[Explained] Why are Alkali Metals so reactive?

 Alkali Metals are those elements which are found in the first group of the periodic table. 

Alkali metals are very reactive because they have low ionization energy. Their atomic size is large which results in poor shielding and thus low nuclear charge. And they only have a single valence electron which explains their tendency to lose it and attain noble gas electron configuration and thus be reactive.

Cell Phone Batteries are made up of Lithium which is an element of group 1 also known as alkali metals.

 The group 1 or Group 1 A of the periodic table consists if six elements including hydrogen. These elements are 

• Hydrogen
• Lithium 
• Sodium 
• Potassium 
• Rubidium 
• Cesium 
• Francium 

They are collectively known as alkali metals. 

Alkali metals are known to be very reactive in nature.

Alkali Metals show typical silvery white metallic luster when freshly cut. The metallic luster fades rapidly due to oxidation by atmospheric air. They are soft, malleable and ductile. They are so soft that they can be cut with a knife. Lithium is the hardest of all the elements present in the alkali group.

There are various reason for them to be so reactive, we shall discuss the reasons for them to be so reactive.

They have very low ionization energy. They have the lowest among the all the elements of the periodic table.

Ionization energy is the energy that is required to remove the most loosely bound electron from an isolated gaseous atom to form a positively charged ion is known as ionization energy.

One of the most important factors that affect ionization energy is the size of the atom.

The atomic and ionic radii of alkali metals are the largest in their respective periods.

Each alkali metal atom is the first element of its period. As one moves from the left to right in a period, the differentiating electrons are added in the same electron shell and the nuclear charge increases with increase in the atomic number. Thus, in going from left to right in a period, the number of shells remains the  same but nuclear charge increases with each succeeding element. 

Thus, the electrons in the valence shell experience a greater pull towards the nucleus. This results in the successive decrease of the atomic and ionic radii with increase in the atomic number. This is why the atomic and ionic radii of alkali metals are the largest in their respective periods. 

The ionic radii of alkali metals ions are smaller than the atomic radii of the corresponding atoms.  

For example the ionic radius of Na⁺ ion is 102 pm whereas the atomic radius of Na atom is 186 pm.

Alkali metals possess only one electron in their valence shell. During the formation of cation, the valence s electron is lost. The cation thus formed has one electrons shell less than the parent atom. The removal of an electron shell decreases the size. 

      Na           ⟶     Na⁺ + e^-

     1s²2s²2p⁶3s¹              1s²2s²2p⁶         

Moreover, the removal of an electron from the valence shell increases the effective nuclear charge experienced by the remaining electrons. Thus, the remaining electrons are pulled closer to the nucleus resulting in a further decrease in the size of the ion. 

The combined effect of the decrease in the number of the electron shells and an increase in the effective nuclear charge is responsible for the smaller size of alkali metal cations as compared to those of the corresponding alkali metal atoms. 

Due to this large size that their atoms have the effective nuclear charge experienced by the outermost electrons is very low, which explains their tendency to be reactive.

Finally one of the most important reasons why alkali metals are so reactive is that they have only one electron in the s shell. They have a tendency to lose that electron to attain the electron configuration of the nearest noble gas.