Name the elements whose compounds formed the basis of classification in Mendeleev's periodic table. Why did Mendeleev choose these elements ? How the formulae of these compounds had helped Mendeleev in deciding the position of an element in his periodic table ?
Generated by claude-sonnet-4-6 · 2026-06-15 06:52 · grounding rag
Model Answer
Mendeleev chose hydrogen (H) and oxygen (O) as the basis for classification because these two elements form compounds (hydrides and oxides) with most other elements.
He selected them because they are the most widely combining elements — hydrogen is the lightest and oxygen is the most abundant reactive element.
The formulae of hydrides and oxides helped Mendeleev decide the group of an element. For example, if an element X formed an oxide XO and a hydride XH₄, it was placed in the group whose general formula matched. The valency of the element, shown by these formulae, determined its position in the periodic table.
Source: Chapter 5 (Periodic Classification of Elements), Mendeleev's Periodic Table section
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Explanation
- 1 mark each for: naming the two elements (H and O), reason for choosing them, and how formulae determined position — three clear points needed.
- Examiners look for: naming both elements correctly, stating that oxides and hydrides were used, and linking the formula (valency) to group placement.
- Avoid vague statements — be specific that it was the formulae of oxides and hydrides that revealed valency and thus group number.
- Note: The source passages provided are from Chemistry Chapter 4 (Carbon), which does not cover Mendeleev. This answer is based on the correct CBSE Chapter 5 content. Always match your answer to the right chapter.