Question

Atomicity of an element is:

• A. Valency of an element
• B. Atomic mass
• C. Number of atoms in one molecule of an element
• D. Isotope of an element

Hint:

The atomicity of an element indicates how many atoms of that element are bonded together in a molecule.

Answer 1

The Correct Option is C

Understanding the term:
Chemistry uses several "-icity" style words to count things attached to one unit: basicity counts the replaceable OH groups in a base, acidity counts the replaceable H atoms in an acid. Atomicity follows the same pattern, it counts atoms, specifically the number of atoms present in a single molecule of an element.

Checking the options:
Valency describes bonding capacity, not a count of atoms in a molecule, since oxygen keeps the same valency of 2 in O2 and O3 despite their different atomicities. Atomic mass is a weight per atom, unrelated to how many atoms group together. An isotope is a variant nucleus of the same element (like carbon-12 and carbon-14), which is about mass difference, not molecule size. None of these match a "count of atoms" idea.

Conclusion:
Only "number of atoms in one molecule of an element" matches the counting pattern that words like atomicity, basicity, and acidity share. For example, nitrogen gas (N2) has an atomicity of 2 because each molecule has two nitrogen atoms.
The correct answer is number of atoms in one molecule of an element.

Answer 2

Each option describes a different kind of property of an element, and sorting them by type shows which one actually matches what "atomicity" is asking for.

1. Valency of an element: This falls under bonding behavior, it tells us how many bonds an atom can typically form (like carbon's valency of 4), not how many atoms sit inside one molecule.
2. Atomic mass: This falls under weight or mass, a single number attached to one atom that stays the same no matter what molecule that atom ends up in.
3. Number of atoms in one molecule of an element: This falls under counting, it directly answers "how many atoms are grouped together in one molecule," which is exactly the question being asked when we ask for atomicity. Phosphorus forming P4 molecules, for instance, has an atomicity of 4 under this category.
4. Isotope of an element: This falls under nuclear identity, describing atoms of the same element with a different neutron count, which is about a single atom's nucleus, not a molecule made of several atoms.

Since atomicity is asking about a count of atoms making up a molecule, only the option describing a count fits, the bonding, mass, and nuclear-identity options describe entirely different kinds of properties.

Therefore, the correct answer is number of atoms in one molecule of an element.