Atoms, elements and compounds, Year 9

NaCl

HintThe metal takes its symbol from its Latin name, and the two elements join one to one.

The whyThere is one sodium for every chlorine, so no small numbers are needed. A capital letter starts each new symbol, which is how you can tell the formula holds two elements, Na and Cl, and not four.

CO2

HintWrite each element's symbol, then put a number after the one there is more than one of.

The whyA symbol with no number after it stands for a single atom. On paper the 2 is written small and low, straight after the O it counts; typed on one line it sits in the same place.

SO3

HintThe prefix in front of "oxide" is the one that starts "triangle" and "tricycle".

The whyPrefixes in a name count atoms: mono- is one, di- is two and tri- is three. So this compound has three oxygen atoms for each sulfur, just as carbon dioxide has two oxygens for each carbon.

The metal's

HintThe formula keeps the same order as the spoken name: sodium chloride, magnesium oxide.

The whyA formula is written in the same order as the name, and the name puts the metal first. That is why NaCl reads as sodium chloride, and why you never see it written ClNa.

A composite

HintFibreglass and reinforced concrete are both examples.

The whyEach part does a job the other cannot. In fibreglass, thin glass fibres give strength, while the plastic around them holds the fibres in place and stops them snapping.

They are brittle

HintIt is the opposite of being able to bend or change shape under a blow.

The whyCeramics are hard, so they resist scratches and wear, but they cannot bend or stretch to absorb a sudden knock. A crack runs straight through instead. Hard and brittle often go together.

They have a much higher melting point than steel

HintCompare the temperature each solid can take before it gives way.

The whySteel melts at around 1,500 °C, but the ceramics used for furnace linings stay solid well beyond that. They are also poor conductors of heat, which helps to keep the heat inside the furnace.

Steel

HintIt is the metal used for bridge girders and car bodies.

The whyThe two materials cover each other's weakness: concrete resists being squashed and the rods resist being stretched. Together they make a composite that can span a wide gap, which plain concrete could not.

light

HintThink what a cyclist wants from a frame when riding up a hill.

The whySteel is strong but heavy; plastic weighs little but is weak. Carbon fibres set in a plastic resin give a material that is strong for a fraction of the mass of metal.

Nothing at all

HintNot air: air is itself made of particles.

The whyAir is made of particles too, so it cannot be what fills the gaps. In the model there is only empty space between one particle and the next, which is why a gas can be squashed into a smaller volume.

The forces between the particles

HintSomething must be pulling neighbouring balls towards each other, or the solid would fall apart.

The whyThe simple model draws particles as plain balls and leaves out the attractions between them. Those attractions matter: they are stronger in some substances than in others, which is why different substances melt and boil at different temperatures.

It is simple and still explains a great deal

HintThink why a tube map is useful even though it is not a true picture of London.

The whyA model is judged by how useful it is, not by whether it is a perfect copy. Plain balls are enough to account for melting, boiling and diffusion, so a more detailed picture is saved for the questions that need it.

Why a gas can be squashed

HintOne of the two needs you to know how strongly the balls of each substance attract one another.

The whySquashing a gas needs only balls and gaps. Different melting points need something the simple model leaves out: how strongly the particles of each substance pull on one another. Knowing which questions a model can answer is part of using it well.

A scientific model

HintIn everyday speech a globe is one, and so is a toy aeroplane.

The whyParticles are far too small to see, so chemists picture them as tiny balls. The picture is useful because it explains what we observe, and it is improved or replaced when it stops doing so.