See explanation below.
Work Step by Step
1 Concept : Like metal atoms, large numbers of nonmetal and metalloid atoms can form bands of Molecular Orbitals. Band theory explains differences in electrical conductivity and the effect of temperature among these three classes of substances in terms of the presence of an energy gap between their valence and conduction bands. Following are the three types of elements on this basis : Conductors : The valence and conduction bands of a conductor have no energy gap between them, so electrons flow when a tiny electrical potential difference is applied. When the temperature is raised, greater random motion of the atoms hinders electron movement: conductivity decreases when a metal is heated. Semiconductors : In a semiconductor, a small energy gap exists between the valence and conduction bands. Thermally excited electrons can cross the gap, allowing a small current to flow: in contrast to a conductor, conductivity increases when a semiconductor is heated. Insulators : In an insulator, a large energy gap exists between the bands: no current is observed even when the substance is heated. 2 (a) Graphite : Though graphite is an allotrope of Carbon, which is a non-metal, it has delocalized electrons which are free to move throughout the structure of graphite, similar to the properties of metals. Hence, graphite can be classified into the group of conductors. Ans : Conductor. 3 (b) Sulfur : Sulfur is present in group 6A of the modern periodic table. It is a non-metal. In non-metals, the energy gap between the Valence and Conduction bands is very large, making conduction of electricity nearly impossible. Hence, they are poor conductors and can thus be classified as an Insulator. Ans : Insulator. 4 (c) Platinum : Platinum is present in group 8B of the modern periodic table. It is a transition metal. Metals are generally good conductors due to the small gap between Valence and Conduction bands. Hence, it can be rightly classified as a Conductor. Ans : Conductor