Lewis Dot Structures and Covalent Bonding

Atomic Representation: Atoms are represented by their chemical symbol, with valence electrons shown as dots around four coordination sites.
Electron Placement: Place one electron in each site before pairing them up.
Group Similarities: Elements in the same group have similar dot structures because they possess the same number of valence electrons.
Carbon Configuration: Carbon has the electron configuration $1s^2 2s^2 2p^2$ , providing $4$ valence electrons in its outer shell.

Covalent Bond Representation: Unpaired electrons from two atoms combine to form covalent bonds, represented as lines containing $2$ electrons each.
Typical Valence Bonding:
- Carbon: Forms $4$ bonds.
- Nitrogen: Possesses $5$ valence electrons; forms $3$ bonds and $1$ lone pair.
- Oxygen: Forms $2$ bonds and $2$ lone pairs.
- Fluorine: Forms $1$ bond and $3$ lone pairs.

Sigma and Pi Bonds:
- The initial covalent bond between two atoms is a sigma ( $\sigma$ ) bond.
- Double bonds consist of one sigma ( $\sigma$ ) bond and one pi ( $\pi$ ) bond.
- Triple bonds consist of one sigma ( $\sigma$ ) bond and two pi ( $\pi$ ) bonds.
Bond Lengths: Single bonds are the longest, double bonds are shorter, and triple bonds are the shortest.
Formal Charge:
- Occurs when the number of electrons an atom contributes differs from its typical neutral valence count.
- Neutral nitrogen in ammonia contributes $5$ valence electrons ( $3$ covalent bonds plus $1$ lone pair).
- In the ammonium polyatomic ion, nitrogen contributes $4$ electrons ( $1$ per bond), yielding a formal positive charge.

Octet Rule: Elements fill their outer shell with $8$ electrons to complete the $n = 2$ shell.
Hydrogen: Requires only $2$ electrons to fill the $n = 1$ shell.
Expanded Octets: Larger atoms such as phosphorus and sulfur can form $5$ or $6$ covalent bonds.