3.2 Covalent bonding

Question 1

Why is BH3 able to form a coordinate bond but not CH4

Answer 1

Because BH3 has 6 outer electrons in total as 3 pairs are shared, so it has a incomplete outer shell so is able to gain a pair of electrons in a coordinate bond

However, CH4 has 4 bond pairs so 8 outer electrons so is not able to form a coordinate bond

Question 2

What are lone pairs of electrons

Answer 2

Electrons that aren’t part of a covalent bond but are still on one of the elements

Question 3

How do you calculate the lone pairs of electrons

Answer 3

Eg NH3:
Find the group of the central atom (N) which is group 5
Then the number of molecules bonded to it (3) is the number of bond pairs

Do 5-3 then divide by 2 =1 to get the lone pair of electrons

So NH3 has one lone pair of electrons

Question 4

How do bond pairs affect the shape of molecules

How much does the angle between electrons decrease when a lone pair of electrons is present, from the usual angle

Answer 4

. Repulsion between two lone pairs is far greater than repulsion between two bond pairs]. Also, repulsion between a bond pair and lone pair is greater than two bond pairs

This effect can push the other electrons bonded to the central atom away from the lone pairs

SO the bond angle between two bond pairs decreases by 2.5 degrees

Question 5

What shape forms when there are two pairs of bonded electrons around the central atom

What is the bond angle

Answer 5

Linear shape a-b-a

180 degrees because the bond pairs repel so get as far away from each other as possible

Question 6

What shape forms with 3 bond electrons and no lone pairs around a central atom

What is the angle

Answer 6

Trigonal planar

Angle 120 degrees as the electrons in the bond pairs repel so get as far away from each other as possible

Question 7

What shape forms if there are 2 bond pairs and one lone pair around a central atom

What is the bond angle

Answer 7

Bent v shape occurs as the lone pair repels the two bond pairs more than they repel each other

Bond angle decreases by 2.5 degrees from original trigonal planar
so bond angle is 117.5

Question 8

What shape forms when there are 4 pairs of electrons around a central atom, and no lone pairs

What is the bond angle

Answer 8

Tetrahedral shape forms for all molecules
Except for molecules in the group with platinum in it

Bond angles are 109.5 degrees

Question 9

What shape forms when there are 3 bond pairs and 1 lone pair around a central atom

What is the bond angle

Answer 9

Trigonal pyramidal
With the lone pair centred at the top

SO angle is 107 degrees

Question 10

What shape forms when there are 2 bond pairs and 2 lone pairs bonded around a central atom

What is the bond angle

Answer 10

A bent v shape forms

104.5 degrees

Question 11

What shape forms when there are 5 bond pairs of electrons centred around an atom

What are the bond angles

Answer 11

Forms a trigonal bi-pyramidal
With bonds top and bottom, and two on right side and one on left

Form 90 degrees and 120 degrees

Question 12

What shape forms with 4 pairs bonded and one lone pair of electrons centred around an atom

What are the bond angles

Answer 12

See saw, with the lone pair at the top

Bond angles are 117.5 and 87.5 degrees

Question 13

What shape forms with 3 bond pairs and 2 lone pairs around a central atom

Answer 13

T shape or trigonal planar depending on where the lone pairs go

If they go opposite each other, the charges cancel out so bond angles are 120 degrees, so it forms a trigonal planar shape

If they go next to each other, they do not cancel out so the bond angle is less than 90 degrees

Question 14

What shape forms with 6 bond pairs and no lone pairs

What are the bond angles

Answer 14

An octahedral shape

With all bond angles being 90 degrees

Question 15

What shape forms with 5 bond pairs and one lone pair

Answer 15

Square pyramidal, and the bond pair goes at the top

Bond angles are 87.5 degrees

Question 16

What shape forms with 4 bond pairs and 2 lone pairs

Answer 16

Square planar where the bond pairs are at the top and bottom so the charge repulsion cancels out because it is symmetrical

angles are all 90 degrees

Question 17

What is electron pair repulsion theory

Answer 17

Each pair of electrons around an atom will repel the other electrons, so therefore the pairs will take positions as far apart as possible to minimise repulsion

Question 18

Why does oxygen typically form double bonds

Answer 18

Oxygen is in group 6 so needs to gain two electrons for a full outer shell, so only having a single bond will gain it 1 electron

So double bond allows it to gain 2 electrons

Question 19

What shape forms when there are two bond pairs and 3 lone pairs

Answer 19

Linear

180 degrees