Top of the Bench 2021

Their hydroxides are alkalis

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

为什么Drop 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

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

为什么Seven 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

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

为什么A 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

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

为什么Columns 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

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

为什么Group 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

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

为什么Three 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.

6

提示Work out how many of the bracketed groups there are, then how much each one brings.

为什么The 2 outside the bracket doubles the whole NO3 group, so one unit holds 1 calcium, 2 nitrogen and 6 oxygen. Mg(OH)2 works the same way, and in Al2(SO4)3 the 3 outside makes 12 oxygens.

Lithium

提示The lightest metal of all, at the top of group 1.

为什么Its 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

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

为什么Iron 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

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

为什么Yellow-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

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

为什么Chlorine-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

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

为什么Every metal conducts electricity and almost no non-metal does, so a bulb, a battery and two crocodile clips are the quickest first test. But one property is never proof: graphite is a shiny grey solid with a very high melting point that conducts too, yet it burns to an acidic gas and is a non-metal. Confirm with a second property — metals are dense, hammerable (malleable) and form basic oxides, while non-metals are brittle and form acidic ones. Given a data table, high density and malleability point the same way.