Topic 1 of 7
Read position from electron arrangement
Proton number orders the table; outer electrons explain recurring behaviour.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Read position from electron arrangement
Proton number orders the table; outer electrons explain recurring behaviour.
The Periodic Table orders elements by increasing proton number. A period is a horizontal row; a group is a vertical column. For the first twenty elements, occupied electron shells give the period. Main-group elements in one group have the same number of outer electrons, which explains similar chemical reactions.
Worked example
Locate an element
An atom has 17 protons and electron arrangement 2,8,7. Locate it and predict its character.
- Seventeen protons identify chlorine.
- Three occupied shells put it in period 3.
- Seven outer electrons place it in group 17; it is a non-metal.
Period 3, group 17. It behaves similarly to other halogens because their outer shells have the same electron count.
Across a period, character generally changes from metallic on the left to non-metallic on the right. Metals with few outer electrons tend to lose them; non-metals with nearly full outer shells tend to gain or share electrons. Group 18 has full outer shells, including helium with two rather than eight.
| Outer electrons | Typical change | Examples |
|---|---|---|
| 1 or 2 | Lose 1 or 2 | Na+, K+; Mg2+, Ca2+ |
| 3 in aluminium | Lose 3 | Al3+ |
| 5, 6 or 7 in suitable non-metals | Gain 3, 2 or 1 | N3-; O2-, S2-; F-, Cl- |
| Full shell | No usual simple-ion formation | He, Ne, Ar |