MAKING ORDER OUT OF CHAOS – MENDELÉEV’S PERIODIC TABLE
- Dmitri Ivanovich Mendeleev (1834-1907, a Russian chemist) was the most important contributor to the development of the Periodic Table.
- Mendeleev’s Periodic Table proved to be the unifying principle in chemistry. It was the motivation for the discovery of new elements.
- Mendeleev started his work with 63 known elements. He examined the relationship between the atomic masses (fundamental property) of the elements and their physical & chemical properties.
- Mendeleev concentrated on the compounds (hydrides & oxides) formed by elements with oxygen & hydrogen. He selected these elements as they are very reactive and form compounds with most elements. The formulae of the hydrides & oxides were treated as a basic property.
- He wrote down the properties of each element on 63 cards and sorted them with similar properties. He found that most elements were arranged in the increasing order of atomic masses.
- There was a periodic recurrence of elements with similar physical & chemical properties. Thus, he formulated a Periodic Law. It states that ‘the properties of elements are the periodic function of their atomic masses’.
- In a few cases, Mendeleev had placed an element with slightly greater atomic mass before an element with slightly lower atomic mass. The sequence was inverted to group the elements with similar properties together. E.g., Cobalt (atomic mass 58.9) appeared before nickel (atomic mass 58.7).
MENDELEEV’S PERIODIC TABLE
- Mendeleev’s Periodic Table contains groups (vertical columns) and periods (horizontal rows).
- R = elements.
Achievements of Mendeleev’s Periodic Table
- Mendeleev left some gaps in the Periodic Table and predicted the existence of some elements.
- He named them by prefixing Eka (one) to the name of the preceding element in the same group.
- E.g., scandium, gallium & germanium, discovered later, have properties similar to Eka–boron, Eka–aluminium and Eka–silicon, respectively.
- It proves the correctness and usefulness of Mendeleev’s Periodic Table.
- Noble gases like helium (He), neon (Ne) & argon (Ar) were discovered very late because they are very inert and rarely present in the atmosphere. They could be placed in a new group without disturbing the existing order.
| Properties of Eka–aluminium and Gallium | ||
|---|---|---|
| Property | Eka-aluminium | Gallium |
| Atomic Mass | 68 | 69.7 |
| Formula of Oxide | E2O3 | Ga2O3 |
| Formula of Chloride | ECl3 | GaCl3 |
Limitations of Mendeleev’s Classification
- No fixed position for hydrogen because:
- Its electronic configuration resembles that of alkali metals. Like alkali metals, hydrogen combines with halogens, oxygen & sulphur to form compounds having similar formulae. E.g.,
Compounds of H HCl H2O H2S Compounds of Na NaCl Na2O Na2S - Like halogens, hydrogen exists as diatomic molecules and combines with metals and non-metals to form covalent compounds.
- Its electronic configuration resembles that of alkali metals. Like alkali metals, hydrogen combines with halogens, oxygen & sulphur to form compounds having similar formulae. E.g.,
- Isotopes of an element have similar chemical properties, but different atomic masses. So they cannot be placed in the same slots. E.g., Cl-35 & Cl-37 are isotopes of chlorine.
- Atomic masses do not increase in a regular manner from one element to the next. So it was not possible to predict how many elements could be discovered between two elements, especially in the case of heavier elements.
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1. Early Attempts of Classification 2. Mendeleev's Periodic Table 3. Modern Periodic Table
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