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Physical Properties of Group 3B Elements

Chemical Bonding
Group III B includes boron, aluminum, gallium, indium and thallium. All the members of group IIIB contain three electrons in the outermost orbit. Hence, they have an outer electronic configuration of ns2np1. As the outer most electrons are present in the p-shell they are included in the P-block. Here is a list of physical properties by these elements.

Physical properties:

Atomic and ionic radius: atomic radius increases from boron to thallium. From boron to aluminum, the atomic radius increases greatly. This increase is due to greater screening effect caused by the eight electrons present in the penultimate shell. This is not seen in case of boron as it has only two electrons in the penultimate shell. In case of ionic radius, it increases from boron to thallium.

Density:

Density increases from B to TI. This is due to increase in the size of the atom. Of all the elements, aluminum is of very low density and is widely used as a structural material.

Melting and boiling points:

Melting point decreases from B to Ga and then it gradually increases. Boron has a very high melting point due to its existence as a giant covalent polymer in both solid and liquid states. Gallium has a very low melting point and remains as liquid up to 2000ο.

Ionization energy:

Though the members of p-block have large nuclear charge and small size, the first ionization energies of these elements are less than the corresponding values of s-block elements. The electrons are far away from the nucleus and are held less tightly. Hence, they can be removed very easily. However, the average of the first three ionization energies is very high and decrease as we move down the group.


Electropositive character:

Electropositive nature increases from B to TI. Boron acts as a semi-metal while all the other elements show metallic nature.

Oxidation states:

As there are three electrons in the outermost orbit, they show oxidation states of +1 and +3. Boron exhibits only the +3 oxidation state and all the other elements show both the oxidation states. The stability of +1 oxidation state increases as we move down the group. Thus, thallium which shows a +1 oxidation state is highly stable. This is due to the inert pair effect. (Inert pair effect means the two s-electrons in the outer shell remain paired and do not participate in compound formation).

Ability to form ionic compounds:

The tendency to form ionic compounds increases as we move down the group. Boron forms only covalent compounds. This is mainly due to its high ionization energy and small size. The small size of boron enables it to exert strong polarizing effect on neighboring atoms and pulls off the electrons from neighboring atoms. As the ionic size increases as we move down the group, the tendency to form covalent bond decreases.
Oxidation potential or reducing property: the oxidation potentials of the elements of group III B are very high. This is due to high heat of hydration which is due to high charge and small radius of trivalent ions M3+. Aluminum is a very strong reducing agent.
Complex formation: the smaller size and greater charge of group IIIB elements enable them to have a greater tendency to form complexes than the s-block elements.



Solved problems
  • Which of the following have a greater tendency to form complexes?

    • P-block elements
    • S-block elements
    • Both
    • Both do not form complexes
    Answer: a
  • Which of the following elements can only form covalent compounds

    • Boron
    • Aluminum
    • Gallium
    • Indium
    Answer: a
  • The high stability of +1 oxidation state of thallium is due to

    • Small size
    • Inert pair effect
    • Larger atomic radius
    • All the above
    Answer: b
  • Which of the following has a very low melting point

    • Boron
    • Gallium
    • Aluminum
    • Beryllium
    Answer: b

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