The values of conductivity of some materials at 298.15 K in S m$$^{-1}$$ are $$2.1 \times 10^3$$, $$1.0 \times 10^{-16}$$, $$1.2 \times 10$$, $$3.91$$, $$1.5 \times 10^{-2}$$, $$1 \times 10^{-7}$$, $$1.0 \times 10^3$$. The number of conductors among the materials is

The electrical ability of a substance is expressed through its conductivity $$\kappa$$, measured in $$\text{S m}^{-1}$$.

Empirically the following ranges are used at 298 K:

• Good conductors (metals, concentrated electrolyte solutions): $$\kappa \ge 1\; \text{S m}^{-1}$$
• Semiconductors: $$10^{-6}\; \text{S m}^{-1} \le \kappa \lt 1\; \text{S m}^{-1}$$
• Insulators: $$\kappa \lt 10^{-10}\; \text{S m}^{-1}$$

Hence any material whose conductivity is at least $$1\; \text{S m}^{-1}$$ will be counted as a conductor.

The given conductivities are

$$2.1 \times 10^{3},\; 1.0 \times 10^{-16},\; 1.2 \times 10^{1},\; 3.91,\; 1.5 \times 10^{-2},\; 1.0 \times 10^{-7},\; 1.0 \times 10^{3}\; \text{S m}^{-1}$$

Now compare each value with $$1\; \text{S m}^{-1}$$:

• $$2.1 \times 10^{3}\; (\;2100\;) \gt 1$$ ⇒ conductor
• $$1.0 \times 10^{-16} \lt 1$$ ⇒ insulator
• $$1.2 \times 10^{1}\; (\;12\;) \gt 1$$ ⇒ conductor
• $$3.91 \gt 1$$ ⇒ conductor
• $$1.5 \times 10^{-2}\; (\;0.015\;) \lt 1$$ ⇒ semiconductor
• $$1.0 \times 10^{-7} \lt 1$$ ⇒ insulator
• $$1.0 \times 10^{3}\; (\;1000\;) \gt 1$$ ⇒ conductor

Counting the conductors: $$2100,\; 12,\; 3.91,\; 1000$$ gives a total of $$4$$ substances.

Therefore, the number of conductors among the given materials is $$\mathbf{4}$$.