The periodic table

Metalloids, or semi-metals

HintThey straddle the zig-zag boundary and behave partly like each side of it.

The whyBoron, silicon, germanium, arsenic, antimony and tellurium all sit along the staircase. Silicon is shiny like a metal but brittle like a non-metal, and it conducts far better than sulfur yet far worse than copper — which is precisely what makes it a semiconductor and the material every computer chip is cut from.

Malleability

HintA blacksmith relies on it every time a hammer meets hot iron.

The whyMetals can be beaten into sheets and pulled out into wires — that second property is called ductility — because their layers of atoms slide over one another without the structure breaking apart. Solid non-metals such as sulfur are brittle and shatter under the same blow. Metals also carry heat and electricity well and are shiny when freshly cut.

Gas

HintOxygen, nitrogen and helium all belong here, and none of them can be picked up.

The whyThe solid non-metals, such as sulfur, are dull and snap easily instead of bending. Bromine is the one that is liquid at room temperature.

Brittle

HintChalk and glass both do it; a paperclip does not.

The whyPrecision matters here: glass is very hard, meaning difficult to scratch, yet it shatters under a blow because it cannot bend to absorb the force.

A group

HintElements in the same column behave like a family; the rows carry a different label.

The whyMembers of a column react in similar ways, which is why the alkali metals all behave alike. The horizontal rows are called periods.

The alkali metals

HintDrop one into water and the solution left behind turns universal indicator blue.

The whyThey are soft enough to cut with a knife and react vigorously with water, producing hydrogen and a solution of high pH — which is where the family name comes from.

It increases

HintLithium fizzes gently, sodium darts about the surface, and potassium bursts into a lilac flame.

The whyPotassium reacts far more violently with water than lithium does. The pattern runs the other way in Group 7, where reactivity fades as you go down.

The halogens

HintChlorine, bromine and iodine all belong to it, and every one of them is coloured.

The whyChlorine is a green gas, bromine an orange liquid and iodine a grey solid — one family changing steadily down the column, with reactivity falling as it goes.

They hardly react at all

HintHelium fills party balloons and argon fills light bulbs because nothing happens to either of them.

The whyArgon protects a hot filament from burning away, and helium in a balloon cannot catch fire the way hydrogen can. Being unwilling to join in is the whole point.

non-metals

HintBy the far end of the row you reach substances like chlorine and argon.

The whyThe shift is gradual, with semi-metals such as silicon sitting along the staircase line. Sodium at the start of the row is a soft metal; chlorine near the end is a choking gas.

For elements not yet discovered

HintHe trusted his pattern more than he trusted the list of substances known at the time.

The whyHe even predicted the properties that would fill each gap, and when gallium and germanium turned up they matched his description closely — which is what convinced chemists the table was right.

More violently

HintReactivity builds steadily as you travel down this particular column.

The whyThe power of the table is that a group trend lets you predict how an element behaves before anyone has handled it — the very reasoning Mendeleev used for his gaps.