Of these gases, which has the fastest-moving molecules (on average) at a given temperature?

I believe we are choosing between HBr, NO2, and C2H6.

The molar masses of these compound are:

Molar mass of HBr = 80.9119 
Molar mass of NO2 = 46.005 
Molar mass of C2H6 = 30.0694 

The speed of the molecule actually is inversely proportional to its molar mass. Hence, the lighter the molecule, the faster it moves.

mark him as !

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.