Question

Atomicity is given by:

• A. Mass/molecular mass
• B. Mass of the element
• C. Molecular mass X atomic mass
• D. Molecular mass / atomic mass

Hint:

Atomicity is calculated as the molecular mass of the substance divided by the atomic mass of the element.

Answer 1

The Correct Option is D

Step 1: What the question is asking.
We need to find which expression correctly calculates atomicity, the number of atoms present in one molecule of an element.

Step 2: Set up the right formula.
Since one molecule is made of a whole number of identical atoms, dividing the mass of one whole molecule (the molecular mass) by the mass of one single atom (the atomic mass) should give exactly that whole number.
\[ \text{Atomicity} = \frac{\text{Molecular mass}}{\text{Atomic mass}} \]

Step 3: Test it on nitrogen gas.
Nitrogen exists as N2, with a molecular mass of about 28 and an atomic mass of about 14.
\[ \text{Atomicity} = \frac{28}{14} = 2 \]
This matches the known fact that nitrogen gas has two atoms per molecule.

Step 4: Final answer.
The formula that correctly gives atomicity is molecular mass divided by atomic mass.
\[ \boxed{\text{Atomicity} = \dfrac{\text{Molecular mass}}{\text{Atomic mass}}} \]

Answer 2

A good way to check a formula is to see if it still makes sense in the simplest possible case, an element that exists as single free atoms, like neon.

1. Mass/molecular mass: For neon, the mass of a sample divided by its molecular mass just gives the number of moles present in that sample, a quantity that depends on how much neon you happen to have, not a fixed atomicity of 1.
2. Mass of the element: The mass of the element on its own carries no reference to a molecule at all, so it cannot tell us that neon's atomicity is 1 rather than some other number.
3. Molecular mass X atomic mass: For neon, both the molecular mass and atomic mass are about 20, since neon molecules are single atoms. Multiplying these gives about 400, nowhere near the correct atomicity of 1.
4. Molecular mass / atomic mass: For neon, the molecular mass and atomic mass are the same number, about 20, since one molecule is just one atom. Dividing them gives 20 divided by 20, which equals 1, correctly matching neon's atomicity of 1.

Only the option "molecular mass divided by atomic mass" survives this simplest possible check, giving the correct atomicity of 1 for a single-atom element like neon, while every other option gives a number that has nothing to do with atomicity.

Therefore, the correct answer is molecular mass divided by atomic mass.