Chemistry- Chapter 8-> Section 8.1

Lewis structures show [blank_start]bonds[blank_end], but Lewis structures can't [blank_start]predict[blank_end] so they came up with two new models: the [blank_start]valence[blank_end] bond theory and the [blank_start]molecular[blank_end] orbital theory.

Valence bond theory: a model of chemical bonding that states that atoms fill their valence shells by filling vacancies in particular [blank_start]orbitals[blank_end]. Partially filled orbitals of two atoms [blank_start]overlap[blank_end], and a [blank_start]covalent[blank_end] bond is formed. This theory is also called the [blank_start]localized[blank_end] electron theory.

Sigma bonds: The orbitals overlap [blank_start]end[blank_end] to end. The orbital overlap is located [blank_start]between[blank_end] the nuclei. They can form from various orbital combinations: two s orbitals, two p orbitals, or an s and a p orbital. Sigma bonds always form [blank_start]first[blank_end].

Pi bonds: The orbitals overlap [blank_start]side[blank_end] to side. The orbital overlap is located above and [blank_start]below[blank_end] the nuclei. Pi bonds can combine with [blank_start]sigma[blank_end] bonds and other pi bonds to form [blank_start]double[blank_end] and triple bonds. One sigma and one pi= (double bond.) One sigma and two pi= ([blank_start]triple[blank_end] bond.) Pi bonds always form [blank_start]after[blank_end] sigma bonds.

Sigma bonds are the [blank_start]strongest[blank_end]. Pi bonds are [blank_start]weaker[blank_end] than sigma bonds because the electrons are spread out over a greater volume of space around the bond axis. The combination of a sigma and a pi bond is [blank_start]stronger[blank_end] than either bond by itself.

Resonance: the state of molecules that can have [blank_start]two[blank_end] or more possible Lewis structures. No [blank_start]single[blank_end] Lewis structure can completely describe electron distribution in resonance molecules. There are NO single or double bonds in these molecules- the bonds are experimentally identical. A resonance molecule is depicted with a [blank_start]hybrid[blank_end] Lewis structure.

Some molecules form without an octet for each bonded atom: this happens when there is an [blank_start]odd[blank_end] number of valence electrons, electron [blank_start]deficiency[blank_end], and hypervalent molecules.

Some molecules have an odd number of valence electrons to share. Substances with these unpaired electrons are called [blank_start]free[blank_end] [blank_start]radicals[blank_end] and tend to be fairly [blank_start]reactive[blank_end].

Electron Deficiency: elements with [blank_start]less[blank_end] than 8 valence electrons.

Hypervalent Molecules: The central atom has [blank_start]more[blank_end] than 8 valence electrons. This occurs only in Period [blank_start]3[blank_end] and higher. These molecules are described as having [blank_start]expanded[blank_end] octets.

The Molecular Orbital Theory: the orbitals of an atom are replaced by [blank_start]new[blank_end] orbitals when a molecule forms. Each molecule has a [blank_start]unique[blank_end] set of molecular orbitals equal to the [blank_start]sum[blank_end] of the atomic orbitals of the original atoms. Molecular orbitals are much [blank_start]larger[blank_end] than atomic orbitals, up to the size of the entire molecule. Orbitals are ranked in order of [blank_start]increasing[blank_end] energy.

[blank_start]Paramagnetism[blank_end]: the attraction of a molecule to a magnetic field

Factors affected by the arrangement of a molecule's electrons: - bond [blank_start]formation[blank_end] (bonding and antibonding) - bond [blank_start]type[blank_end] (single, double, triple bonds) - molecular properties (like paramagnetism)

Bonding orbitals: - [blank_start]constructive[blank_end] interference - [blank_start]between[blank_end] nuclei - [blank_start]lower[blank_end] in energy than the atomic orbitals from which they form - stable molecules

Antibonding orbitals: - [blank_start]destructive[blank_end] interference - [blank_start]outside[blank_end] of the molecule, far away from the nuclei - [blank_start]higher[blank_end] in energy than the atomic orbitals - unstable