Which of following statement is not true?

Options

A. HF is more polar than HBr
B. CuCl is more covalent than NaCl
C. HF and `B_2` are isoelectronic species having `sigma` bond
D. Chemical bond formation takes place when forces of attraction overcome the forces of repulsion

Explanation

To determine which statement is not true, let's analyze each one step by step.

### Step 1: Analyze the first statement
**Statement:** HF is more polar than HBr.

- **Explanation:** The polarity of a bond depends on the difference in electronegativity between the two atoms involved. Fluorine (F) has a higher electronegativity than bromine (Br). Therefore, the bond in HF is more polar than that in HBr because the difference in electronegativity between H and F is greater than that between H and Br.

- **Conclusion:** This statement is **true**.

### Step 2: Analyze the second statement
**Statement:** CuCl is more covalent than NaCl.

- **Explanation:** According to Fajans' rules, the covalent character of a bond increases with the polarizing power of the cation. Copper (Cu) in CuCl has a +1 charge and a pseudo-noble gas configuration (2-8-18), which gives it a high polarizing power. Sodium (Na), on the other hand, has a +1 charge and a configuration of 2-8. Since Cu+ has a higher polarizing power than Na+, CuCl is indeed more covalent than NaCl.

- **Conclusion:** This statement is **true**.

### Step 3: Analyze the third statement
**Statement:** HF and B2 are isoelectronic species having a sigma bond.

- **Explanation:** Isoelectronic species have the same number of electrons. HF has 10 electrons (1 from H and 9 from F), and B2 also has 10 electrons (5 from each B atom). However, while HF contains a sigma bond, B2 does not have a sigma bond; instead, it has a pi bond due to the molecular orbital configuration. The last two electrons in B2 fill the pi orbitals (π2p_x and π2p_y), leading to the presence of a pi bond instead of a sigma bond.

- **Conclusion:** This statement is **not true**.

### Step 4: Analyze the fourth statement
**Statement:** Chemical bond formation takes place when forces of attraction overcome the forces of repulsion.

- **Explanation:** This statement is correct. A chemical bond forms when the attractive forces between atoms (or ions) are greater than the repulsive forces. When atoms come close together, the attractive forces (due to opposite charges) must overcome the repulsive forces (due to like charges) for a bond to form.

- **Conclusion:** This statement is **true**.

### Final Conclusion
The statement that is **not true** is the third statement: "HF and B2 are isoelectronic species having a sigma bond."

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