氧分子會下降嗎?

會, 如果以大氣層大約含有 20% 的氧及 80% 的氮計, 則空氣的平均摩爾質量為 28.8 g/mol
而純氧氣的摩爾質量為 32 g/mol.
所以, 在同一溫度及氣壓下, 純氧氣的密度比空氣的密度為高, 因為它們的摩爾容積均相同.
所以氧分子會下降, 或者說氧氣會下沉.

No. Oxygen molecules would not "sink" because they possess thermal energy.

The thermal energy possessed by an oxygen atom
= (3/2).k.T
where k is the Boltzman's constant (=1.38 x 10^-23 J/K)
T is the temperature in Kelvin scale

thus, thermal energy of oxygen atom at 300 K
= (3/2).(1.38x10^-23).(300) J = 6.21 x 10^-21 J

Taking the mass of an oxygen atom to be about 2.7 x 10^-26 kg (i.e. a weight of 2.7 x 10^-25 N) , such thermal energy is sufficient for an atom to "fly" to a height of 6.21x10^-21/2.7x10^-25 m = 23 000 m or 23 km above the earth surface.

Since oxygen is heavier than air, if the oxygen is trapped inside a balloon, it will sink. So as a helium gas balloon will rise. (Helium is the lightiest gas.) But if oxgygen is left free in the environment, it will not sink. It will move freely in the environment until it is evenly spaced within the environment.