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You know how the table is built and how it is used. Now ask why. Why did Moseley's X-rays settle an old argument? Why does the energy to remove an electron dip twice in each period? In this course we read further, solve harder problems and take an olympiad style test.

What's inside

1. Extended reading: Moseley and the atomic number: In 1913 Moseley measured the X-rays of the elements and found a straight line. It showed that the atomic number is the true order of the elements, and that four places in the table were still empty.
2. Extended reading: ionisation energy and electronegativity: The first ionisation energy is the energy to remove the outer electron of an atom. It rises across a period, with small dips, and falls down a group. Electronegativity is the pull of an atom on the electrons of a bond. The difference in electronegativity tells the kind of bond.
3. Olympiad practice: predicting an element from its neighbours: Mendeleev predicted missing elements by averaging the four elements around a gap: above, below, left and right. For germanium the average gives the atomic mass within 0.3 percent and the density almost exactly. The method works where properties change smoothly, and fails at the edges of the table and across the line between metals and non-metals.