Q.Among NaCl, BeCl2 and AlCl3, which is the compound with the most covalent character? Justify your answer.

Anomalous Behaviour of Beryllium

You already know that elements in the same group have similar valence electron configurations, so you expect them to behave alike. For Group 2 (alkaline earth metals), that expectation mostly holds — except for the very first member: beryllium. It is the odd one out, and the reason is written in its size.

The root cause: extreme smallness

Beryllium is tiny. Its atomic radius is about 112 pm, while the next element, magnesium, is already 160 pm. That small size means two things:

1. Very high ionisation enthalpy — pulling off an electron from such a small atom takes a lot of energy (899 kJ/mol for the first, 1757 kJ/mol for the second). Compare that to magnesium (737 and 1450 kJ/mol).
2. High charge density — the Be²⁺ ion is incredibly small and doubly charged, so it polarises any nearby electron cloud very strongly.

These two consequences cascade into every chemical difference you see.

What actually happens differently

Covalent character, not ionic. Most Group 2 metals form ionic compounds (MgO, CaCl₂, etc.). Beryllium cannot. Its high ionisation enthalpy means forming Be²⁺ costs too much energy, and its tiny size means even if it did form Be²⁺, that ion would distort any anion's electron cloud so severely that the bond becomes covalent. Beryllium chloride (BeCl₂) is a covalent molecule, not an ionic lattice. Beryllium oxide (BeO) has high covalent character — it is hard, high-melting, and does not dissolve in water like MgO does.

Amphoteric nature, not basic. The oxides and hydroxides of Mg, Ca, Sr, Ba are all basic — they react with acids to give salt and water, but not with bases. BeO and Be(OH)₂ react with both acids and strong bases. With a base, beryllium hydroxide forms the beryllate ion:

This amphoteric behaviour is typical of elements near the diagonal in the periodic table (beryllium resembles aluminium in this respect).

No direct reaction with water. Magnesium reacts slowly with hot water; calcium reacts even with cold water. Beryllium does not react with water at all — its covalent oxide layer protects it, and the metal itself is too reluctant to lose electrons.

Complex formation. Because Be²⁺ is so small and has high charge density, it readily forms complexes by accepting lone pairs. Beryllium compounds dissolve in excess alkali to form tetrahydroxoberyllate(II) ions, and BeCl₂ forms adducts like BeCl₂·2H₂O. The heavier Group 2 metals rarely form such complexes.

The precise statement

Anomalous behaviour of beryllium refers to the fact that beryllium differs significantly from the rest of Group 2 elements due to its exceptionally small atomic size and high ionisation enthalpy. These properties cause its compounds to be predominantly covalent rather than ionic, its oxide and hydroxide to be amphoteric rather than basic, its inability to react with water, and its tendency to form complex ions — all of which are absent or much weaker in the heavier alkaline earth metals. …