4.3 Covalent structures

Based on formation of covalent bonds in a molecule
Important to show:
- Bonding pairs of electrons (showing covalent bond as single, double, or triple bonds
- Non-bonding pairs of electrons (often called lone pairs, not involved in bonding)
Each line represents 1 pair of electrons (2 electrons)
Each dot represents 1 electron
The Octet Rule states that elements tend to lose electrons, or gain electrons, or share electrons in order to acquire noble gas configuration (8 electrons in valence shell)

When you have a bond between cation [NH₄]⁺ and anion [NO₃]^–, you have ionic bond between the ions, however the cation and anion are covalent in nature
Here is diagram of ammonium nitrate NH₄NO₃

Lewis structures don’t tell anything about actual shape. They are 2D
VSEPR theory takes into account 3D shape and bond angles
Theory is: since electrons are negatively charged subatomic particles, pairs of electrons repel one another to be as far apart as possible in space
Every pair of electrons are described as occupying an electron domain
Electron Domain Geometry based on total number of electron domains
Molecular Geometry gives shape of molecule

Lone pairs occupy more space than bonding pairs, so they decrease bond angle between bonding pairs

Occurs when there is more than one possible arrangement for a double bond in a molecule
These multiple possible arrangements contribute to an overall structure called resonance forms.
The actual electronic structure of the species is called a resonance hybrid of these resonance forms.
In order to show this idea of resonance, the contributing resonance forms are linked via a double arrow
One best known example of resonance is benzene C₆H₆ which forms delocalized benzene ring

Whether a molecule itself is polar or not
Easy method:
- If there are non-polar bonds, molecule is non polar
- If there are polar bonds and molecule is symmetrical, molecule is non-polar
- If there are polar bonds and molecule is asymmetrical, molecule is polar

Allotropes of same element can vary in both chemical and physical properties
Carbon has 4 allotropes: graphite, diamond, graphene, C₆₀ fullerene
Covalent Network Solids is one with atoms held together by covalent bonds in 3D lattice structures.
Covalent Network Solid examples include graphite, diamond, graphene, and SiO₂
C₆₀ is molecular because had weak London forces between molecules

Silicon dioxide in its most common crystalline form is called quartz. This is another example of a three-dimensional covalent network solid