for the first 20 elements.
What is sodium’s electronic configuration? 2,8,1.
How do I find the group from a configuration? The number of outer-shell electrons.
How do I find the period? The number of occupied shells.
What group is 2,8,6 in? Group VI, Period 3.
Why do elements in a group behave similarly? They have the same number of outer electrons.
What is the configuration of Al³⁺? 2,8 — three electrons have been lost.
Does the proton number change when an ion forms? No — only the electrons.
Why do many ions have the configuration 2,8? They have achieved a full outer shell, like neon.
Do isotopes have the same configuration? Yes — which is why their chemistry is identical.
Quick revision checklist
- I know the shell capacities 2, 8, 8
- I can write configurations for the first 20 elements
- I check the total equals the proton number
- I can find the group from the outer electrons
- I can find the period from the number of shells
- I can go backwards from group and period to a configuration
- I know outer electrons determine chemical properties
- I can predict the ion an element forms
- I adjust the electrons before writing an ion’s configuration
- I know the proton number is unchanged in an ion
- I know isotopes share a configuration
These notes cover electronic configuration in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and data booklet for your own exam series.
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