Which property can differ between allotropes due to different atomic arrangements?

• A. Mass number
• B. Atomic number
• C. Molecular identity changes
• D. Hardness and electrical conductivity

Answer: (D)

Different atomic arrangements in allotropes change how atoms bond and how electrons move, which alters physical properties that depend on bonding and electron mobility. A classic example is carbon: in diamond, each carbon forms a stiff three-dimensional network of strong covalent bonds, making it extremely hard and a poor electrical conductor because electrons are tied up in bonds. In graphite, carbon atoms arrange in planar layers with delocalized electrons within the sheets, so it conducts electricity along the layers and is much softer overall due to weaker forces between layers. Mass number and atomic number stay the same for all allotropes of an element, since the identity of the element doesn’t change, and referring to a change in molecular identity isn’t the defining feature here. Thus, hardness and electrical conductivity can differ between allotropes because of the different atomic arrangements.