State Mendeleev's periodic law. Describe his periodic table and give two merits and two defects.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Law. The properties of elements are a periodic function of their atomic masses (1869).
2 · Arrangement. Elements are in order of increasing atomic mass. Rows are periods; columns are groups I to VIII with A and B subgroups, plus a zero group for the noble gases.
3 · Merit: predictions. Gaps were left for undiscovered elements; eka-silicon was found as germanium (72.6), as predicted.
4 · Merit: corrections. The atomic masses of beryllium, indium and gold were corrected from their positions.
5 · Defect: anomalous pairs. Cobalt (58.9) is placed before nickel (58.7).
6 · Defect: isotopes and hydrogen. There are no separate places for isotopes, and hydrogen's position is unclear.
7 · Close. These defects were removed when atomic number replaced atomic mass.
Scoring tip: Give a named example for every merit and defect; a bare list earns about half the marks.
