Trends in Physical Properties of Group 13 Elements

• Group 13 of the long form of the periodic table consists of five elements: Boron (B), Aluminium (Al), Gallium (Ga), Indium (In) and Thallium (Tl).
• All these elements collectively are named as boron group elements, because Boron (B) is the first element of this group.
• These elements form the first group of the p-block elements.
• Some important physical properties of these elements are given below:

I) Atomic and ionic radii

• The atomic and ionic radii (for M³⁺ ions) increase in going from boron (B) to thallium (Tl) i.e., down the group.
• This is because of the introduction of an additional electron shell with each subsequent element down the group.
• The atomic and ionic radii of Group 13 elements are smaller than those of the corresponding elements of Group 2.
• As we move from Group 2 to Group 13 in any period, the nuclear charge increases.
• The added electrons however, enter the same shell. The electrons in the same shell do not screen each other effectively from the attraction of the nucleus.
• As a result, the electrons in the outermost shell experience greater inward pull towards the nucleus, i.e., the effective nuclear charge increases.
• For that reason, the atomic radii and ionic radii of Group 13 elements are smaller than those of Group 2 elements.

II) Density

• Boron and Aluminium have relatively low densities, whereas the higher members have high densities like gallium, indium and thallium.
• The densities of Group 13 elements are higher than those of Group 2 elements. Due to higher effective nuclear charge, the atomic and ionic radii of Group 13 elements are smaller than those of Group 2 elements. As a result, the densities of Group 13 elements are higher than those of Group 2 elements.
• Density of Group 13 elements increase in going down the group. This occurs due to more closely packed structure of higher elements of the group.

III) Melting and Boiling points

• The melting and boiling points of Group 13 elements are much higher than those of Group 12 elements. It is due to stronger interatomic attractions in these elements.
• The melting and boiling points of Boron are very high as compared to those of the other elements of the group. This is due to network structure of Boron.
• Gallium has the lowest melting point in this group due to the weak binding forces in the system in which Ga exist as Ga₂.
• The elements of this group exhibit high boiling points which steadily decrease in going from B to Tl.

IV) Ionization energy

• There are three electrons in the outermost shell of Group 13 elements. So, these elements are characterized by three ionization energies.
• Some characteristic features of the ionization energies of Group 13 elements are:

a) The first ionization energies (I1) of these elements are lower than the corresponding values for Group 2 elements. However, the second (I2) and third (I3) ionization energies are much higher.

• This is because the first electron to be lost by Group 13 elements is a p-electron.
• The p-electrons are less penetrating than s-electrons. Therefore, lesser energy is required to remove a p-electron.
• The second and the third electrons to be removed from an atom of any Group 13 elements are from s-orbital. Removal of s-electrons, particularly from a positively charged ion (after the removal of p-electron) requires more energy. Therefore, second and third ionization energies of Group 13 elements are higher.

b) Down the group, the first ionization energy of the Group 13 elements decreases sharply from B to Al. Thereafter, the energy shows a slight increase.

• The initial decrease in the first ionization energy may be because the atomic size of Al is much larger than that of B.
• A slight increase in the ionization energy in going from Al to Tl is because the inner d-electrons in and after gallium do not shield the outer electrons from the nuclear charge effectively.
• This causes a slight increase in the effective nuclear charge down the group.
• Therefore, the ionization energies increase in going from Ga to Tl as one goes down the group.
• Since, the total ionization energy (I1 + I2 + I3) for boron is very high, hence Boron does not exist as B³⁺ ion.

V) Electropositive character ( or metallic character)

a) Group 13 elements are less electropositive than the corresponding alkali and alkaline earth metals.

This is due to smaller atomic radii and higher effective nuclear charge of the Group 13 elements as compared to Group 1 and 2 elements.

b) The electropositive (or metallic character) of Group 13 elements increases in going from boron (B) to aluminium (Al) and then decreases to thallium (Tl). Boron is a semi-metal and closer to non-metals than to the metals in its properties e.g., boron is a poor conductor of electricity, while other elements of this group are typical metals.

• Aluminium (Al) is more electropositive than Boron (B). This is due to

1. The atomic radius of Al is much larger than that of B.
2. The ionization energies of Al are lower than those of B.

• The decrease in the electropositive character from Al to Tl may be due to,

1. A gradual increase in the ionization energies in going from Al to Tl.
2. Lowering of hydration energies because the oxidation state of +1 gets stabilized relative to +3 oxidation sate as we go down the group (due to inner-pair effect).

Image source: toppr

VI) Oxidation state

• Boron does not form B³⁺ ions due to high ionization energy. So it gives mainly covalent compounds.
• Al has a good tendency to form Al³⁺ ion in the solution due to its high hydration energy.
• Gallium, indium and thallium show +1 and +3 oxidation states. The +1 state becomes more stable than +3 in going from Ga to Tl.

1. As we go down the group, the tendency of s-electrons in the valence shell to take part in bonding decreases.
2. This resistance of a pair of s-electrons to be lost or to participate in covalent bond formation is known as inert-pair effect.
3. The inert-pair effect becomes more operative towards the bottom of the group.
4. Thus, for heavier elements of the group (e.g., Ga, In and Tl), the +1 state becomes more stable than +3 state.
5. Thus, Tl³⁺ should have strong tendency towards reduction e., the reaction

   Tl³⁺ + 2e^– → Tl⁺ is favourable in the forward direction.

Image source: Braincart

VII) Reducing character

• The reducing power of Group 13 elements is lower than that of the Group 2 elements.
• For Group 13 elements, the reducing power decreases as we go down the group which is mainly due to lowering of hydration energy down the group.

References:

i) https://www.britannica.com/science/boron-group-element

ii) https://www.vedantu.com/iit-jee/group-13-elements

iii) https://www.askiitians.com/iit-jee-s-and-p-block-elements/boron-family.html

Trends in Physical Properties of Group 13 Elements