Which one of the following molecules has a coordinate bond

Options

A. `Al_2Cl_6`
B. `BCl_3`
C. `NaCl`
D. `O_2`

Answer

To determine which molecule has a coordinate bond, we need to analyze the bonding in the given options. Here’s a step-by-step solution:

### Step 1: Understand Coordinate Bonds
A coordinate bond (or dative bond) is formed when one atom donates both of the electrons to be shared in a bond. This typically occurs between an electron-deficient atom and an atom with a lone pair of electrons.

### Step 2: Identify the Compounds
Let's consider the molecule Al2Cl6 (aluminum chloride dimer) and its formation from AlCl3 (aluminum chloride).

### Step 3: Analyze AlCl3
AlCl3 is an electron-deficient compound. Aluminum (Al) has three valence electrons (3s² 3p¹) and can form three covalent bonds with chlorine (Cl), which has seven valence electrons (3s² 3p⁵). After bonding with three Cl atoms, Al has six electrons but still requires two more to complete its octet.

### Step 4: Dimerization of AlCl3
To achieve a stable electronic configuration, two AlCl3 molecules can dimerize to form Al2Cl6. In this process, one of the Cl atoms from one AlCl3 donates a pair of electrons to the Al atom of the other AlCl3, forming a coordinate bond.

### Step 5: Structure of Al2Cl6
In Al2Cl6, there are three normal covalent bonds formed by sharing electrons between Al and Cl, and there are two coordinate bonds where Cl donates a pair of electrons to Al. This allows aluminum to complete its octet.

### Conclusion
Thus, Al2Cl6 contains coordinate bonds due to the donation of electron pairs from chlorine atoms to aluminum.

### Final Answer
The molecule that has a coordinate bond is **Al2Cl6**.

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