# Evaluate Covalent Bonding

In this worksheet, students will learn what covalent bonding is, and how to show it on a dot and cross diagram. Students will also learn about the structure of polymers, and evaluate different representations of covalent structures.

Key stage:  KS 4

GCSE Boards:   AQA, AQA Trilogy, AQA Synergy, OCR 21st Century, OCR Gateway, Pearson Edexcel

Difficulty level:

### QUESTION 1 of 10

Ionic bonding works brilliantly when a compound has a metal and a non-metal in it. The metal atom loses unwanted electrons, the non-metal takes electrons it does want, and everything works well. What if there isn't a metal and a non-metal? We need other types of bonding; covalent bonding happens between two non-metal atoms. It's easy to mix up ionic and covalent bonding, so watch out for the ways they are similar and different.

A lot of very important substances are covalently bonded. make sure you can remember the names and formulas of these ones:

 Name Formula water H2O ammonia NH3 methane CH4 carbon dioxide CO2

If both atoms are non-metals, there isn't a source of spare electrons to complete electron shells, so ionic bonding can't happen. For example, imagine having two fluorine atoms. Each atom has 7 out of 8 positions filled in its outermost shell, and neither atom is going to give up any of its electrons. To solve this problem, the two atoms share one electron each. The shared electrons sit in parts of the shell between the two atoms. A tiny fairy sitting in the middle of each atom would see a complete outer shell, but this only works if the two atoms are exactly the right distance between each other, making a bond between the atoms. This type of bond is called a covalent bond, and we can draw a dot-and-cross diagram for this sort of bonding as well. The covalent bond is the part where the circles touch or overlap. Each of the fluorine atoms has put one of its electrons into the shared part of the shell, so this is called a single covalent bond, which can be written as F-F.

You can do the same thing with compounds. Carbon dioxide is carbon and oxygen, and they are both non-metals. Carbon needs another four electrons, and oxygen needs another two. To make carbon dixoide, each bond between carbon and oxygen has two electrons from the carbon and two from the oxygen. If you look round the oxygen, they seem to have eight electrons, and if you look round the carbon, it seems to have eight electrons. The carbon and oxygen each put two electrons in the shared part of the shell, so this is a double covalent bond, which we can write as C=O.

In this diagram, the electrons from the different atoms are shown in differerent colours. Another way of showing this is to use different symbols- for example dots and crosses.

Be really careful not to mix up the different dot-and-cross diagrams. Remember:

If it's a metal with a non-metal, you have ionic bonding. One or more electrons move to the non-metal, but the atoms stay separate.

If it's a non-metal with a non-metal, you have covalent bonding. Some electrons are shared, so the atoms are drawn touching or overlapping.

Polymers are an important group of materials with covalent bonding. Polymers are the basis for plastics, and many parts of the human body. They are very long, with a chain of carbon atoms down the middle. This picture shows polyethne, which is the plastic used for shopping bags.

Shutterstock 177624044

It's not worth drawing out all the atoms, and often we don't need to draw the atoms with the bonds at their exact angles. Instead, scientists simplify the diagram like this:

This makes it simpler to see which atoms are bonded to which. The repeating part of the structure is put in brackets, and the n tells us that there are many repeated copies of that making the complete polymer. You'll learn a lot more about polymers in another activity.

We've seen several different ways of drawing these structures. If you draw the atoms in their true positions, the picture is more realistic, but it can be hard to see enough of the structure to understand how the different atoms link together. If we simplify the diagram, it is easier to understand, but the disadvantage is that we lose some of the information about the structure.

Only some of these compounds have covalent bonding between the atoms. Tick the covalently bonded compounds

Ammonia

Iron oxide

Methane

Sodium chloride

Hydrogen chloride

Water

Match up the names and formulas of these compounds.

## Column B

water
CH4
ammonia
CO2
methane
NH3
carbon dioxide
H2O

Look at this diagram of the hydrogen molecule. Match the half-sentences about hydrogen.

1080108920

## Column B

Hydrogen atoms have
one electron in the shared part of the shell.
Each atom has
covalent bonding.
This means that there are
a complete first shell.
Each hydrogen atom "sees"
two electrons in the shared part of the shell.

Which of these is the correct dot-and-cross diagram for an O2 molecule?

picture 1

picture 2

picture 3

Look at this dot-and-cross diagram. What is the formula of this molecule? Don't worry about typing in the subscripts.

