Cambridge O Level Chemistry · Syllabus 5070 · 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 O Level Chemistry.
Period in context
The Periodic Table is an arrangement of elements in order of increasing proton number, laid out so that rows (periods) share the same number of occupied electron shells and columns (groups) share the same number of outer-shell electrons. Because chemical reactions happen at the outer shell, elements in the same group share an outer-shell arrangement and therefore share similar chemistry and form ions of the same charge — for example every Group I element has one outer electron and forms a \(+1\) ion, while every Group VII element has seven outer electrons and forms a \(-1\) ion.
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 the mark is then for recognising it. “The period tells me there are four shells, but not the group, so I cannot predict the ion” is a full-credit answer. Inventing the rest is not.
Questions students ask about Period
What is the difference between a period and a group?
A period is a horizontal row, and every element in it has the same number of occupied electron shells — that number is the period number. A group is a vertical column, and every element in it has the same number of outer-shell electrons, which is why elements in the same group share similar chemistry.
How do you find the period and group from an electronic configuration?
Count the number of entries in the configuration to get the period, and read the last entry to get the group. For \(2,8,7\), there are three entries, so the element is in Period 3, and the last entry is 7, so it is in Group VII.