Why BeF2 and BeCl2 are covalent not ionic: Fajans' rule

I hope now you have become quite familiar with atoms and able to predict their behaviour and motives. You can predict which element would prefer ionic bonding and which one likes to choose covalent bonding. Do you ever try to understand how they feel? Let’s try to understand them more closely. Atoms are like us, and as you understand them, you will find many more similarities with your behaviour.

Do you remember the first day of your friendship? When you meet someone for the first time, you like some of his qualities and dislike a few things. That means you are partly attracted towards him and at the same time something repels you too. Similar things happen to the ions when they come to bond together.

When the positive ion comes closer to the negative ion, it attracts electrons of negative ion but repels its nucleus. Similarly negative ion attracts the nucleus of positive ion and simultaneously repels its electrons. Thus each of them disturbs the harmony of the other. This disturbance is called the polarization and the susceptibility to get polarized is called the polarisibility.

You know that in covalent bond the bonding pairs of electrons (electron density) are located in between the bonded atoms and in ionic bond electron density is located near the negative ion (more electronegative element). If positive ion is strong enough to polarise negative ion, it will be able to pull electron density away from negative ion, as a result of which covalent character is introduced in their ionic bond.

The introduction of covalent character in ionic bond depends on the polarizing power of positive ion and polarisibility of negative ion. Let’s try to understand it.

Positive ion

• Formed by removal of electron from atom.
• That's why the number of protons are more than the number of electrons (i.e. higher effective nuclear charge)
• That’s why nucleus becomes more powerful and binds electrons more tightly.
• Powerful nucleus can pull electrons of negative ion and at the same time can defend its electrons to get polarised by negative ion.
• Hence it is less susceptible to get polarised and has more polarising power.
• Smaller positive ions have more polarising power.

Negative ion:

• Formed by addition of electron to the atom
• That’s why the number of electrons are more than the number of protons (i.e. less effective nuclear charge)
• Nucleus wouldn’t be able to bind electrons tightly as a result of which size of the ion increases.
• Because of the larger size of the negative ion nucleus is far way to be able to polarise positive ion.
• Due to less effective nuclear charge, nucleus is not able to defend electrons to get polarised by positive ions.
• Hence negative ion is weak in polarising ability and susceptible to get polarised.
• Larger negative ions are easy to get polarised.

Fajans gave four rules which summarise the factors favouring polarization and covalency.

1. A small positive ion favours covalency: Smaller ion has more polarising power and will be able to pull electron density away from negative ion.
2. A large negative ion favours covalency: Larger negative ions are easy to get polarised.
3. Large charges on either ion or on both ions favour covalency: High charge on positive ion increases its polarising power and high charge on negative ion increases its polarisibility.
4. Polarization, and hence covalency, is favoured if the positive ion does not have a noble gas configuration: Noble gas configuration is the most stable configuration (more stable than the full filled state) and most effective at shielding the nuclear charge, so the ions which do not have noble gas configuration (such as Tl ⁺¹,Pb⁺² and Bi⁺³) are highly polarising because their nuclei are able to radiate their power more efficiently due to lesser shielding.

In BeF₂ molecule Be⁺² ion is smaller and also have higher charge on it, hence it will be able to pull electron density away from F^-1 ions and introduce the covalence character. That’s why BeF₂ is a covalent molecule not an ionic. And for the similar reasons BeCl₂ is also a covalent molecule not an ionic molecule.

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