Collision with a stationary Hydrogen atom Can some explain me what happens when an electron or neutron collides with stationary Hydrogen atom?
What are conditions for elastic and inelastic condition?
For example, how to tackle following problem- 
A neutron collides head on with a stationary hydrogen atom in ground state.
(A) If kinetic energy of the neutron is less than 13.6eV, collision must be elastic
(B) if kinetic energy of the neutron is less than 13.6eV, collision may be inelastic.
(C) inelastic collision takes place when initial kinetic energy of neutron is greater than 13.6eV.
(D) perfectly inelastic collision cannot take place.
This theory is not given in my book. Explain me or link me to some source so that I can read it myself.
 A: You can only have an inelastic collision between two bodies if one or both of the bodies have some internal degrees of freedom that can absorb energy.
For example if you have a rigid sphere then the only type of energy it can possess is kinetic energy. If we collide two rigid spheres then conservation of energy means the sum of the kinetic energies before must be equal to the sum of the kinetic energies afterwards because there isn't anywhere else for the energy to go.
At everyday energies (i.e. a few electron volts) the neutron behaves like the aforementioned rigid sphere and the only type of energy it can have is kinetic. So in these circumstances a collision between two neutrons would always be elastic.
However a hydrogen atom is a composite object containing a proton and an electron, and it does have internal degrees of freedom because it's possible to excite the electron into a higher energy state. So if a neutron collides with a hydrogen atom it's possible for some of the original kinetic energy to go into exciting the electron. In that case the sum of kinetic energy after the collision is less than the sum of the kinetic energy before the collision and we have an inelastic collision. The missing energy has been used in exciting the electron and is released when the electron decays back into the ground state and emits a photon.
In this case, to answer the question you need to think about how the electron can absorb energy. The figure of 13.6 eV is mentioned because it's the ionisation energy of hydrogen i.e. the energy needed to knock the electron completely out of the hydrogen atom. But you need to think about whether this is the minimum amount of energy that the electron could absorb.
