Chemistry · Periodic Classification Of Properties · NEET
There are four main merits. 1) He left EMPTY GAPS for elements not yet discovered and predicted their properties. 2) His predictions came true - eka-aluminium turned out to be gallium and eka-silicon turned out to be germanium, with almost the exact properties he wrote. 3) He CORRECTED wrong atomic weights of some elements (like beryllium, indium and gold) so they fit their group. 4) Noble gases (discovered later) fit neatly into a new group without breaking the table. For NEET, remember: prediction + correction = his two strongest merits.
The four common defects are: 1) ISOTOPES - atoms of the same element with different atomic weights had no separate place, but Mendeleev's law is based on atomic weight, so this was a contradiction. 2) HYDROGEN - it resembles both Group I (alkali metals) and Group VII (halogens), so its position was not fixed. 3) ANOMALOUS PAIRS - a few pairs like Ar-K, Co-Ni and Te-I were placed against the order of increasing atomic weight to keep similar elements together. 4) No clear reason was given for placing chemically DIFFERENT elements (like coinage metals Cu, Ag, Au and alkali metals Na, K) in the same group. For NEET, isotopes and anomalous pairs are the most-asked defects.
Mendeleev's periodic law says properties are a periodic function of ATOMIC WEIGHT. But atomic weight is NOT the true fundamental property of an element - atomic number is. Because of this weight-based rule, isotopes (same element, different weight) could not be separated, and some heavier elements had to be placed BEFORE lighter ones to match properties. Moseley later showed atomic number is the real basis, which fixed all these issues in the modern table.
He arranged known elements by atomic weight and grouped ones with similar properties. When a spot did not have a matching element, instead of forcing a wrong element into it, he LEFT THE GAP EMPTY and named the missing element 'eka' + the element above it (eka-aluminium sat below aluminium). Using the properties of nearby elements, he estimated the missing element's atomic weight, density and formula of its oxide. When gallium was found, its real properties matched his eka-aluminium prediction almost perfectly. This boldness made his table famous.
Isotopes are atoms of the SAME element with DIFFERENT atomic weights (for example, chlorine-35 and chlorine-37). Mendeleev's table places elements strictly by atomic weight. If we followed the rule strictly, the two isotopes would go into two different positions - but they are the same element with the same chemistry. So the table had no way to give isotopes one shared place. This is a clear defect caused by using atomic weight instead of atomic number.
Hydrogen has one electron, like alkali metals (Group I), so it can lose an electron and form H+ like Na+. But it also needs one more electron to fill its shell, like halogens (Group VII), and forms H- (hydride). Because it behaves like BOTH families, Mendeleev could not give it one fixed position. This 'anomalous position of hydrogen' is still discussed today and is a favourite NEET point.
Three classic pairs: Argon-Potassium (Ar = 39.9 is heavier than K = 39.1, but Ar comes first), Cobalt-Nickel (Co = 58.9 is heavier than Ni = 58.7, but Co comes first), and Tellurium-Iodine (Te = 127.6 is heavier than I = 126.9, but Te comes first). Mendeleev placed the heavier element first so each element sat with its own chemical family. These are called anomalous pairs and are covered in detail on the next concept page.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Atomic weight. His law states that properties of elements are a periodic function of their atomic weights. The modern periodic law later changed this to atomic number.
A merit is what the table did well - leaving gaps, predicting elements, correcting atomic weights, fitting noble gases. A defect is a failure - no place for isotopes, unclear hydrogen position, and anomalous pairs placed against weight order.
No. When he made his table, the internal structure of the atom was unknown. Atomic number was discovered later by Moseley in 1913, which fixed the defects of Mendeleev's table.
'Eka' is a Sanskrit word meaning 'one'. Mendeleev used it for a missing element by naming it after the element just above the gap, for example eka-aluminium (later found to be gallium) and eka-silicon (later found to be germanium).
It is usually placed in Group 1, but it is still called anomalous because it also shows halogen-like behaviour. This unresolved position started as a defect in Mendeleev's table.
