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

• Option 1: Valency of an element - This is incorrect. The valency of an element refers to its ability to combine with other elements, typically based on the number of electrons in its outer shell. Atomicity is a different concept, relating to the number of atoms in a molecule of an element. 
• Option 2: Atomic mass - This is incorrect. The atomic mass refers to the mass of an atom, usually measured in atomic mass units (amu). Atomicity, on the other hand, refers to the number of atoms in one molecule of an element, not its mass.
• Option 3: Number of atoms in one molecule of an element - This is the correct answer. The atomicity of an element refers to the number of atoms that make up a single molecule of that element. For example, the atomicity of oxygen (O₂) is 2, as it consists of two oxygen atoms per molecule.
• Option 4: Isotope of an element - This is incorrect. An isotope refers to different forms of the same element that have the same number of protons but different numbers of neutrons. It is unrelated to the concept of atomicity, which deals with the number of atoms in a molecule.

Explanation:

The atomicity of an element refers to the number of atoms that make up a single molecule of that element. For example, oxygen (O₂) has an atomicity of 2 because each molecule of oxygen consists of two oxygen atoms. The atomicity helps define the molecular structure of elements, whether they exist as diatomic molecules (like O₂), tetraatomic molecules (like phosphorus, P₄), or monatomic molecules (like noble gases such as neon, Ne).

Answer 2

To work out what "atomicity" means, it helps to test each option against elements whose atomicity we already know: helium exists as single free atoms (atomicity 1), oxygen exists as O2 (atomicity 2), ozone as O3 (atomicity 3), phosphorus as P4 (atomicity 4), and sulfur as S8 (atomicity 8). A correct definition of atomicity must fit every one of these cases at once.

1. Valency of an element: Valency is fixed for a given element (oxygen's valency is 2, whichever molecule it forms), but atomicity changes from element to element and even between allotropes (O2 vs O3). Since a single fixed valency cannot explain why oxygen has atomicity 2 while sulfur has atomicity 8, valency and atomicity cannot be the same quantity.
2. Atomic mass: Atomic mass is also a fixed property of one atom (about 16 for oxygen, about 32 for sulfur) and does not depend on how many atoms group together to form a molecule. Both O2 and O3 are built from atoms of the same atomic mass, yet their atomicities (2 and 3) are different, so atomic mass cannot be what atomicity measures.
3. Number of atoms in one molecule of an element: Checking this against the examples above: He has 1 atom per molecule (atomicity 1), O2 has 2 (atomicity 2), O3 has 3 (atomicity 3), P4 has 4 (atomicity 4), and S8 has 8 (atomicity 8). Every known case matches perfectly, so this is the definition being described.
4. Isotope of an element: An isotope describes atoms of the same element with different numbers of neutrons (for example, carbon-12 and carbon-14). This is a property of the nucleus of a single atom and has nothing to do with how many atoms combine to form a molecule, so it cannot define atomicity.

Only the definition "number of atoms in one molecule of an element" stays consistent across every example, from single atoms like helium to eight-atom molecules like sulfur.

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