Lesson 1 of 8
Read position from electron arrangement
Proton number orders the table; outer electrons explain recurring behaviour.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
I can order elements by proton number.Syllabus
Syllabus K326 / K328, 8.1(a). Order elements by proton number
I can relate position and electron arrangement.Syllabus
Syllabus K326 / K328, 8.1(b). Relate position and electron arrangement
I can explain group similarity.Syllabus
Syllabus K326 / K328, 8.1(c). Explain group similarity
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.
Group number and period number come straight from the electron arrangement. The number of occupied shells gives the period. The number of outer electrons gives the group, for groups 1, 2 and 13 to 17 (group 13 has 3 outer electrons, group 14 has 4, and so on; group 18 has a full outer shell). So an atom with arrangement 2,8,2 has three shells and two outer electrons: period 3, group 2, which is magnesium. Elements in the same group react in similar ways because they have the same number of outer electrons, and outer electrons are the ones lost, gained or shared in reactions.
Worked exampleLocate an element
An atom has 17 protons and electron arrangement 2,8,7. Locate it and predict its character.
Try it first. Then show the worked stepsHide the worked steps
- 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.
Check your understanding
Why do sodium and potassium have similar chemistry, even though their proton numbers differ?
Show the answer
Both have one outer-shell electron, which they lose in reactions.
Elements in the same group have the same number of outer electrons, so they react in similar ways. Potassium has one more occupied shell, which helps explain why it is more reactive.
Check your understanding
An atom has the electron arrangement 2.8.8.2. Where is it in the Periodic Table?
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Period 4, Group 2
Four occupied shells put it in Period 4. Two outer electrons put it in Group 2. It is calcium, proton number 20.