Covalent Bonds, Polarity, and Electron Counting (Notes)

Covalent Bonds and Polarity

Covalent bonds are defined as the sharing of a pair of electrons between atoms for each bond.
In a covalent bond, two electrons are involved in the shared pair (one contributed by each bonded atom).
Polarity of a molecule depends on charge distribution and whether there is a dipole moment; polarity is not just about having bonds but about how those bonds are arranged and whether their dipoles cancel or reinforce.
Dipole: a separation of charge within a molecule leading to a molecule having a positive end and a negative end.
If a molecule is symmetrical with identical substituents, the bond dipoles can cancel, potentially making the molecule nonpolar; if asymmetrical, the molecule tends to be polar.
In discussion of polarity, the term ipole (dipole moment) is central to determining polar vs nonpolar character.

Consider a central atom with three covalent bonds and one lone pair.
Three bonds contribute a total of 3 \times 2 = 6 electrons to the bonding framework.
There is one lone pair around the central atom, contributing 2 electrons as non-bonding electrons.
Total electrons around the central atom = 6 + 2 = 8 electrons (octet).
This arrangement involves four electron domains around the central atom: three bonding pairs and one lone pair.
Geometry implications (using VSEPR):
- Electron-domain geometry: tetrahedral (four electron domains).
- Molecular geometry: trigonal pyramidal when there are 3 identical substituents and 1 lone pair (e.g., NH3).
Bond angles are slightly less than the ideal tetrahedral angle due to lone-pair repulsion, typically around \approx 107^{\circ} for AX3E1 species.
The phrasing in the transcript (our different types of electrons) reflects counting three bonding electron pairs and one lone pair as distinct electronic regions.

When constructing Lewis structures, there can be multiple valid ways to draw the structure that satisfy the same overall electron count and octet rules.
The question raised in the transcript about how to obtain the last six electrons after removing two electrons highlights the idea that you count electrons in bonds (six) and add back in the lone pair (two) to reach the octet (eight) around the central atom.
There are many valid ways to write structures sometimes; this can refer to different representations of the same molecule (e.g., placing electrons differently while preserving total count), and it can lead into concepts like resonance in molecules where electrons are delocalized across multiple structures.
Key takeaway: Lewis structures are a convenient bookkeeping tool; while there can be multiple drawings for the same molecule, the total number of electrons and formal charges must balance.

The presence of a lone pair on the central atom (as in AX3E1) typically contributes to molecular polarity because the electron distribution is asymmetrical.
Polarity influences physical properties such as boiling/melting points, solubility, and interactions with other molecules.
Understanding the combination of covalent bonding, lone pairs, and geometry helps explain real-world behavior of molecules (e.g., ammonia NH3 is polar; CO2 is nonpolar due to symmetry even though it has polar bonds).

Covalent bonds involve sharing of electron pairs; each bond uses two electrons.
Polarity depends on charge distribution and dipole interactions; symmetry can cancel dipoles to yield nonpolar molecules.
A central atom with 3 bonds and 1 lone pair has 4 electron domains, leading to a tetrahedral electron geometry and a trigonal pyramidal molecular geometry with a typical bond angle around 107^{\circ}.
Electron counting around the central atom includes both bonding electrons (6) and lone pair electrons (2), totaling 8 to satisfy the octet.
There can be different valid ways to draw Lewis structures for the same molecule; resonance and alternative representations are possible when electrons are distributed differently while keeping the same overall count.

Bond electrons per bond: 2
Lone pair electrons (one lone pair): 2
Total electrons around central atom (for AX3E1 example): 6 + 2 = 8
Number of electron domains around central atom: 4
Ideal tetrahedral angle: 109.5^{\circ}; actual AX3E1 angle: ~107^{\circ} due to lone-pair repulsion.