As we move from left to right in a period, the outer shell remains the same, while the nuclear charge increases effectively that makes the removal of an electron difficult and hence the value of ionization energy increases.
Position of Hydrogen over VII A-Group Elements
Hydrogen has certain similarities with halogens due to this it can be placed above VII A-group elements.
But it also shows certain differences with this group. Therefore, it is difficult to decide its position in the periodic table.
Hydrogen is a gas like most of halogens.
Hydrogen exists in diatomic form (H2) like F2, Cl2, and I2.
Hydrogen requires one electron to complete its outermost shell just like halogens.
Both hydrogen and halogens form stable ionic compounds with alkali metals.
Hydrogen forms H- ion by gaining one electron like F-, Cl-, Br-, etc.
Hydride ion H- is unstable in aqueous solution while halides ions, i.e,. F-, Cl-, Br- etc, are stable.
H- + H2O → H2 + OH-
Hydrogen has just one electron in its outermost shell while halogens have seven electrons in their outermost shells.
Hydrogen forms very stable compound H2O with oxygen while halogens do not form.
Alkali metal hydrides e.g. NaH are mostly basic while alkali metals halides e.g. NaCl are neutral.
Hydrogen can form H+ ion by losing an electron, while halogens do not form positive ions.