chemistry - 4.1 Flashcards
When atomic orbitals combine, what are formed?
Molecular orbitals.
What is the number of molecular orbitals formed equal to?
The number of atomic orbitals that combine.
What does the combination of two atomic orbitals result in?
The formation of a bonding molecular orbital and an antibonding orbital.
What does the bonding molecular orbital encompass?
Both nuclei.
What is the basis of bonding between atoms?
The attraction of the positively charged nuclei and the negatively charged electrons in the bonding molecular orbital.
What can each molecular orbital hold?
A maximum of two electrons.
In a non-polar covalent bond, where is the bonding molecular orbital?
Symmetrical about the midpoint between two atoms.
What do polar covalent bonds result from?
Bonding molecular orbitals that are asymmetric about the midpoint between two atoms.
In a polar covalent bond, which atom has the greater share of the bonding electrons?
The atom with the greater value for electronegativity.
What are an extreme case of asymmetry?
Ionic compounds, with the bonding molecular orbitals being almost entirely located around just one atom, resulting in the formation of ions.
What are sigma molecular orbitals or sigma bonds?
Molecular orbitals that form by end-on overlap of atomic orbitals along the axis of the covalent bond.
What are pi molecular orbitals or pi bonds?
Molecular orbitals that form by side-on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bond.
How can the bonding and shape of molecules of carbon be explained?
By hybridisation.
What is hybridisation?
The process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals.
What are these hybrid orbitals?
Degenerate.
In alkanes, what occurs during hybridisation?
The 2s orbital and the three 2p orbitals of carbon hybridise to form four degenerate sp3 hybrid orbitals. These adopt a tetrahedral arrangement. The sp3 hybrid orbitals overlap end-on with other atomic orbitals to form sigma bonds.
How can the bonding in alkenes be described?
In terms of sp2 hybridisation.
The 2s orbital and two of the 2p orbitals hybridise to form three degenerate sp2 hybrid orbitals. These adopt a trigonal planar arrangement. The hybrid sp2 orbitals overlap end-on to form
sigma bonds. The remaining 2p orbital on each carbon atom of the double bond is unhybridised and lies perpendicular to the axis of the sigma bond. The unhybridised p orbitals overlap side-on to form pi bonds.
How can the bonding in benzene and other aromatic systems be described?
In terms of sp^2 hybridisation. The six carbon atoms in benzene are arranged in a cyclic structure with sigma bonds between the carbon atoms. The unhybridised p orbitals on each carbon atom overlap side-on to form a pi molecular system, perpendicular to the plane of the sigma bonds. This pi molecular system extends across all six carbon atoms. The electrons in this system are delocalised.
How can the bonding in alkynes be described?
In terms of sp hybridisation. The 2s orbital and one 2p orbital of carbon hybridise to form two degenerate hybrid orbitals. These adopt a linear arrangement. The hybrid sp orbitals overlap end-on to form sigma bonds. The remaining two 2p orbitals on each carbon atom lie perpendicular to each other and to the axis of the sigma bond. The unhybridised p orbitals overlap side-on to form two pi bonds.
What can molecular orbital theory be used to explain?
Why organic molecules are colourless or coloured.
What do electrons fill?
Bonding molecular orbitals, leaving higher energy anti bonding orbitals unfilled.
What is the highest bonding molecular orbital containing electrons called?
The highest occupied molecular orbital (HOMO).
What is the lowest anti bonding molecular orbital called?
Lowest unoccupied molecular orbital (LUMO).
What can cause electrons to be promoted from HOMO to LUMO?
Absorption of electromagnetic energy.
