Some Atoms Can Form Molecules with Expanded Octets
Expanded Octets and Their Connection to the Periodic Table
Definition
Octet rule
Atoms are driven by their quest for stability, often achieved by attaining a noble gas electron configuration, commonly referred to as the octet rule.
- However, some elements in Period 3 and beyond (such as phosphorus, sulfur, and xenon) can exceed this limit.
- These atoms can accommodate more than eight electrons in their valence shell due to the availability of vacant d orbitals in addition to their s and p orbitals.
Why Only Period 3 and Beyond?
- Electron Configuration:
- Atoms in Period 3 and beyond have a principal quantum number $n \geq 3$, which means they have d orbitals in their valence shell (e.g., 3d for Period 3 elements).
- These d orbitals can participate in bonding, allowing the atom to hold more than eight electrons.
- Atomic Size:
- Larger atoms, like sulfur and phosphorus, have more space in their valence shell to accommodate additional electrons, reducing electron-electron repulsion.
Elements in Periods 1 and 2 cannot expand their octet because they lack d orbitals and have smaller atomic sizes, limiting their ability to accommodate extra electrons.
Visualizing Lewis Structures for Expanded Octets
To understand expanded octets, let’s explore how to draw Lewis structures for molecules where the central atom has more than four bonds (or electron domains).
Steps to Draw Lewis Structures for Expanded Octets:
- Count Valence Electrons:
- Add up the valence electrons of all atoms in the molecule.
- For ions, adjust for the charge (add electrons for negative charges or subtract for positive charges).
- Arrange Atoms:
- Place the least electronegative atom in the center.
- Distribute Electrons:
- Connect atoms with single bonds and distribute the remaining electrons as lone pairs to satisfy the octet rule for peripheral atoms.
- Expand the Octet:
- Place any leftover electrons on the central atom, allowing it to exceed eight electrons if it belongs to Period 3 or beyond.
Phosphorus Pentafluoride (PF₅)
Phosphorus, being in Period 3, can expand its octet.
- Count Valence Electrons: Phosphorus has 5 valence electrons, and each fluorine atom contributes 7. Total = $5 + (7 \times 5) = 40$.
- Arrange Atoms: Place phosphorus in the center and bond it to five fluorine atoms.
- Distribute Electrons: Each P–F bond uses 2 electrons, leaving none unused. Phosphorus now has 10 electrons in its valence shell.
Sulfur Hexafluoride (SF₆)
Sulfur, also in Period 3, can expand its octet to accommodate six bonds.
- Count Valence Electrons: Sulfur has 6 valence electrons, and each fluorine contributes 7. Total = $6 + (7 \times 6) = 48$.
- Arrange Atoms: Place sulfur in the center and bond it to six fluorine atoms.
- Distribute Electrons: Each S–F bond uses 2 electrons, leaving none unused. Sulfur now has 12 electrons in its valence shell.
