State the modern periodic law. Describe the long form of the periodic table in terms of groups and periods, and explain why it is better than Mendeleev's table.

1. 1 · Law. The properties of elements are a periodic function of their atomic numbers (Moseley, 1913).
2. 2 · Groups. 18 vertical columns (1 to 18); the same number of valence electrons, so similar properties and the same valency.
3. 3 · Periods. 7 horizontal rows; the same number of shells; period number = the shell being filled.
4. 4 · Period lengths.
   • Period 1: 2 elements
   • Periods 2 and 3: 8 each
   • Periods 4 and 5: 18 each
   • Periods 6 and 7: 32 each
5. 5 · Better than Mendeleev's. Isotopes share one place, and anomalous pairs such as cobalt and nickel fall in order of atomic number.
6. 6 · Reason for periodicity. Similar outer electronic configurations recur, so properties repeat.
7. 7 · Close. So the long form links each element's place to its electrons.

Scoring tip: Quote 18 groups, 7 periods and the period lengths; they are quick, sure marks.