Final answer:
Nitrogen (N2) has the lowest density at 298 K and 101.3 kPa among the given options due to its lowest molar mass.
Step-by-step explanation:
The element with the lowest density at 298 K and 101.3 kPa among argon (Ar), nitrogen (N2), fluorine (F2), and oxygen (O2) is hydrogen (H2). However, hydrogen is not one of the options provided. Since the question may contain a typo, assuming the correct answer must be chosen from the given elements, we should compare their molar masses to determine the least density, as all gases at the same temperature and pressure will occupy the same volume according to Avogadro's law.
Given molar masses are approximately: Ar = 39.95 g/mol, N2 = 28.01 g/mol, F2 = 38 g/mol, O2 = 32 g/mol. Since density is directly proportional to molar mass for gases at equal temperature and pressure, nitrogen (N2), with the lowest molar mass, will have the lowest density among the options provided.