Where is an electron located?

There is no certain place but the place(s) which has the highest probability to contain electrons is called the orbital(s) of a certain atom/ion.

Though it is believed that electrons are located in some certain places around an atom earlier it is now believed that electron has no certain place as an electron has the qualities of both a particle and that of a wave. So it is said now that electrons are found in the cloud around the nucleus (in the orbitals)

There are four main types of orbits - s,p,d,f

By using these orbitals  for each atom is formed under the principal of Aufbau.

As Hydrogen contains only 1 electron, that electron can be found in the s orbital. But as the number of electrons grow: - more than 2 --- electrons starts filling the p orbital ( eg: Oxygen, Lithium, Boron etc.) - more than 20 --- electrons starts filling the d orbital ( eg: Zinc, Copper etc.) - more than 57 --- electrons starts filling the f orbital ( eg: Mercury, Polonium etc.)

(You can clearly understand this with electron configurations of atoms) This is clearly shown in attached below :

Hope this helped!