Peculiar Behavior of Beryllium

Beryllium, the first element of Group 2, exhibits unique properties that distinguish it from the other typical alkaline earth metals. This is primarily due to its small atomic size and relatively high electronegativity. Key differences include:

1. Hardness: Beryllium is the hardest element among the elements in Group 2.
2. Melting and Boiling Points: Beryllium has the highest melting and boiling points in its group.
3. Formation of Covalent Compounds: Beryllium tends to form covalent compounds because the electronegativity difference between beryllium and other elements is not large enough to form ionic bonds.
4. Reaction with Water: Beryllium does not react with water, even at high temperatures, unlike other alkaline earth metals which decompose water, liberating hydrogen gas (H₂).
   Mg + H₂O → MgO + H₂
5. Reaction with Hydrogen: Beryllium does not react directly with hydrogen gas at room temperature to form its hydride. However, beryllium hydride (BeH₂) can be prepared indirectly at high temperatures and pressures. Other alkaline earth metals readily form ionic hydrides with hydrogen, while the hydrides of beryllium and magnesium are covalent.
6. Reaction with Alkalis: Beryllium reacts with alkalis to produce hydrogen gas.
   BeO + 2NaOH → Na₂BeO₂ + H₂
   Other alkaline earth metals do not react with alkalis.
7. Behavior of Oxides and Hydroxides: The oxides and hydroxides of beryllium are amphoteric, meaning they dissolve in both acids and alkalis to form salts.
   Be(OH)₂ + 2HCl → BeCl₂ + 2H₂O
   Be(OH)₂ + 2NaOH → Na₂BeO₂ + 2H₂O
8. Behavior of Carbides: Beryllium carbide decomposes in water to produce methane (CH₄).
   Be₂C + 4H₂O → 2Be(OH)₂ + CH₄
   The carbides of other alkaline earth metals decompose in water to produce acetylene (C₂H₂).
   CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂
9. Behavior of Nitrides: Beryllium nitride (Be₃N₂) is volatile, while the nitrides of other alkaline earth metals are non-volatile.
10. Number of Molecules of Water of Crystallization: Salts of Be²⁺ ions typically have no more than four molecules of water of crystallization. This limitation is due to the availability of only four orbitals to accept lone pairs from oxygen atoms of water molecules. In contrast, other Group 2 metal cations, like magnesium (Mg²⁺), can extend their coordination number to six by using one 3s, three 3p, and two 3d orbitals from their outermost shell.
11. Formation of Complex Compounds: Due to its small size, Be²⁺ forms stable complex compounds such as [BeF₄]²⁻. Other M²⁺ ions from typical alkaline earth metals form very few complex compounds.

These distinctive properties highlight the unique chemical behavior of beryllium compared to other elements in Group 2.