Concepts explained
Why Does a Pattern Appear When You Line Up the Elements?
The kinds of grain were not simply many. Line them up by weight and the properties repeat every eight steps. One man trusted that rhythm enough to leave gaps — and the gaps were filled within fifteen years.
Lined up by weight
Several people tried the same thing: lining the up from lightest to heaviest. Something odd appeared. The properties repeated.
The third element explodes in water. Eight steps on, the eleventh explodes in water. Eight more, the nineteenth, the same. Like the chorus of a song, something similar came back at regular steps.
Tool From the Atom to the Periodic Table Blow an atom up to a football pitch and zoom to the grain of rice at its centre, change protons, neutrons and electrons one at a time, count each element's hands to see why water is H2O, then fold the line into the table and sing through all 118. (Korean only)The places that did not fit
There was a problem. At certain places the chorus slipped: the property due at the eighth step arrived at the ninth. Others stopped there, or forced the pieces to fit.
A Russian teacher did something else. He left the awkward places empty. The song was not wrong; an element simply had not been found yet. Then he went further and wrote down, in advance, how heavy each missing element would be and how it would behave.
Within fifteen years three were found, close to what he had written. Once, the discoverer's measurement disagreed with the prediction; he wrote asking him to measure again. On the second try the prediction was right.
A little further in
Why eight steps?
He did not know why the repeating happened. The reason came fifty years later. Piece two ended by noting there are smaller things inside the atom; the electrons among them sit on the outside in layers, and each layer holds a fixed number. The first holds two, the next eight.
Chemistry is the work of the outermost electrons. Elements with one electron in the outer layer give it up easily and explode in water; elements whose outer layer is full have nothing to give or take and mix with no one. Every eight steps the outer layer empties and fills again, so the properties come round every eight steps.
The second board of the Elements tool shows exactly this. Raise the atomic number and electrons take seats one by one; when a layer fills, the row changes. That is where the odd numbers 2·8·8·18 come from.
Outer electrons 'give and take', we said. That is the next piece: how do atoms actually hold on to each other?
The question that remainsLeaving a gap means trusting the rule more than the facts in front of you. How do you tell when that trust is misplaced?