Groups and trends in the periodic table
Chemistry · Key Stage 3 · The School
Groups and periods
A GROUP is a column and a PERIOD is a row. Elements in the same group behave alike because they have the same number of electrons in their outer shell — Group 1 has one, Group 7 has seven, Group 0 has a full shell. The table is arranged so that the pattern of behaviour is the pattern of the layout.
Down Group 1 and Group 7
Down GROUP 1, the alkali metals get MORE reactive: lithium fizzes on water, sodium darts about, potassium ignites. Down GROUP 7, the halogens get LESS reactive: fluorine is ferocious, iodine is sluggish. The trends run in opposite directions, and knowing which is which is worth more than any single fact about one element.
Group 0
The NOBLE GASES in Group 0 have a full outer shell already, so they have no tendency to gain, lose or share electrons and are almost entirely unreactive. That is why helium fills balloons rather than hydrogen, and why argon fills light bulbs — an unreactive atmosphere protects the filament.
GROUP 1: lithium fizzes on water, sodium darts about and may ignite, potassium bursts into lilac flame. Reactivity RISES down the group, because the single outer electron is further from the nucleus and easier to lose. GROUP 7: fluorine is ferocious, chlorine strong, bromine less so, iodine sluggish. Reactivity FALLS down, because a bigger atom attracts an incoming electron less well. Losing versus gaining — that is the whole difference.