[Explained] Why do Alkali Metals Impart Colour to the Flame?

Today we shall look into why alkali metals impart characteristic colour to the flame?

Alkali metals impart colour to the flame because when an alkali metal or its salt is heated in a flame, its electrons get excited to higher energy levels due to absorption of energy. The excited states are short lived. When the excited electrons return back to their normal states, the energy is emitted. The emitted energy corresponds to the visible region and therefore a characteristic color is imparted to the flame.

Alkali metals and their salts show characteristic colors when heated in a non-luminous flame. The color imparted to the flame darkens on moving down the group.

Element	Color
Li	Crimson
Na	Golden Yellow
K	Pale Violet
Rb	Violet
Cs	Violet

Alkali metals have low ionization energies, that is why they get easily excited to a higher energy level.

Factors affecting ionization energy

Ionization enthalpy or ionization energy

The ionization energy values follows the following trends:

Alkali metals possess very low values of ionization energy. The ionization energy of an alkali metal atom is lowest in the period.

The alkali metal atoms possess electronic configuration of the type [Noble Gas] ns¹.

The noble gas core shields the valence s-electron from the nucleus. Therefore in alkali metals the valence electron is loosely held by the nucleus and can be removed easily by supplying a small amount of energy. This is why alkali metals possess quite low ionization energies.

The ionization enthalpy of alkali metals decrease progressively in going from Li to Cs.

In going from Li to Cs. the distance of the valence electron from the nucleus increases progressively due to the addition of a new shell at each succeeding element. The increase in the number of shells causes an increase in the screening effect which consequently decreases the effective nuclear charge experienced by the valence electron. 

This facilitates an easier removal of the valence electron. This is why the ionization energies of alkali metals decrease on moving down the group. 

The second ionization energies of the alkali metals are very high

When and electron is removed from an alkali metal atom, the cation formed has a stable noble gas configuration. For example. 

Li+ = 1s²

Na+= 1s² 2s² 2p⁶

The noble gas configuration is a very stable configuration. The removal of an electron from such as configuration is very difficult and requires a large amount of energy. This why the second ionization energies of alkali metals are very high.

Alkali Metal ion	Electron Configuration
Li+	1s2
Na+	1s2 2s2 2p6
K+	1s2 2s2 2p6 3s2 3p6

 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.

The alkali metals with their symbols, atomic number, electron configuration is given in the table below for reference.

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

Due to the large size of the atoms, the alkali metals gets excited easily into the higher energy level.

Atomic and Ionic Radii

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

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. 

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.