Question

How did Heisenberg contribute to the atomic theory?

Answer 1

Heisenberg contributed through his uncertainty principal.

He stated that an electron's velocity and location can not possible be known simultaneously. The reason for this is because an electron is such a tiny tiny amount of mass, the act of observing it with any kind of light (radiation) would move the particle in a different direction/velocity.

When you want to locate your phone across a room, you look at it and light bounces off of your phone to your eye. If you look at an electron, light (photons) hit the electron, thus moving it in a different direction and speed.

The exact formula for calculating the uncertainty of an electron goes:
Δx > h/4πmΔv

Where Δx = the uncertainty
h = 6.626 x 10-34 J-s
m = mass of electron (9.109 x 10 -31 kg)
Δv = the degree of certainty you are given (e.g. "speed is known to within 0.01m/s")

Sometimes the uncertainty can be bigger than the atom itself which is why electrons have never been observed directly.

Complete presentation in simple terms here: http://chemistry.tutorvista.com/organic-chemistry/werner-heisenberg-atomic-theory.html