21. Give reason for the following
(i) The Size of Cl^- ion is  greater than the size of a Cl atom
(ii) Argon atom is bigger than chlorine atom.
(iii) Ionization potential of the element increases across a period
​iv) Alkali metals are good reducing agents.

Cl atomic number is 17(17 electrons, 17 protons), when it gains electron then becomes Cl- having 18 electrons and 17 protons. Due to excessive of electron, electron-electron repulsion becomes dominant as compared to electron-proton attraction. Due to this repulsion Cl- size becomes larger.

As we move from left to right in in a periodic table, the atomic radius decreases. This is because (while moving from left to right) no of protons increases which results Protons pulling electrons closer to the nucleus.

Now this behaviour breaks when we reach at Noble elements (such as Ne, Ar) where the size of atoms abruptly increases.

This is because in Noble elements the last shell (and therefore all its orbitals) are completely filled. Now due to this fullness the electrons from the lower energy shells are not able to excite to the outermost higher energy shell, thus generating a repulsion between energy levels, which in turn results in increament of size of atom.

(iii) As you move across a period, the atomic radius decreases, that is, the atom is smaller.  The outer electrons are closer to the nucleus and more strongly attracted to the center.  Therefore, it becomes more difficult to remove the outermost electron.

(iv) The process of addition of electrons is called reduction and element that gives electrons(s) is called reducing agent.
Alkali metals have one electron in their valence shell. These metals participate in chemical reaction with the loss of electron to get nearest noble gas configuration. 
During reactions alkali metals redely gives their valence electron, thus these metals are considered as strong reducing agents.

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