Why Alkali Metals(Group 1) and Halogens (Group 17) are highly reactive ?

Alkali metal(Group 1 ) and halogens(Group 17) are highly reactive because of some factors which I am going to discuss here.

Alkali Metals(Group 1)

First let me clear that Group 1 Element is also called alkali Metal because when they react with water they form strong alkaline(Strong Base with water) . Elements like Lithium , Sodium , Potassium , Rubidium , Caesium form strong bases with water 

Halogens(Group 17)

Group 17 elements are called halogen because the word " Halogen" came from Greek word which means " to make salt " . Element like Fluorine , Chlorine , Bromine and Iodine form salts when reacts with metals .

High reactivity of Alkali Metals(Group 1)

Coming to point , Group 1 elements are highly reactive because of their large atomic Size and low ionization enthalpy . Atomic Size and ionization enthalpy are inversely proportional to each other . 

Ionization enthalpy is simply amount of energy requires to remove any electron from the outermost shell (atom must be in gases and isolated form ) . Ionization enthalpy is always positive because we have to supply energy (endothermic)

From the above , we can say that larger the size , electron will be less bounded (attracted) to the nucleus of an atom . So , less energy will be required to remove an electron from the outermost shell. Therefore group 1 metals easily donate thier electrons and form stable noble gas configuration ns2 np6) 

That's why elements of group 1 like Lithium , Sodium , Potassium ,Rubidium , Caesium have low ionization enthalpy . Element Caesium has the lowest ionization enthalpy in the periodic table .

High reactivity of Halogens(Group 17)

Group 2 elements are also highly reactive because of thier smaller size and highly negative electron gain enthalpy .

Electron gain enthalpy is the energy change take place when a electron is added to an atom ( atom must be in gases state and isolated). It's simple means that how easily can elements accept electron . 

Electron gain enthalpy can be positive or negative but for group 17 it is negative because group 17 Element release energy when an electron is added to their atom

Due to small size of group 17 elements. , they have large nuclear attraction on the outermost electron . When electron approaches an atom it tries to attract toward itself so that , they can accept this electron and acquire noble gas configuration (ns2 np6) and become highly stable .That's why group 17 elements accept electron and become stable .

In short ,low  Ionization enthalpy and  large atomic Size is responsible for high reactivity of Group 1 (Alkali Metals) elements and highly negative electron gain enthalpy and small size is responsible for high reactivity of Group 17(Halogens)

So because of this reason group 1 and group 17 elements are highly reactive in nature