[Explained] Why do alkali metals have different reactivities?

 

Cell Phone Batteries are made up of Lithium

 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.

Why do Alkali Metals have different oxides?

Why do Alkali Metals form Crystalline compounds?

Element	Symbol	Atomic Number	Electron Configuration	Brief Representation of Electron Configuration
Lithium	Li	3	1s2 2s1	[He] 2s1
Sodium	Na	11	1s2 2s2 2p6 3s1	[Ne] 3s1
Potassium	K	19	1s2 2s2 2p6 3s2 3p6 3d10 4s1	[Ar] 4s1
Rubidium	Rb	37	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s1	[Kr] 5s1
Cesium	Cs	55	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1	[Xe] 6s1
Francium	Fr	87	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 7s1	[Rn] 7s1

Why do alkali metals have different reactivities?

Alkali metals have different reactivities because they are differ in size. Alkali metals are part of the same group 1 of the periodic table therefore, they have similar chemical properties but different reactivities. They have the same number of valence electron that is, one. It can be seen in the table above. 

What determines the reactivity of alkali metals?

The fact that larger is the size of the atom, greater is its reactivity, is what       determines the reactivity of alkali metals. As the distance between the nucleus and the valence electron increases, effective nuclear charge decreases and that causes alkali metal to easily lose its valence electron.

What happens to the reactivity of the alkali metal as you go down the group?

Alkali metals tend to be more reactive as we go down the group because the size of the metal atom keeps on increasing. Lithium has the smallest size among the alkali metals and francium the largest. Therefore, with size reactivity also increases.

Now, let us look into some details about the atomic size of alkali metals in terms of atomic and ionic radii and see why they are important.

Atomic and Ionic Radii

The atomic and ionic radii of alkali metals show the following characteristics:

Why do Alkali Metals form Ionic Compounds?

Why are Alkali Metals more reactive than Alkaline Earth Metals?

Why are Alkali Metals strong reducing agents?

Why do Alkali Metals impart colour to the flame?

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. 

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 atomic and ionic radii of alkali metals increase on moving down the group i.e. they increase in going from Li to Cs.

As one moves from Li to Cs in group 1, a new electron shell is added at each element and the nuclear increases in the atomic number. The addition of an electron shell at each element tends to increase the size of the atom but the increase in the nuclear charge has a tendency to decrease the size of the atom or ion. Thus, the two factors oppose each other. 

Why is Hydrogen not considered to be an Alkali Metal?

Why are Alkali Metals so reactive?

Physical and Chemical Properties of Alkali Metals

The increase in the number of shells increases the screening effect of the inner electrons on the valence s-electron. This results in the expansion of the electron cloud. As the screening effect is quite  large, it over weighs the contractive effect of the nuclear charges with increase in the atomic number. The net result is an increase in the atomic and ionic radii of the alkali metals in going from Li to Cs.

Table Salt contains Sodium as a constituent