The periodic table: trends you must know

The Periodic Table chapter rewards understanding over memory. If you can explain why properties change across a period and down a group, you can work out most answers instead of recalling them, and the reasoning is always the same two ideas: nuclear charge (how strongly the positive nucleus pulls) and number of occupied shells (how far the outer electrons sit). Get comfortable with those, and the trends fall into place. Here are the ones SPM examines most.

First, what the table’s structure tells you

Two facts unlock everything else. The group number tells you the number of valence electrons (for Groups 1, 2 and 13–18), and elements in the same group have similar chemical properties because they have the same number of outer electrons. The period number tells you the number of occupied electron shells. So sodium, in Group 1 Period 3, has one valence electron and three occupied shells. Being fluent at reading position → electron arrangement is the foundation for every trend below.

Trend 1: atomic radius

Across a period (left to right), atomic radius decreases. The number of protons increases, so the nuclear charge gets stronger, but the electrons are being added to the same shell. The stronger pull draws the electron cloud in closer, so atoms get smaller.

Down a group, atomic radius increases. Each element down the group has one more occupied shell than the one above, so the outer electrons sit further out and the atom is larger, even though nuclear charge is also rising.

Trend 2: electronegativity

Electronegativity is the tendency of an atom to attract electrons in a bond. It follows directly from radius. Across a period it increases (higher nuclear charge, smaller atom, stronger pull on bonding electrons). Down a group it decreases (the outer shell is further from the nucleus and shielded, so the pull is weaker). This single trend explains a lot of Group 1 and Group 17 behaviour below.

Trend 3: Group 1, the alkali metals

Lithium, sodium and potassium are soft, low-density metals that react with water to give an alkali (a metal hydroxide) plus hydrogen gas. The reactivity trend is the one to lock in: reactivity increases down the group. The explanation is pure radius reasoning, as you go down, the atom is larger and the single outer electron is further from the nucleus and more shielded, so it is lost more easily, and losing that electron is what makes these metals react. Potassium therefore reacts more vigorously with water than sodium, which reacts more vigorously than lithium.

Trend 4: Group 17, the halogens

The halogens, chlorine, bromine and iodine at SPM level, show two trends worth memorising. Their physical state changes down the group: chlorine is a greenish-yellow gas, bromine a reddish-brown liquid, and iodine a purplish-black solid, with colour darkening and melting and boiling points rising as the molecules get larger. Their reactivity decreases down the group, the mirror image of Group 1. A halogen reacts by gaining one electron; going down the group the atom is larger, so the incoming electron is attracted less strongly and gained less easily. This is why chlorine can displace bromine and iodine from their salts, but not the other way round, a favourite exam application.

Trend 5: Group 18, the noble gases

Helium, neon and argon are chemically inert (unreactive), and the reason is the heart of the whole chapter: they already have a stable, full valence shell, a duplet for helium, an octet for the others. With no tendency to lose, gain or share electrons, they do not readily form compounds. Their stability is exactly what every other atom is “trying” to reach through bonding, which is why this group is worth understanding rather than just listing. Their uses follow from being unreactive: argon fills light bulbs, helium fills balloons, neon glows in signage.

Trend 6: the transition elements

Across the middle of Period 4 sit the transition elements. You do not need their individual chemistry, but you should know their characteristic properties as a group: they form coloured compounds and ions, show variable oxidation states, act as catalysts, and have high melting points and densities. A question naming these as special metal properties is really asking you to recognise the transition block.

Putting the trends to work

The examiner’s favourite move is to give you an unfamiliar element by position and ask you to predict its properties. That is answerable every time from the trends: locate the group and period, deduce the electron arrangement, and reason from nuclear charge and shells. Practising this on the interactive SPM periodic table makes position-to-property automatic, and precise wording, “reactivity”, “electronegativity”, “nuclear charge”, is worth checking in a chemistry glossary so your explanations earn full marks. Work through the whole Periodic Table chapter with the “nuclear charge and shells” lens and you will find you are reasoning, not memorising, which is exactly what this chapter is designed to reward.