Cambridge O Level Chemistry · Syllabus 5070 · The Periodic Table
Group I
What is Group I?
The first vertical column of the Periodic Table, whose elements are the alkali metals. Each atom has one electron in its outer shell, so each forms an ion with a charge of one plus. Lithium, sodium and potassium are the three required by the syllabus; all three are relatively soft metals.
This definition is part of the The Periodic Table chapter in Cambridge O Level Chemistry.
Group I 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.
Down Group I the alkali metals become more reactive and lower-melting as the single outer electron is lost more easily, while down Group VII the halogens become less reactive as it becomes harder to gain the extra electron needed to fill the outer shell. A more reactive halogen displaces the halide ion of a less reactive one from solution. Transition elements, between Groups II and III, show variable oxidation numbers rather than one fixed charge, and Group VIII noble gases are unreactive because their outer shell is already full.
Common mistakes with Group I
- 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.
- 4 · Treating the density trend as exact Defect Writing that density increases down Group I as though every step were guaranteed. Fix The syllabus word is generally. Density generally increases, and the data has a wobble in it — potassium is slightly less dense than sodium. Keep the word “generally” and the statement is safe; drop it and the statement is falsifiable from a data table the examiner may well print.
- 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 7 · “Density increases down Group I.” Defect A missing qualifier turns a true statement into a false one. Chemistry The data is not perfectly regular: potassium \((0.86\;\mathrm{g/cm^3})\) is slightly less dense than sodium \((0.97\;\mathrm{g/cm^3})\). The syllabus wording uses generally for exactly this reason. Repair Insert one word: density generally increases down Group I. Transfer Which of the three trends carries a qualifier? Density.
- Error 8 · “Melting point increases down Group I.” Defect Direction reversed. Chemistry The data runs \(181\,{}^\circ\mathrm{C}\), \(98\,{}^\circ\mathrm{C}\), \(63\,{}^\circ\mathrm{C}\) for lithium, sodium and potassium — falling at every step. Repair Melting point decreases down Group I. Transfer Rubidium is below potassium. Higher or lower melting point? Lower.
- Error 9 · “Reactivity decreases down Group I, like Group VII.” Defect One group's trend copied onto the other. Chemistry The two run in opposite senses. Group I reactivity increases down the group; Group VII reactivity decreases. Repair Learn them as a contrasting pair, never separately. Transfer Which is more reactive, potassium or lithium? Potassium. Chlorine or iodine? Chlorine.
Questions students ask about Group I
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.