I need to get a nice picture about how electron moves around nucleus? I find concept of probability and orbitals quite difficult to understand?
Dear Ashok, it is an experimentally established fact that the motion of electrons inside atoms requires us to speak about the probabilistic distributions - and wave functions whose absolute value has to be squared to obtain the probability density. The equation for the wave functions is known and nicely enough, its solutions - the orbitals - can be drawn. This is much more "user-friendly" a situation than in many other portions of physics in which the objects can't be even drawn.
Before quantum mechanics was born, Niels Bohr had a simpler model of the Hydrogen atom that was similar to the Solar System: the electrons were just rotating along circles. He postulated that the circumference of the circle couldn't have been arbitrary but had to satisfy a "discrete" condition. This captured many properties of the Hydrogen atom but wasn't right in details: the details require quantum mechanics and the probabilistic language.
You're not the only one who has problems with it - many people, including famous physicists (not to speak about thousands of not so famous physicists today, and many physics fans), didn't want to accept the probabilistic description as fundamental. They thought there had to exist a more "understandable" description, similar to classical physics. However, these people - including Einstein - have been proved wrong.
I find Wikipedia explanation quite explicit: an atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus.
Actually, nobody "knows" what the electron does around the nucleus, but what we know is that the absolute value of the wave function is the probability density that the electron can be found in that point of space. Plot it and you get the nice pictures of the orbitals. Otherwise, the concept of "movement" is a classical one that you cannot extend directly to small scales.
Electron around nucleus is described by complex wave function with both real and imaginary parts.The electron is no longer described as moving around the nucleus but is found with a certain probability around the nucleus as given by the Schrodinger's equation. Here probability is a measure of chance of finding an electron over a region. If one can measure (but off course no one can in real) the position of electron from the nucleus then he or she will find the electron with some probability and if one goes on to populate all the positions of electron measured over the period of time then he or she will find a shape called orbital. This will help you visualize better.