Unit 3) molecular orbitals 1 -formation,sigma + pi bonds, hybridisation, bonding continuum Flashcards
(37 cards)
VSEPR (valence shell electron pair repulsion) cannot explain the bonding in all compounds. how can bonding be explained in more complex molecules
in more complex molecules the bonding cam be explained in terms of molecular orbital theory
what does molecular orbital theory describe
- describes how atomic orbitals merge and overlap when atoms bond to form molecules
- explains how the valence electrons are distributed within molecular orbitals spread across the whole molecule.
electrons in atoms occupy ….
electrons in molecules occupy….
….atomic orbitals
…..molecular orbitals
molecular orbitals are populated by____, beginning with the orbital with the lowest ____.
a molecular orbital is full when it contains ____ electrons of opposite _____
electrons
energy
2
spins
how are molecular orbitals generated
by combining atomic orbitals
the number of molecular orbitals formed is always equal to the number of…
atomic orbitals that combine
eg if two atomic orbitals combine , then 2 molecular orbitals will be formed
diagram of formation of molecular orbitals
- bonding molecular orbital
- antibonding molecular orbital
look in notes
bonding molecular orbitals are lower in energy than either of the two atomic orbitals used to produce it.
antibonding molecular orbitals are of a higher energy than either of the two molecular orbitals used to produce it
h2 molecule has only 2 electrons , the bonding molecular orbital occupies both of them , the antibonding molecular orbital is empty
how is a sigma bond formed
- when the atomic orbitals overlap along the axis of the bond , a covalent bond called a sigma bond results.
- this is normally referred to as ‘end on’ overlap
the three different types of sigma bond formation
- s and s atomic orbitals overlapping
- p and s atomic orbitals overlapping
- p and p atomic orbitals overlapping
LOOK IN NOTES FOR DIAGRAMS
how does a pi bond arise and how is it formed
- arises when atoms form multiple bonds
- pi bonds are formed by the overlap of two parallel p atomic orbitals, that overlap ‘side on’ or laterally
pi bonds are ____ than sigma bonds
weaker
- shown by the fact that the carbon-carbon double bond (612) is not twice as strong as the carbon-carbon single bond
what is hybridisation
hybrid orbitals are______
- process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals
- degenerate
how many bonds does carbon form
four covalent bonds
electron promotion - which electrons can be promoted
Because of the very small energy gap between the 2s and 2p sublevels, it is relatively easy for an electron to be promoted form 2s 🡪 2p
what is the electron configuration of carbon in its ground state
1s2 2s2 2p2
so has 2 half filled orbitals
if carbon has 2 half filled orbitals why does it
- form 4 covalent bonds
- have a tetrahedral structure
- carbon has one empty 2p orbital and we can obtain more half filled orbitals by promoting one of the paired 2s electrons into the empty 2p orbital. the carbon now has 4 unpaired electrons and can form 4 bonds
- to explain why they adopt a tetrahedral arrangement we introduce the concept of hybridisation:the process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals . in the case of a carbon atom, the one 2s orbital can mix with the 3 2p orbitals to form 4 hybrid orbitals known as sp3 hybrid orbitals
the number of hybrid orbitals formed is always equal to the number of atomic orbitals that were ____
mixed
the four sp3 orbitals are ______ as they are identical in shape and they will point towards the corners of a tetrahedron to minimise _____
degenerate,
repulsion
bonding in alkanes ;hybridisation in saturated compounds
- how are carbon atoms in alkanes hybridised
- explain how binding in ethane and methane takes place(how the molecular orbitals form)
- what arrangement do the hybridised orbitals take
- we already know that carbon has four sp3 hybrid orbitals (sp3 hybridised)
- in an alkane (take c2h6-ethane for example) the four sp3 hybrid orbitals on each carbon will overlap end on with four other orbitals.
- so in ethene the 2 carbons will bond as follows: each carbon with three hydrogen 1s orbitals and one sp3 hybrid orbital on the other carbon atom. Four bonds will be formed and they will adopt a tetrahedral arrangement
- in methane molecular orbitals form between carbon and hydrogen- the carbon bonds with four 1s hydrogen orbitals, overlapping end on
bonds in alkanes eg in ethene and methane are…
sigma bonds
- all 7 bonds made in ethene are sigma bonds
- all 4 bonds in methane are sigma bonds
bonding in alkenes
- how are the carbon atoms in alkenes hybridised
- what arrangement do the hybridised orbitals take
- after electron promotion to create more half filled orbitals,the 2s orbital and 2 out of 3 of 2p orbitals mix om each carbon to form three sp2 hybrid orbitals
- to minimise repulsion these orbitals form a trigonal planar arrangement-they lie in the same plane with an angle of 120 degrees between them
bonding in alkenes
explain how bonding in ethene takes place (how do molecular orbitals form)
- the 2 / or each carbon atoms in ethene use its three sp2 hybrid orbitals to form a sigma bond with 2 hydrogen atoms and a sigma bond with the other carbon atom
- the unhybridised 2p orbitals left on the carbon overlap side on to form a pi bond
are all carbon atoms in alkenes sp2 hybridised?
no
- although in ethene both carbon atoms are sp2 hybridised , this isnt true for all carbon atoms in alkenes. in propene (CH3CH–CH2)for example,only 2carbon atoms of the double bond are sp2 hybridised , the third carbon atom is sp3 hybridised
bonding in alkynes
all alkynes contain a carbon to carbon_____ bond
triple
