Cambridge IGCSE Chemistry · Syllabus 0620 · The Periodic Table
Period
What is Period?
A horizontal row of the Periodic Table. All the elements in one period have the same number of occupied electron shells, and that number is the period number, so an element in Period 3 has three occupied shells.
This definition is part of the The Periodic Table chapter in Cambridge IGCSE Chemistry.
Common mistakes with Period
- 1 · Rows and columns swapped Defect Calling a vertical column a period, or a horizontal row a group. Fix Period is the horizontal row — think of a sentence running left to right and ending in a full stop. Group is the vertical column — a family standing in a line. Period number = number of occupied shells; group number = number of outer electrons.
- Error 1 · “The groups are the horizontal rows.” Defect Row and column swapped. Chemistry A group is a chemical family, and family resemblance comes from a shared outer-shell arrangement, which is what the columns share. The rows share only the number of shells. Repair Period \(=\) row. Group \(=\) column. Transfer Sodium and potassium are in the same what? Group.
- Error 3 · “The period number tells you the number of outer-shell electrons.” Defect The two readings of a configuration have been exchanged. Chemistry In \(2,8,7\) the number of entries \((3)\) is the count of occupied shells, which is the period; the last entry \((7)\) is the count of outer electrons, which is the group. Repair Period \(=\) how many numbers. Group \(=\) the last number. Transfer \(2,8,8,2\): period and group? Period 4, Group II.
Examiner tips on Period
- What the two incomplete cases are teaching. Real questions do sometimes give you less than you need, and recognising that is the answer. “The period tells me there are four shells, but not the group, so I cannot predict the ion” is a complete answer. Inventing the rest is not.