Oxygen shows anomalous behaviour due to its :

(A) Small size

(B) Covalency of four

(C) High electronegativity

(D) Tendency to form weak H-bonds

Choose the correct answer from the options given below:

(A) and (C) only

The correct answer is option 4. (A) and (C) only.

The anomalous behavior of oxygen is primarily attributed to its high electronegativity and small size.

1. High Electronegativity (C):
Electronegativity is the ability of an atom to attract electrons towards itself. Oxygen has a high electronegativity, making it one of the most electronegative elements in the periodic table.
The high electronegativity of oxygen affects the nature of the compounds it forms. It tends to attract electrons strongly, leading to the formation of polar covalent bonds in compounds.

2. Small Size (A):
Oxygen is a small-sized atom, especially when compared to other elements in its period (second period of the periodic table).
The small size of oxygen atoms contributes to the following anomalous behavior:

• Formation of Double Bonds: Oxygen can form stable double bonds in molecules like \(O_2\). The small size allows the formation of multiple bonds, leading to the existence of \(O_2\) with a double bond.
• High Ionization Energy: The small size results in a higher ionization energy for oxygen, making it relatively difficult to remove electrons from oxygen atoms.

Other Options:

• Covalency of Four (B): Oxygen typically has a covalency of two, forming two covalent bonds. For example, in water (\(H_2O\)), each oxygen atom forms two bonds.
• Tendency to Form Weak H-bonds (D): While oxygen can participate in hydrogen bonding, this behavior is not necessarily anomalous and is common for other elements like nitrogen and fluorine.

In summary, the anomalous behavior of oxygen is primarily due to its high electronegativity and small size. These factors influence its bonding characteristics and reactivity, leading to unique properties not shared by other elements in its group. Therefore, the correct answer is (4) (A) and (C) only.