Question

Which property of an element is not variable?
[A] Valency
[B] Atomic mass
[C] Equivalent mass
[D] None of these.

Answer 1

Hint: Valency is the number of electrons needed by the element to gain a stable configuration. Atomic mass is the mass of subatomic particles in an element. Equivalent mass is the mass of one equivalent of an element and it is dependent upon valency.

Complete step by step solution:
To solve this question firstly let us discuss the properties mentioned to us in the question.
In the first option, we have valency.
We know that in chemistry, valency is the combining power of an element with other elements which leads to the formation of chemical compounds or molecules. We determine the affinity or the combining power of an atom by the number of hydrogen atoms it combines with. For example, the valency of oxygen in a water molecule is 2 as it combines with two hydrogen atoms.
In the next option, we have atomic mass. As we can understand from the name itself, atomic mass is the mass of an atom of an element. Every element has its own unique atomic mass.
And in the last option, we have equivalent mass.
Equivalent weight is the mass of one equivalent I.e.it is the mass of a given substance which will combine or displace a fixed amount of another substance. Equivalent weight may be easily calculated from the molar mass.
Equivalent weight is given by the formula-
Equivalent weight= $\dfrac{Molecular\text{ }weight}{~valency\text{ }factor}$
The valency of an element can vary for example in transition metals like copper and iron. They have variable oxidation states. Therefore, valency is variable.
Also, the equivalent weight is dependent upon valency thus it is variable too.
The atomic mass of a compound is constant and it is unique for every element.

Therefore, the correct answer is option [B] Atomic mass.

Note: We often think that the valency and oxidation number is the same thing but it is not. Oxidation number gives us the number of electrons the atom or molecule has gained or lost. Elements in a periodic table are arranged according to their atomic masses thus give us a particular trend which makes it easier for us to study periodic table elements.
We should not be confused between the equivalent weight and the number of equivalents. Equivalent weight is the gram molecular weight of the substance divided by the valency factor of the substance.