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Periodic Classification of Elements — Science STD 10 — Question

CBSE BoardEnglish MediumSTD 10SciencePeriodic Classification of Elements3 Marks
Question
Based on the group valency of elements write the molecular formula of the following compounds giving justification for each:
  1. Oxide of first group elements.
  2. Halide of the elements of group thirteen.
  3. Compound formed when an element, A of group 2 combines with an element, B of group seventeen.
✓

Answer

i. Oxides of the first group elements have the common formula of $M _2 O$.

Example- $Na _2 O , K _2 O$. This is because, the first group elements have a common valency of 1 , and the valency of Oxygen is 2 so, to satisfy the combining capacity of Oxygen two 1st group metals are required.


ii. Halides of group 13 elements have a common formula of $MX _3$, where M -metal and X -halogen element.

Example- $AlCl _3, BF _3$. This is because the valency of group 13 elements is 3 and that of halogens is 1 so, to satisfy the combining capacity of aluminum or other group 13th elements three of halogens are required in the molecular formula.


iii. The general formula for those kinds of compounds would be $AB _2$.

Example- $MgCl _2, CaCl _2$. This is because the valency of group 2 elements is 2 and that of group 17 th elements if 1 so to satisfy the combining capacity of group 2 elements two of group 17 elements are required in the molecular formula.

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