PROBLEM 15Q
Explain the difference between a Bohr orbit and a quantummechanical orbital.
Solution 15Q
The difference between a Bohr orbit and a quantummechanical orbital :
Bohr orbit 
Quantum  mechanical orbital 
Bohr orbit has got an electron orbiting in a circle around the nucleus. The electrons can only orbit stably, without radiating, in certain orbits (called by Bohr the "stationary orbits") at a certain discrete set of distances from the nucleus. It is like a planet’s path around the sun in the solar system. 
There is no single location of the electron, but instead there is a probability density function, called the wave function, which gives the chance that you will find the electron at any given point. i.e, an atomic orbital is a mathematical function that describes the wavelike behavior of either one electron or a pair of electrons in an atom.

Here, there are only discrete energy levels, corresponding to one sinusoidal wave as the electron goes around a single orbit, then two sinusoidal waves as the electron goes around a single orbit, then three and continues. 
Here, the wave function is smooth and nonzero all around the nucleus rather than just nonzero in a little ring around the nucleus. It stretches on forever, although the probability drops off drastically as the electron gets much further away from the nucleus than the radius determined by the Bohr model. There are still discrete energy states, which correspond well to the Bohr model.

It represents two dimensional motion of electrons.

It represents three dimensional motion of electrons. 
Orbitals are directional in nature except ‘s’ orbital.

Orbits are non directional 