Cambridge O Level Chemistry · Syllabus 5070 · The Periodic Table
Group
What is Group?
A vertical column of the Periodic Table. Elements in the same group have the same number of electrons in their outer shell, and because chemical reactions involve the outer shell, those elements have similar chemical properties and form ions of the same charge.
This definition is part of the The Periodic Table chapter in Cambridge O Level Chemistry.
Group 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
- 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.
- 3 · “Group number = ion charge” applied everywhere Defect Deducing a \(+4\) ion for carbon, or a \(+7\) ion for chlorine, by reading the group number as the charge. Fix The relationship holds for Groups I, II and III (charge \(=\) group number, positive) and for Groups V, VI and VII (charge \(=\) group number \(-\,8\), negative). Group IV and the transition elements do not follow it, and Group VIII normally forms no ions at all.
- 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.
- 7 · Displacement run the wrong way Defect Predicting that iodine displaces chloride, usually justified by “iodine is bigger”. Fix Only a more reactive halogen displaces a less reactive halide, and reactivity decreases down Group VII, so the order is \(\mathrm{Cl_2 > Br_2 > I_2}\). Iodine is the least reactive of the three and displaces nothing from the other two. “No reaction” is a complete and correct answer worth full marks.
- 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.
- Error 4 · “Carbon is in Group IV, so it forms \(\mathrm{C^{4+}}\) ions.” Defect The group-to-charge rule applied outside its range. Chemistry Four outer electrons is equally far from losing all of them and from gaining four. Group IV elements normally share electrons instead, forming covalent bonds as in \(\mathrm{CO_2}\) and \(\mathrm{CH_4}\). Repair The rule covers Groups I–III and V–VII. Group IV is an exception, and so are the transition elements. Transfer What ion does silicon \((2,8,4)\) form in its simple compounds? None — it shares.
- Error 5 · “Iron is in the eighth column, so its ion is \(\mathrm{Fe^{8+}}\).” Defect Main-group reasoning applied to a transition element. Chemistry Transition elements show variable oxidation numbers, so no single charge follows from their position. Iron forms \(\mathrm{Fe^{2+}}\) and \(\mathrm{Fe^{3+}}\). Repair For a transition element, read the charge from the Roman numeral in the name or from the formula — never from a column count. Transfer What is the charge on the copper ion in copper(II) sulfate? \(+2\).
- Error 6 · “Group VIII elements form ions with a charge of \(8+\).” Defect The pattern continued into a group where it does not apply. Chemistry Ions form in order to reach a full outer shell. A noble gas already has one, so there is no reason to lose or gain anything. Group VIII normally forms no ions at all. Repair Group VIII: no ion. Write it as an atom — \(\mathrm{Ar}\), \(\mathrm{Ne}\). Transfer What ion does neon form? None.
- 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.
- Error 12 · “Chlorine is a green liquid; bromine is a brown gas.” Defect States shuffled between elements. Chemistry At r.t.p. the three states run gas, liquid, solid going down the group: chlorine is a pale yellow-green gas, bromine a red-brown liquid, iodine a grey-black solid. Repair Fix the order gas → liquid → solid to the order Cl → Br → I, and the rest follows. Transfer Which halogen is a liquid at r.t.p.? Bromine.
- Error 15 · “Iodine displaces chloride because iodine atoms are bigger.” Defect Displacement run backwards, on an irrelevant reason. Chemistry Displacement needs the added halogen to be the more reactive, and reactivity decreases down Group VII. Iodine is the least reactive of the three and displaces neither of the others. Atomic size is not a Topic 8 explanation. Repair Compare positions first: only a halogen above the halide can displace it. Transfer Does bromine displace chloride? No. Does bromine displace iodide? Yes.
Examiner tips on Group
- 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.
- A sentence worth memorising. “Going down the group, [property] [increases / decreases / generally increases], from [first value with unit] to [last value with unit], so [element] will be [above / below] [nearest value], probably in the region of [range].” Fill in the brackets and you have written a full-mark prediction, every time.
- Two directions, one comparison. Whenever a question asks about two halogens, there is only ever one fact in play: which is higher in the group. From it come the reactivity order, the direction of any displacement, the density comparison, and often the state. Find that one fact first and the rest of the answer writes itself.
- Marking yourself honestly. Award a mark only where your answer contains the underlined idea, not merely something adjacent to it. “It goes brown” is not “the colourless solution turns brown”. “Density increases” is not “density generally increases down the group”. The gap between those pairs is where the marks in this topic actually live.
Questions students ask about Group
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.
Why do elements in the same group have similar chemical properties?
Chemical reactions happen at the outer shell of an atom, and elements in the same group share the same number of outer-shell electrons. Lithium \((2,1)\) and sodium \((2,8,1)\) both have one outer electron, so both lose it to form a \(+1\) ion and undergo the same type of reaction, even though their inner shells and masses differ.
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.
Why does Group IV not fit the "lose or gain electrons to form an ion" pattern?
The group-to-charge rule covers Groups I–III and V–VII, but Group IV is an exception. Four outer electrons is equally far from losing all of them and from gaining four, so Group IV elements normally share electrons instead, forming covalent bonds, as in \(\mathrm{CO_2}\) and \(\mathrm{CH_4}\).
Why can't you predict a transition element's ion charge from its group position?
Transition elements show variable oxidation numbers, so no single charge follows from their position between Group II and Group III. Iron forms both \(\mathrm{Fe^{2+}}\) and \(\mathrm{Fe^{3+}}\), so the charge must be read from the Roman numeral in the compound's name or from its formula, never from a column count.
Why are the noble gases unreactive?
Noble gases already have a full outer electron shell, so there is nothing to gain by losing, gaining or sharing electrons. This is also why they exist as single, unbonded atoms rather than forming molecules, and why Group VIII elements normally form no ions at all.