The periodic table

Metals

提示Only a small cluster in the top right-hand corner is not.

为什么Around three-quarters of the known elements sit on the left and in the centre of the table. A staircase line marks the boundary where behaviour changes.

Good conductors

提示Every wire in the house has a copper core for a reason.

为什么Metals carry both electricity and heat well, which is why cables have metal cores and saucepans metal bases. They are also shiny when freshly cut and bend rather than snap.

Gas

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

为什么The 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

提示Chalk and glass both do it; a paperclip does not.

为什么Precision 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

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

为什么Members 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

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

为什么They 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

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

为什么Potassium 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

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

为什么Chlorine 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

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

为什么Argon 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

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

为什么The 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

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

为什么He 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

提示Reactivity builds steadily as you travel down this particular column.

为什么The 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.