[Explained] Why do Alkali Metals form Crystalline Compounds?


Alkali metals form crystalline compounds due to it's metallic character. It has very low values of ionization energy which also attributes to the formation of ionic compounds. It is a characteristic of ionic compounds to form crystalline compounds.

Why do Alkali Metals have different oxides?

Why do Alkali Metals have different reactivities?

Uses of Crystalline Solids

Let us look into the characteristics of ionic compounds.

Properties of Ionic Compounds

Now lets look at the properties of ionic compounds. They exhibit some characteristic properties that we will discuss one by one. 

  • They have a crystalline form in room temperature.
  • They have high melting and boiling points
  • They are hard and brittle 
  • They are soluble in water.
  • They are poor conductors of electricity in solid state but good conductors in fused or dissolved state.
  • They do not exhibit stereo-isomerism.
  • They undergo very fast reactions known as ionic reactions.
The section below explains why the alkali metals form ionic bonds. 

They form ionic bonds due to low ionization energy, low electronegativity and high metallic character.

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. 

ElementSymbolAtomic NumberElectron ConfigurationBrief Representation of Electron Configuration
LithiumLi31s2 2s1[He] 2s1
SodiumNa111s2 2s2 2p6 3s1[Ne] 3s1
PotassiumK191s2 2s2 2p6 3s2 3p6 3d10 4s1[Ar] 4s1
RubidiumRb371s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s1[Kr] 5s1
CesiumCs551s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1[Xe] 6s1
FranciumFr871s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 7s1[Rn] 7s1



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 ionElectron Configuration
Li+1s2
Na+1s2 2s2 2p6
K+1s2 2s2 2p6 3s2 3p6


Why do Alkali Metals form Crystalline Compounds?
Bananas contain a lot of Potassium

 

Electronegativity

Alkali metals possess low values of electronegativity. In general, the electronegativity of alkali metals decrease in going down the group. 

Due to large size and low nuclear charge, the alkali metal atoms are unable to attract electrons towards them. This is why they possess low values of electronegativity. In going from Li to Cs, the size of atom increases further. Consequently, the electronegativity of alkali metals decreases on going down the group. 

Electropositive Character (also known as Metallic character)

 The alkali metals are strongly electropositve. Each alkali metal atom is the most electropositive atom in its period. Due to strong electropositive character alkali metals exhibit strong metallic character. The electropositive character of alkali metal metals increases in going from Li and Cs.

The tendency of an atom to form positive ions by losing its valence electron s determine its electropositive character. Since alkali metals possess very low values of ionization energies, they have strong tendency to lose their valence electrons. This is why they show strong electropositive or metallic character. 

As the ionization energies of alkali metals decrease progressively in going from Li to Cs, their tendency of losing valence electrons also increases progressively. Consequently, the electropositive character increases on moving down the group.

Why do Alkali Metals impart colour to the flame?

Why is Hydrogen not considered to be an Alkali Metal?

Why are Alkali Metals so reactive?

Physical and Chemical Properties of Alkali Metals

Oxidation State

An alkali metals exhibit only +1 oxidation state in their compounds. They do not show variable oxidation states as shown by several other elements of their periods.

The alkali metal atoms possess only one electron ns1 in their valence shells and can lose it readily due to low ionization energies. On losing the valence electron they form a monopositive cation and thus exhibit +1 oxidation state.

    M ⟶  M+ + e-

The monopositive cation formed has the configuration of a nearest noble gas. As the noble gas configuration is a very stable configuration, the cation formed does not allow the further removal of electrons easily. This is why alkali metal atoms do not exist in higher oxidation states and exhibit only =+1 state.

The alkali metal cations (M+) have no unpaired electrons. Therefore, they are colorless and diamagnetic in nature. 



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