Which of the following gases has a density of 0.715g/L at STP
a) He
b) CO2
c) CO
d) CH4

CO2 is the gas which has high density of 0.715/L at STP.

Explanation:

PV=nRT

at STP,

P,T=constant

V=Km/M

1/K=m/V*1/M=S*1/M

therefore,

The molar mass is more, as well as the density is also greater

Hence Only CO2 has greater mass and greater density.

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.