Top of the Bench 2021

Their hydroxides are alkalis

HintUniversal indicator dipped in afterwards goes a colour from the top end of the chart.

The whyDrop lithium, sodium or potassium into water and you get hydrogen gas plus the metal hydroxide, which dissolves to give a solution well above pH 7. The metals themselves are not alkalis; it is what they leave behind in the water that earns the family its name. Curie’s picture: metal + water → metal hydroxide + hydrogen, every time, just faster as you go down.

Diatomic

HintHydrogen gas is H2, not H — the small number in the formula tells you how many atoms travel together.

The whySeven elements go about in pairs: hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine — H2, N2, O2, F2, Cl2, Br2, I2. The noble gases are the opposite: single atoms (monatomic). Sulfur, phosphorus and carbon are neither. A quiz that asks "which of these exist as diatomic molecules?" is testing whether you know this list.

Fluorine

HintThe halogens get less reactive going down — which one sits at the very top?

The whyA pale yellow gas so reactive it attacks glass and was not isolated until 1886, after several chemists had been poisoned trying. Group 7 runs the opposite way to group 1: the smallest halogen is the most reactive, the smallest alkali metal the least. Fluoride in toothpaste is the tamed ion, not the wild element.

A period

HintRows run left to right; the number tells you how many electron shells the atoms have.

The whyColumns are groups (elements with similar properties); rows are periods (elements with the same number of shells). Across a period the elements change from metals on the left to non-metals on the right; down a group the atoms get bigger. Sodium, magnesium, aluminium, silicon, phosphorus, sulfur, chlorine and argon share period 3.

oil

HintThe liquid in the jar is there to keep two things off the metal — think what it reacts with.

The whyGroup 1 metals tarnish within seconds of being cut, reacting with oxygen and with water vapour in the air. Storing them under a liquid that contains neither keeps them shiny. The teacher fishes the lump out with tongs, cuts it on a tile, and the fresh face goes dull while you watch — the quickest demonstration of reactivity there is.

softer

HintCaesium can be cut like butter; lithium needs a proper push on the knife.

The whyThree trends travel together down group 1: softer, lower melting point, more reactive (and, for most of the group, denser). So given a table of unnamed metals, the one with the lowest melting point and lowest hardness is the one furthest down. The halogens run the other way on both counts: reactivity falls down group 7, but melting point RISES.

K

HintIt comes from the Latin kalium, not from the English name.

The whyThe Latin-name symbols to know: K potassium, Na sodium, Fe iron, Cu copper, Ag silver, Au gold, Pb lead, Sn tin, Hg mercury. P is phosphorus, not potassium.

Lithium

HintThe lightest metal of all, at the top of group 1.

The whyIts 2019 Nobel Prize was for exactly these batteries: the metal gives up its single outer electron easily and is light, so a battery built on it stores a lot of energy for its weight. Lithium is the least dense metal — it floats on oil — and the least reactive of its group, though still lively enough to fizz on water.

Iron

HintThe metal in steel, and the one that makes blood red too.

The whyIron oxide is red-brown whether it is on a garden gate, in a rusty nail or in the dust of Mars. Rust itself is HYDRATED iron oxide — iron, oxygen and water together. A planet, a nail and a gate all go the same colour for the same reason: iron that has reacted with oxygen.

gas; liquid; solid

HintPicture the swimming-pool smell, the smoky brown bottle, and the dark crystals in a jar.

The whyYellow-green gas, orange-brown liquid, grey-black solid: the states go gas → liquid → solid down the group because the molecules get bigger and stick to each other more, so melting and boiling points rise. Iodine warmed gently skips the liquid and gives a purple vapour. Bromine is the only liquid non-metal.

Isotopes

HintChlorine-35 and chlorine-37 are two versions of one thing.

The whyChlorine-35 and chlorine-37 are both chlorine (17 protons); one has two extra neutrons. Because they are the same element they react identically — the difference is weight, which is why a periodic table shows chlorine as 35.5: an average over the natural mix. Isotope questions appear on the Older paper most years.

It conducts electricity

HintShiny is not enough — ask what a wire is made of, and why.

The whyEvery metal conducts electricity; almost no non-metal does (graphite is the famous exception). So a bulb, a battery and two crocodile clips settle it. Good heat conduction, high melting point and being hammerable (malleable) are metal properties too, but "conducts electricity" is the one a quiz accepts as the deciding test. Given a data table, a high melting point and high density point the same way.