The periodic table

Metalloids, or semi-metals

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

为什么Boron, 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

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

为什么Metals 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

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.