Atoms, elements and compounds, Year 9

NaCl

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

为什么There 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

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

为什么A 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

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

为什么Prefixes 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

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

为什么A 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

提示Fibreglass and reinforced concrete are both examples.

为什么Each 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

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

为什么Ceramics 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

提示Compare the temperature each solid can take before it gives way.

为什么Steel 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

提示It is the metal used for bridge girders and car bodies.

为什么The 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

提示Think what a cyclist wants from a frame when riding up a hill.

为什么Steel 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

提示Not air: air is itself made of particles.

为什么Air 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

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

为什么The 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

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

为什么A 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

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

为什么Squashing 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

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

为什么Particles 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.