3.1 Molecular Orbitals Flashcards
When do molecular orbitals form?
when atomic orbitals combine
What is the number of molecular orbital 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 positively charged nuclei and the negatively charges electrons in the bonding molecular orbital is the basis of bonding between atoms.
How many electrons can each molecular orbital hold?
2
Explain bonding molecular orbitals in non-polar covalent bonds?
the bonding molecular orbitals is symmetrical about the midpoint between two atoms
Explain bonding molecular orbitals in polar covalent bonds?
the bonding molecular orbitals are asymmetric about the midpoint between two atoms
explain differences between electronegitivity and bonding electrons
The atom with the greater value for electronegativity has the greater share of the bonding electrons.
Explain bonding molecular orbitals in ionic bonds?
Ionic compounds
are an extreme case of asymmetry,
with the bonding molecular orbitals being almost
entirely located around just one atom, resulting in the formation of ions.
How are sigma bonds formed?
end-on overlap of molecular orbitals along the axis of the covalent bond
How are pi bonds formed?
side-on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bond
How can the electronic configuration of an isolated carbon atom be explained?
- cannot explain the number of bonds formed by carbon atoms in molecules.
- The bonding and shape of molecules of carbon can 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.
- These hybrid orbitals are degenerate.
Explain bonding in alkanes
- 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.
Explain bonding in alkenes
- can 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.
Explain bonding in benzene and other aromatics
- can be described in terms of sp2
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.
Explain bonding in alkynes
- can 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.
Why can molecular orbitals be used to explain weather organic molecules are coloured or colourless?
Electrons fill bonding molecular orbitals, leaving higher energy antibonding orbitals unfilled.
What does HOMO mean?
The highest bonding molecular orbital containing electrons is called the highest occupied molecular orbital
What does LUMO mean?
The lowest antibonding molecular orbital is
called the lowest unoccupied molecular orbital (LUMO).
What could cause electrons to be promoted from HOMO to LUMO
absorption of electromagnetic energy
What causes most organic molecules to appear colourless and what does it result in?
- the energy difference between HOMO and LUMO is relatively large
- results in absorption of light from the ultraviolet region of the spectrum
What is a chromophore?
a group of atoms within a molecule that is responsible for absorption of light in the visible region of the spectrum.
