Octet Rule and Exceptions - UNSOLVED PRACTICE SET

Q7. State the octet rule. Why do atoms tend to achieve an octet in their outermost shell?

Q8. Give two examples of molecules that obey the octet rule and two examples that violate it.

Q9. What is an incomplete octet? Give two examples of molecules with incomplete octets.

Q10. What is an expanded octet? Why can phosphorus form PCl₅ but nitrogen cannot form NCl₅?

Q11. Your little brother collects marbles and always wants to have exactly 8 marbles in his favourite pouch to feel "complete." One day, he finds a special pouch that can hold 12 marbles, and he's thrilled. How is your brother's "8-marble rule" like the octet rule, and how is the "12-marble pouch" like the expanded octet in elements like sulphur and phosphorus?

Q12. Why does the octet rule fail for hydrogen and helium? What rule do they follow instead? 

Q13. Explain the octet rule and its significance in chemical bonding. Discuss the three main types of exceptions to the octet rule with suitable examples:

(a) Incomplete octet

(b) Expanded octet

(c) Odd-electron molecules

Why is the octet rule not a universal law but rather a useful guideline?

Q14. Describe why some elements can have an expanded octet while others cannot. Explain the role of available d-orbitals in this phenomenon. Compare the chemistry of nitrogen and phosphorus, or oxygen and sulphur, to illustrate why period 3 elements can expand their octet but period 2 elements cannot.

Q15. During a class debate on the octet rule, students present the following cases:

(a) Student A argues that BF₃ is unstable because boron has only 6 electrons. Student B says BF₃ is perfectly stable. Who is correct, and what concept explains this stability despite the incomplete octet?

(b) A student asks why XeF₂, XeF₄, and XeF₆ exist even though xenon is a noble gas. Explain how xenon can expand its octet and why this was a surprising discovery.

(c) NO and NO₂ are called odd-electron molecules. Draw their Lewis structures and explain why they cannot satisfy the octet rule for all atoms.

(d) Why does the octet rule work so well for period 2 elements (C, N, O, F) but becomes less reliable for heavier elements? What does this tell us about the limitations of simple bonding theories?

Q16. For each of the following molecules, determine whether the central atom satisfies the octet rule. If not, identify the type of exception:

(a) BeCl₂

(b) SF₄

(c) ClO₂

(d) XeF₄

(e) AlCl₃

For each molecule, count the total electrons around the central atom and classify the structure.

Q17. The following table lists several molecules:

(a) Complete the table by filling in the number of valence electrons around the central atom and classifying the octet status.

(b) Which of these molecules have the same central atom but different octet status in different compounds? Give examples.

(c) For PCl₅, calculate how many electrons the phosphorus atom is using from its 3s, 3p, and 3d orbitals.

Q18. The concept of formal charge helps determine the most stable Lewis structure.

(a) Calculate the formal charge on each atom in the following structures of HNO₃:

Structure I: H-O-N(=O)-O (with one N=O double bond and one N-O single bond)

Structure II: H-O-N(-O⁻)(=O⁺) (with different arrangement)

(b) Which structure is more stable? Explain using formal charge minimization.

(c) In SO₂, draw two resonance structures and calculate formal charges. Which structure contributes more to the resonance hybrid?

(d) Why does the most stable structure not always satisfy the octet rule for all atoms (e.g., in expanded octet molecules)?