That equal volumes of gas under the same conditions have equal numbers of molecules. this concept is a very important one, as it will be used often in future studies.

Avagadro’s law states at constant temperature and pressure, volume is directly proportional to the number of molecules of the gas.

For example  

At STP that is standard temperature and pressure at which T = 273K and P = 1 atm

Volume occupied by 1 mole ( gas molecules) of any gas is equal to 22.4L

That is

1 mole = 22.4L

2 mole = 44.8 L

3 mole = 67.2 L

So 1 mole of gas =22.4L ( gas molecules)

1 mole of gas =22.4L ( gas molecules)

1 mole of gas =22.4L ( gas molecules)

the seeing the electronic configuration we can say that the element is fluorine.

fluorine is the most electronegative element in the periodic table.

but by only seeing the configuration we cannot say about its reactivity because we don't know if its configuration after excitation or by addition of electrons.

but if not then, it is the lightest halogen and exists as a highly toxic pale yellow diatomic gas at standard conditions. as the most electronegative element, it is extremely reactive: almost all other elements, including some noble gases, form compounds with fluorine.