Showing Covalent Bonding Using Dot Cross Diagrams

• If you work through this powerpoint you should be able to draw a dot cross diagram for any covalent molecule provided you are given the displayed formula of the molecule.

(Displayed formula-atoms shown as letters and covalent bonds shown as sticks)

C

H

H

HH

Methane CH4

C

H

H

HH

Using the stick model as a guide, draw the atoms as circles. Wherever there is a bond, they overlap.

Wherever there is a bond draw in a pair of electrons, a cross from the C and a dot from the H.

Now check all of the carbon’s electrons have been used up. C has 4 electrons in its outer shell and there are 4 crosses in our diagram. They are all accounted for so we do not need to draw any more crosses.

Finally, check how many electrons the carbon “thinks” it has in its outer shell-8-so that’s ok. Each H atom” thinks” it has a full outer shell so that is OK also. The diagram is complete.

N

H

HH

Ammonia NH3

N

H

HH

Using the stick model as a guide, draw the atoms as circles. Wherever there is a bond, they overlap.

Draw in a pair of electrons wherever the atoms over lap.; a dotfrom the N and a cross from the H

We have only drawn in 3 dots but N has 5 electrons in its outer shell. We need to draw in another 2 dots. We always draw these as a pair (lone pair).

Finally check. Does the N think it has a full outer shell (i.e. 8)? Yes it does so that is OK. Also each H “thinks” it has a full outer shell so that’s Ok. The diagram is complete.

O

H

H

Water H2O

O

H

H

Using the stick model as a guide, draw the atoms as circles. Wherever there is a bond, they overlap.

Draw in a pair of electrons wherever the atoms over lap.; a dot from the O and a cross from the H

We have only drawn in 2 dots but O has 6 electrons in its outer shell. We need to draw in another 4 dots. We always draw these as 2 pairs (lone pairs).

Finally check. Does the O think it has a full outer shell (i.e. 8)? Yes it does so that is OK. Also each H “thinks” it has a full outer shell so that’s Ok. The diagram is complete.

FH

Hydrogen Fluoride HF

FH

Using the stick model as a guide, draw the atoms as circles. Wherever there is a bond, they overlap.

Draw in a pair of electrons wherever the atoms over lap.; a dot from the F and a cross from the H

We have only drawn in 1 dots but F has 7 electrons in its outer shell. We need to draw in another 6 dots. We always draw these as 3 pairs (lone pairs).

Finally check. Does the F think it has a full outer shell (i.e. 8)? Yes it does so that is OK. Also each H “thinks” it has a full outer shell so that’s Ok. The diagram is complete.

ClCl

Chlorine Cl2

ClClDraw in a pair of electrons wherever the atoms over lap.; a dot from one Cl and a and a cross from the other

We have only drawn in 1 electron for each chlorine but each one has 7 electrons in its outer shell. We need to draw in another 6 electrons for each chlorine. We always draw these as 3 pairs (lone pairs).

Each chlorine ”thinks” it has 8 electrons in its outer shell. The diagram is complete

Oxygen O2

O O

OOThe stick model shows a double bond, so where the oxygens overlap there should be 2 pairs of electrons. 2 crosses come from the oxygen on the right, 2 dots from the one on the left.

The oxygen on the left only has 2 electrons in its outer shell. It should have 6 so we need to draw in 2 more pairs

Doing the same for the one on the right...

Each oxygen “thinks it has 8 electrons in its outer shell. The diagram is complete

Ethene C2H4

C C

CCH H

H

H

H

H H

HFill in the electrons in the double bond.

Fill in the electrons for the C-H bonds.. Use a different symbol for the H electrons (red circle)

C atoms have 4 electrons each already. We do not need to draw any more in

All atoms “think” that they have a full outer shell of electrons. The diagram is complete.

N N

NN

Nitrogen N2

There is a triple bond , so a total of 3 pairs of electrons are shared

Each N only has 3 of its electrons accounted for. N has 5 in its outer shell, so there must be one pair outside the bond.

