How is an atmosphere created, if possible?

Question

as in the earths atmosphere

Answer 1

Atmosphere is just gases trapped by gravity of a planet.  Since everything has an escape velocity that depends on the size of the planet, how much atmosphere (or gases) that a planet can hold depends on its size.  Earth could not hold any helium in its atmosphere.  

To create an atmosphere, just pump gas onto a sizable atmosphere and it will simply hang around.  Earth's atmosphere use to be mostly CO2 but plant life changed that by converting most of it to oxygen.

Answer 2

Ours was created by volcanic gasses cooling off.

Answer 3

Or it could be created by outgassing: the hot materials, in the formations of the planet, would burn or evaporate, creating as atmosphere.

Answer 4

An oxygen molecule (O2) collides with another oxygen molecule in the sky. As a result the 2 oxygen atoms are broken apart (creating O). When one of these oxygen atoms collides with another oxygen molecule (O2) it connects with it making an ozone molecule (O3).