Lewis Symbols and Structures for Chemical Bonding

The Octet Rule

Most main group elements follow the octet rule, which states that atoms tend to form structures where they are surrounded by eight electrons. This count includes both the electrons involved in shared bonds and the electrons in lone pairs.

Exam Tip: When checking if a structure follows the octet rule, remember to count every electron touching the atom—both the dots (lone pairs) and the lines (bonds, where each line represents two electrons).

How to Draw a Lewis Structure

Drawing a Lewis structure for a molecule or a polyatomic ion requires a systematic approach. Follow these five steps:

1. Calculate Total Valence Electrons

Sum the valence electrons of all atoms in the molecule.

• For neutral molecules: Simply add the valence electrons of each atom.
• For polyatomic ions: Adjust for the charge. If the ion has a negative charge, add electrons to the total. If it has a positive charge, subtract electrons from the total.

2. Draw a Skeleton Structure

Identify the central atom. This is typically the atom that is least electronegative (or furthest to the left/bottom of the periodic table), though Hydrogen is a special case and is never the central atom. Connect the terminal atoms to the central atom using single bonds. Note: Each bond drawn uses up 2 electrons from your total count.

3. Distribute Electrons to Terminal Atoms

Use the remaining electrons to complete the octets of the terminal atoms.

• Exception: Hydrogen can only ever have a duet (two electrons) and cannot have lone pairs.

4. Place Remaining Electrons on the Central Atom

If there are any electrons left over after completing the terminal octets, place them on the central atom as lone pairs.

5. Optimize the Structure

If the central atom still does not have a full octet, you must rearrange the electrons. Erase a lone pair from a terminal atom and turn it into a double or triple bond with the central atom. Continue this process until the central atom satisfies the octet rule.

Exceptions to the Octet Rule

While the octet rule is a helpful guide, it is not universal. There are several notable exceptions:

• Hydrogen: As mentioned, Hydrogen is stable with only two electrons (a duet).
• Electron-deficient atoms: Some atoms, such as Boron, can form stable structures with fewer than eight electrons.
• Hypervalent atoms: Atoms in row 3 or below on the periodic table can exceed eight electrons because they have access to $d$ orbitals.
  • Important: Row 2 elements (like Carbon, Nitrogen, or Oxygen) can never be hypervalent because they only have access to $s$ and $p$ orbitals, limiting them to a maximum of eight electrons.
• Odd-electron molecules: Also known as free radicals, these molecules contain an odd total number of valence electrons, making it impossible for every atom to have an octet.