Write equations which represent the discharge at the cathode of the following ions: K^+, Pb^2+, Al^3+, and at the anode of: Br^-, O^2-, F^-.

At the cathode:

K = K^+ + 1
e^-1 (e^-1) signifies an electron

Pb = Pb^+2 + 2
e^-1

Al = Al^+3 + 3 e ^-1

At
the cathode the metal ions are being oxidized through the loss of one or more
electrons.

At the anode:

Br +
1 e^-1 = Br^-1

O + 2 e^-1 =
O^-2

F + 1 e^-1 = F^-1

At the
anode, the non-metal ions are being reduced by the gain of one or more
electrons.

These are examples of half-cell reactions.  If
you are doing an oxidation-reduction reaction the loss of electrons at one electrode has
to equal the gain of electrons at the other electrode. The metals are reducing agents
and the non-metals are oxidizing agents.

To remember this: 
"an oxidizing agent is reduced by the gain of electrons"