The periodic table, Year 9

Solid

提示Follow the direction the pattern is already moving in.

为什么Astatine 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

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

为什么Rubidium 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

提示This group runs the opposite way to the alkali metals.

为什么Every 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

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

为什么Lithium, 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

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

为什么Atomic 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

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

为什么Mendeleev 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

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

为什么Any 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

提示Think of how an element looks, behaves and reacts.

为什么Tellurium 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

提示Picture how much of the table lies to the left of the staircase line.

为什么About 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

提示Decide which side of the staircase line each one falls on.

为什么Elements 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

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

为什么Elements 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

提示Think what coal smoke containing this oxide does to rainwater.

为什么The 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.