Changing the volume without changing the temperature or mass only changes the pressure. Volume and pressure are inversely proportional so halving the volume will double the pressure.
P = 1 Atm, T = 0 °C are "standard" temperature and pressure (STP). The volume of 1 mole of gas is 22.4 L under these conditions. That means the amount of hydrogen gas in the cylinder is 1 mole, so has a mass of 2.016 g.
After the volume reduction, the pressure is 2 Atm, and the mass remains 2.016 g.
Atoms form chemical bonds to achieve a full outer energy level, which is the most stable arrangement of electrons. A chemical bond is a force of attraction between atoms or ions
