Group 3: Boron and Aluminium

Boron and Aluminum are Group 3 elements. What does this mean?

They have 3 electrons in their outer shell.

How many covalent bonds can a boron and aluminum atom form?

3 covalent bonds- this gives a total of 6 electrons in the outer shell of the Group 3 atom.

Compounds of boron and aluminum are often described as ‘electron-deficient.’ What does this mean?

A species that has less than 8 electrons in its outer shell.

How may an electron-deficient boron/aluminum atom gain a stable octet?

Aluminum and boron are able to form co-ordinate bonds with a lone pair of electrons from another atom to gain a stable octet.

Therefore, they are called electron acceptors and can form dimers.

What is a dimer?

Formed when 2 of the same molecules join together.

What is a co-ordinate bond?

A covalent bond in which both of the bonding electrons come from the same species.

It is also known as a dative bond.

How is aluminum hexachloride, Al₂Cl₆ formed?

The aluminum in aluminum chloride is electron deficient.

Therefore, two monomers of aluminum chloride, AlCl₃, are able to form a dimer of Al₂Cl₆. They are linked together by two co-ordinate bonds.

Two chlorine atoms share their lone pair of electrons with two atoms of aluminum to form the co-ordinate bonds.

What is a donor acceptor compound and give one example of this type of compound.

Donor acceptor compounds form where a lone pair is donated by one compound and accepted by another, forming a co-ordinate bond.

An example of this is boron trifluoride accepting a lone pair from the nitrogen in ammonia to form NH₃BF_3.