A negative externality is when the…
marginal social cost > marginal private cost
The extra cost borne by society resulting from the last unit of output consumed/produced is greater than the extra cost to the individual/firm.
The socially optimum level of output (where MSB=MSC) is Q1, and price P1. The privately optimum level (where MPC=MPB) of output is Q2 and price P2.When there is a negative externality, the market produces at the privately optimum level at point X, therefore there is over-production. The shaded area represents the welfare loss and the MEC.
If a firm, a factory for example, produces electricity, they also create a negative externality which is pollution. If the firm fails to recognise and act against reducing the pollution, market failure occurs. The incentive function of price breaks down (see word of the day) because the firm is only charging consumers for the output of the good produced in the factory and not the output produced as a negative externality. Therefore the good is under-priced, over-consumed and over-produced. Externalities are a form of market failure.