Chemistry
(a) Explain the terms 'valency' and 'variable valency'.
(b) How are the elements with variable valency named ? Explain with an example.
Answer
(a) Valency is the combining capacity of an atom or of a radical. The valency of an element or of a radical is the number of hydrogen atoms that will combine with or displace one atom of that element or radical.
Variable valency is the ability of certain elements to have more than one valency or different combining capacities. An atom of an element can sometimes lose more electrons than are present in its valence shell, i.e., there is a loss of electrons from the penultimate shell too. Such element is said to exhibit variable valency.
(b) If an element exhibits two different positive valencies, then the suffix "ous" is used for the lower valency and the suffix "ic" is used for the higher valency. Modern chemists use Roman numerals in place of these trivial names.
For example, Iron can exhibit a valency of +2 or +3. The ion with valency +2 is named as Ferrous ion (Fe2+) and with valency +3 is named as Ferric ion (Fe3+). Its oxides will be named as Ferrous oxide or Iron (II) oxide [FeO] and Ferric oxide or Iron (III) oxide [Fe2O3].
Related Questions
Assertion (A): The concentration of free radicals in the atmosphere is high during the night.
Reason (R): Free radicals are produced by the photolysis of molecules by solar radiation.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Atoms being extremely small, cannot be seen or weighed directly. But indirect methods of physics have enabled us to know the absolute mass of nearly all kinds of atoms. The mass of a hydrogen atom is found to be 1.66 × 10-24 g while that of a carbon atom is 1.9926 × 10-23 g. As these masses are too small, it is not convenient to use kilograms or grams as unit. It has, therefore, been considered appropriate to use the mass of some standard atom as a unit and then relate masses of other atoms to it.
(a) What do you understand by the statement "R.A.M. of silver is 108" ?
(b) Which element is considered as the standard for atomic masses ?
(c) What is the difference between R.A.M. and R.M.M.? Give an example to explain.
(d) What is the significance of amu or μ ?
(e) How is 1 amu related to grams ?
(f) Is the atomic mass expressed in amu the actual mass of an atom of that element ?
(g) Why is amu preferred instead of grams or kilogram for calculating the atomic mass of an atom ?
An element X forms a chloride with the formula XCl3, and an oxide with the formula X2O3. This oxide dissolves in water to form a solution that turns blue litmus red.
(a) Identify the element.
(b) Is X a metal or a non-metal ? Justify your answer.
(c) Write the chemical formula for the sulphate of element X.
A compound is found to have a mass ratio of element A to element B as 3 : 8. If the formula of the compound is AB, what is the relationship between the atomic masses of A and B.