Draw the Lewis structures for each of the following molecules or ions. Identify instances where the octet rule is not obeyed; state which atom in each compound does not follow the octet rule; and state how many electrons surround these atoms: (a) PF₆^-, (b) BeCl₂, (c) NH₃, (d) XeF₂O (the Xe is the central atom), (e) SO₄^2- .

Verified step by step guidance

Step 1: For each molecule or ion, count the total number of valence electrons. Remember to add or subtract electrons for any charges present.

Step 2: Draw the skeletal structure of each molecule or ion, connecting atoms with single bonds. Typically, the least electronegative atom is the central atom, except for hydrogen.

Step 3: Distribute the remaining electrons as lone pairs to complete the octet for each atom, starting with the most electronegative atoms. Remember that hydrogen only needs 2 electrons.

Step 4: Check each structure to see if all atoms have a complete octet (8 electrons) or if there are exceptions. Identify any atoms that do not follow the octet rule and count the electrons around them.

Step 5: For each molecule or ion, identify and explain any exceptions to the octet rule, such as expanded octets, incomplete octets, or odd-electron species.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Lewis Structures

Lewis structures are diagrams that represent the bonding between atoms in a molecule and the lone pairs of electrons that may exist. They use dots to represent valence electrons and lines to represent bonds between atoms. Understanding how to draw Lewis structures is essential for visualizing molecular geometry and predicting the behavior of molecules in chemical reactions.

Octet Rule

The octet rule is a chemical rule of thumb that states atoms tend to bond in such a way that they each have eight electrons in their valence shell, achieving a stable electron configuration similar to that of noble gases. However, there are exceptions to this rule, particularly for elements in the third period and beyond, which can accommodate more than eight electrons due to available d-orbitals.

Expanded Octet

An expanded octet occurs when an atom has more than eight electrons in its valence shell, which is common for elements in the third period and beyond, such as phosphorus, sulfur, and xenon. This phenomenon allows these atoms to form more bonds than would be predicted by the octet rule, leading to the formation of complex molecules and ions. Identifying atoms with expanded octets is crucial for accurately drawing Lewis structures.

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Octet Rule

Related Practice

Textbook Question

In the vapor phase, BeCl₂ exists as a discrete molecule. (a) Draw the Lewis structure of this molecule, using only single bonds. Does this Lewis structure satisfy the octet rule?

Textbook Question

In the vapor phase, BeCl₂ exists as a discrete molecule. (b) What other resonance structures are possible that satisfy the octet rule?

Textbook Question

For Group 13–17 elements in the third row of the periodic tableand beyond, the octet rule is often not obeyed. A friendof yours says this is because these heavier elements are morelikely to make double or triple bonds. Another friend ofyours says that this is because the heavier elements are largerand can make bonds to more than four atoms at a time.Which friend is more correct?

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Textbook Question

In the vapor phase, BeCl₂ exists as a discrete molecule. (c) On the basis of the formal charges, which Lewis structure is expected to be dominant for BeCl₂?