Mendeleev's periodic law and table: merits and limitations
Mendeleev (1869) arranged the 63 known elements by atomic mass in rows and columns. He left gaps and predicted new elements, but some elements did not fit.
Mendeleev's law and how he built his table
Dmitri Mendeleev, a Russian chemist, studied all the 63 elements known in his time and their compounds. He arranged them in order of increasing atomic mass, and saw similar properties come back again and again. In 1869 he stated his periodic law: the properties of elements are a periodic function of their atomic masses. 'Periodic' means repeating at regular intervals. He filled a row until he reached an element like the first one, then began a new row below it. Above all, he trusted properties over mass. Where no known element fitted a place, he left it empty. Where needed, he even swapped the order of two elements.
Main features of Mendeleev's table
- The horizontal rows are periods. Along a period, properties change step by step. Your NIOS lesson says six periods in one place and seven in its summary; the last row was only partly filled in Mendeleev's time.
- The vertical columns are groups, numbered I to VIII in Roman numerals.
- Groups I to VII are split into subgroups A and B. Group VIII holds sets of three elements, such as iron, cobalt and nickel.
- Elements in a group are chemically similar, and their properties change gradually from top to bottom.
- A zero group was added later, when the noble gases were discovered.
Merits of Mendeleev's table
- All known elements classified: all 63 elements fitted into one table, which made their study systematic.
- Atomic masses corrected: the masses of beryllium, indium and gold were corrected from their positions. Indium was thought to have mass 76 and valency 2; Mendeleev gave it about 113 and valency 3. Today's values are 114.8 and 3.
- New elements predicted: he left gaps and predicted eka-boron (now scandium), eka-aluminium (gallium) and eka-silicon (germanium).
- Valency: the group number showed the valency, such as 1 for lithium, sodium and potassium in group I.
Eka-silicon predicted, germanium found
| Property | Mendeleev's eka-silicon | Germanium (found in 1886) |
|---|---|---|
| Atomic mass | 72 | 72.6 |
| Density (g cm⁻³) | 5.5 | 5.36 |
| Formula of the oxide | EO₂ | GeO₂ |
| Formula of the chloride | ECl₄ | GeCl₄ |
Defects of Mendeleev's table
| Defect | Example |
|---|---|
| Position of hydrogen is unclear | Hydrogen is like the alkali metals, and also like the halogens |
| No separate places for isotopes | ¹²C and ¹⁴C have different masses but share one place |
| Anomalous pairs: a heavier element before a lighter one | Cobalt (58.9) before nickel (58.7); tellurium (127.6) before iodine (126.9); argon (39.9) before potassium (39.1) |
| Unlike elements grouped together | Copper and silver placed with the alkali metals |
| Like elements kept apart | Gold and platinum placed in different groups |
Germanium, the element whose gap Mendeleev left empty, later became important in early transistors and radios. A good classification can point to discoveries before they happen.
2 marks: “Mendeleev left some places in his table empty. Why? Give one example.” He left them for elements not yet discovered, and predicted their properties. Example: eka-silicon, found later as germanium, with atomic mass 72.6 against his predicted 72.
2 marks: “Why is placing cobalt before nickel called an anomaly in Mendeleev's table? Give one more such pair.” Mendeleev arranged elements by increasing atomic mass, yet cobalt (58.9) comes before the lighter nickel (58.7), to keep similar elements together. Another pair: argon (39.9) before potassium (39.1), or tellurium before iodine.
5 marks: “State Mendeleev's periodic law. Describe his table in terms of groups and periods, and give two merits and two defects.” Law: properties of elements are a periodic function of their atomic masses. Rows are periods; columns are groups I to VIII, split into A and B subgroups, with a zero group added for noble gases. Merits: gaps with correct predictions (eka-silicon became germanium); corrected atomic masses (indium). Defects: anomalous pairs (cobalt before nickel); no places for isotopes; hydrogen's position unclear.
Mendeleev's three Ps: Properties first, Places left empty, Predictions came true.