1. It depends on many factors, like the composition (imagine meteoric iron impacting solid rock), angle of impact, speed of impact, mass of the objects, which not only affects inertia but also the strength of their gravitational fields. Overall, its a very difficult thing to simulate and its not just a matter of a fixed amount being ejected into space every single time. 2. If we look at Earth, the most common elements are iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%) (from Wikipedia). Also hydrogen and helium are the most abundant elements in the Universe. 3. For a planet to have an atmosphere it must have a magnetic field (otherwise it will be stripped away by stellar winds) and sufficient gravity (mass) to keep a hold of it. A magnetic field requires a rotating liquid iron core, which means the planet cannot be too close to the star otherwise it will be tidally locked and rotation will be too slow. Gas giants are a little different as their magnetic fields are generated by their liquid hydrogen cores.
