When an element exists in two or more forms in the same physical state, it exhibits

Step 1: Analyze the question The question asks for the term used when one element exists in two or more different forms in the same physical state (e.g., all solids).

Step 2: Explain the concept This is allotropy. Allotropes are different structural forms of the same element, in the same physical state, with different physical properties and often similar chemical properties.

Examples:

• Carbon: diamond, graphite, fullerene (all solids)
• Oxygen: O₂ and ozone (O₃), both gases
• Sulfur: rhombic and monoclinic sulfur (both solids)
• Phosphorus: red and white phosphorus

Step 3: Check each option

• A. Isotopy: Atoms of the same element with the same number of protons but different numbers of neutrons (e.g., ¹²C and ¹⁴C). This concerns atomic structure, not different structural forms of the element.
• B. Allotropy: Same element, different forms, same physical state. This matches the question.
• C. Isobars: Atoms of different elements with the same mass number but different atomic numbers (e.g., ⁴⁰Ar and ⁴⁰Ca).
• D. Isomerism: Compounds, mainly organic, with the same molecular formula but different structural arrangements (e.g., butane and methylbutane). It applies to compounds, not elements.

Step 4: Answer The correct option is B. allotropy.

Tip: "Allo" means "other," so allotropy means "another form." Link it to carbon: diamond and graphite. Also remember that isotopes differ in neutrons, isobars share mass number, and isomers are compounds with the same formula.