Unit 3) molecular orbitals 1 -formation,sigma + pi bonds, hybridisation, bonding continuum

Question 1

VSEPR (valence shell electron pair repulsion) cannot explain the bonding in all compounds. how can bonding be explained in more complex molecules

Answer 1

in more complex molecules the bonding cam be explained in terms of molecular orbital theory

Question 2

what does molecular orbital theory describe

Answer 2

• 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.

Question 3

electrons in atoms occupy ….

electrons in molecules occupy….

Answer 3

….atomic orbitals

…..molecular orbitals

Question 4

molecular orbitals are populated by____, beginning with the orbital with the lowest ____.
a molecular orbital is full when it contains ____ electrons of opposite _____

Answer 4

electrons
energy
2
spins

Question 5

how are molecular orbitals generated

Answer 5

by combining atomic orbitals

Question 6

the number of molecular orbitals formed is always equal to the number of…

Answer 6

atomic orbitals that combine

eg if two atomic orbitals combine , then 2 molecular orbitals will be formed

Question 7

diagram of formation of molecular orbitals

• bonding molecular orbital
• antibonding molecular orbital

Answer 7

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

Question 8

how is a sigma bond formed

Answer 8

• 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

Question 9

the three different types of sigma bond formation

Answer 9

• s and s atomic orbitals overlapping
• p and s atomic orbitals overlapping
• p and p atomic orbitals overlapping
  LOOK IN NOTES FOR DIAGRAMS

Question 10

how does a pi bond arise and how is it formed

Answer 10

• 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

Question 11

pi bonds are ____ than sigma bonds

Answer 11

weaker

- shown by the fact that the carbon-carbon double bond (612) is not twice as strong as the carbon-carbon single bond

Question 12

what is hybridisation

hybrid orbitals are______

Answer 12

• process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals
• degenerate

Question 13

how many bonds does carbon form

Answer 13

four covalent bonds

Question 14

electron promotion - which electrons can be promoted

Answer 14

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

Question 15

what is the electron configuration of carbon in its ground state

Answer 15

1s2 2s2 2p2

so has 2 half filled orbitals

Question 16

if carbon has 2 half filled orbitals why does it

• form 4 covalent bonds
• have a tetrahedral structure

Answer 16

• 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

Question 17

the number of hybrid orbitals formed is always equal to the number of atomic orbitals that were ____

Answer 17

mixed

Question 18

the four sp3 orbitals are ______ as they are identical in shape and they will point towards the corners of a tetrahedron to minimise _____

Answer 18

degenerate,

repulsion

Question 19

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

Answer 19

• 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

Question 20

bonds in alkanes eg in ethene and methane are…

Answer 20

sigma bonds

• all 7 bonds made in ethene are sigma bonds
• all 4 bonds in methane are sigma bonds

Question 21

bonding in alkenes

• how are the carbon atoms in alkenes hybridised
• what arrangement do the hybridised orbitals take

Answer 21

• 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

Question 22

bonding in alkenes

explain how bonding in ethene takes place (how do molecular orbitals form)

Answer 22

• 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

Question 23

are all carbon atoms in alkenes sp2 hybridised?

Answer 23

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

Question 24

bonding in alkynes

all alkynes contain a carbon to carbon_____ bond

Answer 24

triple

Question 25

bonding in alkynes

• how are the carbon atoms in alkynes hybridised
• what arrangement do these orbitals take

Answer 25

• in a carbon atom the 2s orbital mixes with 1 out of 3 of the 2p orbitals to form two degenerate sp hybrid orbitals. the remaining two 2p orbitals are left unhybridised
• to minimise repulsion , the angle between the two sp hybrid orbitals will be 180 degrees

Question 26

bonding in alkynes

- explain how bonding in ethyne takes place (how do molecular orbitals form)

Answer 26

• in ethyne (c2h2) both carbon atoms are sp hybridised
• each carbon atom uses its two sp hybrid orbitals to form sigma bonds with one hydrogen atom and the other carbon atom
• the unhybridised 2p orbitals left on the carbon atom overlap side on to form two pi bonds

Question 27

what are the 3 types of bonds that exist between atoms

Answer 27

• non polar covalent
• polar covalent
• ionic bonds

Question 28

bonding continuum

Answer 28

• a sort of scale- non polar covalent bonding lies on one end of the continuum and ionic bonding at the other , polar covalent bonding lies between the two extremes

Question 29

non polar covalent bonss

• what are they
• when do they exist

Answer 29

• when bonding electrons are shared equally between the two atoms of the bond
• such bonds exist between two identical atoms of different elements with the same electronegativity values

Question 30

polar covalent bonds

• what are they
• when do they exist

Answer 30

• where the bonding electrons are shared unequally , with the atom with the larger value of electronegativity having the greater share of electrons . this atom will carry a partial negative charge , and the atom with the lower electronegativity carrying a partial positive charge.

Question 31

atoms in polar covalent bonds do not have full fledged charges like ions in ionic compounds, but what does the partial charge imply?

Answer 31

these partial charges imply that polar covalent bonds have some ionic character.
the more polar the covalent bond the greater ionic character

Question 32

ionic bonds

• what are they
• when do they exist

Answer 32

• in ionic bonds the bonding electrons have been transferred from one atom to another resulting in the formation of ions
• in most ionic compounds the bonding electrons are not completely transferred and this implies some degree of electron sharing. ie covalent character
• they exist when two atoms have a very large difference in electronegativity

Question 33

these three different types of bonding can also be described in terms of _____ ______

Answer 33

molecular orbitals

Question 34

bonding molecular orbital in non polar covalent bond

Answer 34

• the bonding molecular orbital in a non polar covalent bond is completely symmetrical about the midpoint between the two atoms of the bond

Question 35

bonding molecular orbital in polar covalent bond

Answer 35

the bonding molecular orbital in a polar covalent bond is asymmetrical about the mid point between the two atoms. it is skewed towards the atom with a greater electronegativity

Question 36

bonding molecular orbital in ionic bond

Answer 36

an ionic bond is an extreme form of a polar covalent bond. the orbital only embraces one atom in the bond (the bonding pair of electrons do occupy the orbital)
- extreme case of asymmetry , an electron has been stripped from one atom and transferred to another resulting in formation of ions

Question 37

hybridisation and how many sigma/pi bonds 
saturated c-c bond 
unsaturated c--c 
unsaturated c---c 
have?

Answer 37

c-c - one sigma , sp3
c–c one sigma , one pi, sp2
c—c one sigma , two pi sp