Each N “thinks” it has 8 electrons in its outer shell, the diagram is complete

C C

CC

Ethyne C2H2

H H

H H

There is a triple bond between the two C atoms , so a total of 3 pairs of electrons are shared between them

The bond between each C and H is single so they share one pair of electrons.

Each carbon now has 4 electrons in its outer shell-all are accounted for, no more need to be added.

Each carbon “thinks” it has 8 electrons in its outer shell, each H ”thinks” it has 2, the diagram is complete.

Carbon Dioxide

C OO

CO OThere is a double bond between each O atom and the carbon. Each bond has 4 elecrons shared.All 4 of the C’s electrons are accounted

for.

Only 2 of the O’s 6 electrons are accounted for. Draw in 4 more electrons for each O.

Now the O’s and the C atoms think they have 8 electrons in their outer shell. The diagram is complete

Carbonic Acid H2CO3

C

O

H CO

O

O

H H

OO

H

Place electrons in the bonds first

All the C’s electrons are accounted for, but the O’s need more electrons outside of the bonds

Each atom “thinks” it has a full outer shell . The diagram is complete.

C

O

H CO

O

O

OO

H

Carbonic Acid Donates Two H+ ions and Becomes a Carbonate Ion CO3

2-

H HH+ H+

The H’s leave the molecule but leave their electron behind. This means the H’s have a +1 charge. The two bottom O’s each have an extra electron and so the ion has a charge of-2 overall.

More than 8 Electrons in the Outer Shell

• Non-metal elements in period 3, (Si, P, S, Cl, Br and I) can have more than 8 electrons in their outer shell in some of their compounds

• Period 2 elements (C, N, O, F) cannot have any more than 8.

• In organic chemistry, the elements C, H, O and N are the most important, so we are usually only dealing with 8 electrons.

Phosphorous Pentachloride PCl5

PCl Cl

Cl Cl

ClClP

Cl

Cl

Cl

ClCl

Fill in the electrons in the bonds. The P atom now has all 5 of its electrons accounted for.

The Cl’s only have 1 of their 7 electrons in the outer shell. We need to draw 3 pairs on each Cl.

Note the P has more than 8 electrons in its outer shell (10). This is allowed as it is a big atom (in period 3).

The diagram is now complete

S

O O

O O

OS

O

O

OH

HH H

Sulphuric Acid H2SO4

Fill in the electrons in the bonds. All 6 of the S’s electrons are accounted for. Note it has 12 electrons in its outer shell. This is allowed because it is in period 3.

The diagram is now complete

Draw in the remaining electrons for the O’s.

S

O O

O O

OS

O

O

OH

HH H

Sulphuric Acid Loses Two H+ ions to Become the Sulphate Ion SO4

2-

The H’s leave but without their electrons. Each H has a +1 charge and a -2 charge is left on the sulphate ion.

Dative Covalent Bonds

• In a dative bond, two electrons are shared as in a normal covalent bond, but they are both provided by the same atom.

• In diagrams they are represented by a stick with an arrow on the end

• The arrow head goes to the atom which doesn’t provide any electrons to the bond

N

O

OO

H

Nitric Acid HNO3

N

O

O O

H

Draw the electrons in the bonds. Take care with the dative bond shown as an arrow. Both electrons come from the nitrogen so both are shown as crosses.

All 5 of the N’s electrons are accounted for. Draw in the O’s remaining electrons.

Each atom “thinks” it has a full outer shell.

Ozone O3

O O O

OO O

Draw the electrons in the bonds. Take care with the dative bond. Both electrons are shown as black dots as they come from the middle O atom.

Fill in the remaining electrons. Each O atom provides 6.

Each oxygen atom thinks it has 8 electrons in its outer shell.

Ammonium Ion NH4+

N

H

H+

HH

H N

H+

H

H

Fill the electrons in the bonds. Take care with the dative bond-both electrons are shown as crosses as both are from the N atom. The H came without its own electron so is positively charged.