Why is the first ionization enthalpy of helium maximum among all the elements?

(i) Why is the first ionisation enthalpy of helium maximum among all the elements? (ii) Arrange thegiven compounds in terms of ascending order of oxidising property: HCl, HBr, HI, HF.

Although the ionisation enthalpy of Li is the highest among all the alkali metals, it is the strongest reducing agent in aqueous medium. Explain with reasons.

Why first ionisation enthalpy of Cr is lower than that of Zn ?

Compared to first ionization enthalpy of an atom, the second is :

The periodicity is related to the electronic configuration that is all chemical and physical properties are a manifestation of the electronic configuration of the elements.the atomic and ionic radii genreally decrease in a period from left to right as a consequence the ionization enthalpies genrally increase and electron gain enthalpies become more negative across a period in other words the ionization enthalpy of the extreme left element element in a period is the least and the electron gain enthalpy of the element on the extreme right is the highest negative.this results into highly chemical reactivity at the two extremes and the lowest in the centre similarly down the group the increase in atomic and ionic radii result in gradual decrease in ionization enthalpies and a regular decrease (with exception in some third period elements)in electron gain enthalpies in the csea of main group elements. reducing and oxidizing behaviour of the elements metallic and non metallic character of elements acidic,basic,amphoteric and neutral charcter of the oxides of the elements The correct order of the metallic character is

Explain why the first ionisation enthalpies of 5d elements are much higher as compared to those of 3d and 4d elements.

_____ is the most abundant among all the elements.