The periodic table, Year 9

Solid

HintFollow the direction the pattern is already moving in.

The whyAstatine is so rare and so radioactive that nobody has collected enough of it to look at. Chemists predict its state from the trend in its group, which is exactly what the periodic table is for.

Below

HintEach step down the group has moved the number the same way so far.

The whyRubidium melts at about 39 °C, so it would turn liquid on a very hot day. A steady trend through three elements gives a prediction you can trust; the more elements that fit a pattern, the more confidence it deserves.

Less reactive

HintThis group runs the opposite way to the alkali metals.

The whyEvery group has its own trends. In Group 1 the metals get more reactive going down; in Group 7 the non-metals get less reactive going down. You have to know the direction for each group before you can predict with it.

group

HintLithium, sodium and potassium share one; so do the halogens.

The whyLithium, sodium and potassium all fizz in water and all form alkalis, because they share a column. Elements side by side in a row can be as different as a metal and a gas.

In order of atomic weight

HintHe used the one number that chemists of his day could measure for each element's atoms.

The whyAtomic weight, now called relative atomic mass, says how heavy an element's atoms are compared with others. When Mendeleev listed the elements from lightest to heaviest, similar ones turned up at regular intervals.

Germanium

HintThe country is in central Europe, and its Latin name begins with G.

The whyMendeleev predicted its atomic weight, its density and the formula of its oxide fifteen years before it was found. The match was so close that few chemists doubted his table afterwards.

It had correctly predicted something nobody yet knew

HintCompare explaining facts already in hand with forecasting a fact still to be found.

The whyAny tidy arrangement can be made to fit the facts already known. A scientific idea earns real trust when it risks a prediction that could have turned out wrong, and the prediction then comes true.

properties

HintThink of how an element looks, behaves and reacts.

The whyTellurium is slightly heavier than iodine, but iodine plainly belongs with chlorine and bromine, so he placed tellurium first. He trusted the pattern of behaviour over the weights, and later discoveries showed he was right to.

Metals

HintPicture how much of the table lies to the left of the staircase line.

The whyAbout three-quarters of the elements are metals. The non-metals are squeezed into the top right-hand corner, even though they include most of the elements that make up the air and living things.

Magnesium

HintDecide which side of the staircase line each one falls on.

The whyElements on the left of a period are metals, and metals conduct electricity. Sulfur lies to the right of the dividing line, so it is a non-metal: a brittle yellow solid that does not conduct.

Silicon

HintIt is used to make computer chips, and a famous valley in California is named after it.

The whyElements on the line are hard to classify. This one is shiny like a metal but brittle like a non-metal, and it conducts electricity only a little, which is exactly what makes it useful in electronics.

Acidic

HintThink what coal smoke containing this oxide does to rainwater.

The whyThe oxides of non-metals are acidic, while the oxides of metals are bases. So the change across a period, from metal to non-metal, shows up in the chemistry of the oxides as well as in how the elements look.