Q.What is the correlation between ionization energies and reactivity of elements?

Group 18, the noble gases (He, Ne, Ar, Kr, Xe, Rn), has the stable, completely filled valence-shell configuration ns2np6 (helium's is 1s2), which gives the group its characteristic chemical inertness -- they have no tendency to gain, lose or share electrons under ordinary conditions, and so are normally zero-valent, monoatomic gases. All except radon occur naturally in the atmosphere (about 1% by volume of dry air, mostly argon); helium is obtained commercially from natural gas, and helium/neon are also found trapped in radioactive minerals such as pitchblende and monazite; xenon and radon are the rarest members, radon being a decay product of radium-226. Physically, noble gases are monoatomic, sparingly soluble in water, and have very low melting/boiling points, since the only force between their atoms is the weak van der Waals force -- helium has the lowest boiling point (4.2 K) of any known substance. Down the group, atomic radius and ionisation enthalpy both follow the same trends as other groups (radius increases, ionisation enthalpy decreases), and it is this falling ionisation enthalpy that lets the heavier noble gases (Kr, Xe, and in principle Rn) be induced to form real compounds, unlike He, Ne and Ar, which remain entirely unreactive. Neon, helium and argon each have distinct practical uses -- from welding atmospheres and MRI cooling (helium) to discharge-lamp signage (neon) to filament-life-extending bulb fills (argon) -- described fully alongside xenon's chemistry.