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

• Option 1: Mass/molecular mass - This is incorrect. Atomicity is not directly calculated by dividing the mass of an element by its molecular mass. 
• Option 2: Mass of the element - This is incorrect. The atomicity is not calculated based on the mass of the element alone. Atomicity refers to the number of atoms in one molecule of the element, not its mass.
• Option 3: Molecular mass X atomic mass - This is incorrect. Atomicity is not calculated by multiplying molecular mass and atomic mass. The calculation involves dividing molecular mass by atomic mass.
• Option 4: Molecular mass / atomic mass - This is the correct answer. Atomicity is calculated by dividing the molecular mass of a compound by the atomic mass of its constituent element. This gives the number of atoms in one molecule of the element.

Explanation:

Atomicity refers to the number of atoms in one molecule of an element. It is calculated using the formula:

Atomicity = Molecular mass / Atomic mass

For example, in the case of oxygen (O₂), the molecular mass is approximately 32 g/mol, and the atomic mass of oxygen is approximately 16 g/mol. Therefore, the atomicity of oxygen is:

Atomicity = 32 / 16 = 2

This shows that one molecule of oxygen contains two oxygen atoms, so the atomicity of oxygen is 2.

Answer 2

Atomicity is just a count, the number of atoms in one molecule, so it must come out as a plain number with no units attached. Checking the units of each option shows which formula can actually produce a pure count.

1. Mass/molecular mass: This divides a sample's mass (in grams) by the molecular mass (in grams per mole), which gives the number of moles, not the number of atoms in a molecule. The units left over are moles, not a plain count, so this cannot be atomicity.
2. Mass of the element: This is just a mass, measured in grams or atomic mass units. A mass by itself carries a unit and tells us nothing about how many atoms are bonded together, so it cannot represent atomicity.
3. Molecular mass X atomic mass: Multiplying two mass-type quantities together gives a result with units of mass squared, which is not even a physically sensible quantity here, let alone a plain count of atoms.
4. Molecular mass / atomic mass: Both molecular mass and atomic mass are measured in the same units (grams per mole), so dividing one by the other cancels the units completely and leaves a pure number. Testing this on sulfur, whose stable form is S8: the molecular mass of S8 is about 256, and the atomic mass of sulfur is about 32, so 256 divided by 32 gives 8, exactly matching the known atomicity of sulfur.

Only "molecular mass divided by atomic mass" produces a unit-free number, and it correctly reproduces the known atomicity of sulfur (8) when tested.

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