What is the dot-and-cross diagram for methane, CH4?

picture 1

picture 2

picture 3

This diagram shows the polymer structure of polychloroethene (PVC), which is the plastic used to make window frames and water pipes. What is its chemical formula?

C2H3Cl

(C2H3Cl)n

(CHCl)n

(C2H2Cl2)n

The formula of an actual polyethylene molecule is C6000H12000. How many C2H4 units (called monomers) have been joined together to make the molecule? (This is the n value in the structural diagram).

What are the advantages of showing covalent structures with diagrams like this?

(skeletal diagram)

You can see which atoms are bonded to each other.

You can measure distances between atoms.

You can work out the formula of the molecule.

The diagram is realistic

What are the advantages of showing structures with diagrams like this?

(ball and spoke type)

You can easily work out the chemical formula.

You can clearly see all the atoms.

You have a clear idea of the shape of the molecule.

The type of bonding is clear.

• Question 1

Only some of these compounds have covalent bonding between the atoms. Tick the covalently bonded compounds

Ammonia
Hydrogen chloride
Water
EDDIE SAYS
Remember that metal with non-metal (like iron oxide) gives ionic bonding. If you have two non-metals, the bonding is covalent.
• Question 2

Match up the names and formulas of these compounds.

## Column B

water
H2O
ammonia
NH3
methane
CH4
carbon dioxide
CO2
EDDIE SAYS
A lot of these molecule names don\\\'t follow any pattern- you just have to learn them by heart. Keep testing yourself, and you will soon know them all.
• Question 3

Look at this diagram of the hydrogen molecule. Match the half-sentences about hydrogen.

1080108920

## Column B

Hydrogen atoms have
covalent bonding.
Each atom has
one electron in the shared part o...
This means that there are
two electrons in the shared part ...
Each hydrogen atom "sees"
a complete first shell.
EDDIE SAYS
Since hydrogen only has electrons in the first shell, that means that two electrons are enough to complete the shell. It means that each hydrogen atom seems to have two electrons. Although the atoms don't really see the electrons, we sometimes talk as if they do.
• Question 4

Which of these is the correct dot-and-cross diagram for an O2 molecule?

picture 2
EDDIE SAYS
For covalent bonding, we need an overlap between the atoms. Each of the oxygen atoms puts two electrons into the shared part, so each atom has four electrons which it doesn\'t share.
• Question 5

Look at this dot-and-cross diagram. What is the formula of this molecule? Don't worry about typing in the subscripts.

CF4
F4C
EDDIE SAYS
The standard answer is CF4; but if you write F4C, you won\'t lose any marks. Naming and counting the atoms is important, but the order you put them in the formula isn\'t important at GCSE.
• Question 6

What is the dot-and-cross diagram for methane, CH4?

picture 1
EDDIE SAYS
Before any bonding happens, carbon has 4 outermost electrons, and each hydrogen has one. Start by thinking about the hydrogens; each of them has to make a single covalent bond with the carbon. That means four bonds C-H bonds, and that means that all four of carbon\'s electrons are being used in covalent bonds.
• Question 7

This diagram shows the polymer structure of polychloroethene (PVC), which is the plastic used to make window frames and water pipes. What is its chemical formula?

(C2H3Cl)n
EDDIE SAYS
Mostly, this isn\'t too hard- you just have to count the different types of atoms. The brackets and the n are important, though; polymers are very long repeating chains, and that is what the n (which means \"some fairly large number that we don\'t know exactly) is telling us.
• Question 8

The formula of an actual polyethylene molecule is C6000H12000. How many C2H4 units (called monomers) have been joined together to make the molecule? (This is the n value in the structural diagram).

3000
EDDIE SAYS
All you have to do is divide the number of carbon atoms in the polymer molecule by the number in the monomer. This is quite small for a good piece of polyethylene; the plastics you use typically have n higher than 10000.
• Question 9

What are the advantages of showing covalent structures with diagrams like this?

(skeletal diagram)

You can see which atoms are bonded to each other.
You can work out the formula of the molecule.
EDDIE SAYS
This sort of diagram is very stylised and simplified. It shows the really important information- what elements are present, and what atoms are bonded together- but not much more.
• Question 10

What are the advantages of showing structures with diagrams like this?

(ball and spoke type)

You have a clear idea of the shape of the molecule.
EDDIE SAYS
Diagrams like this are helpful to imagine what the structure looks like, but sometimes an atom might be hidden behind another one. There are lots of computer programs and web pages which let you rotate the model on the screen, so that you can see exactly what the structure is like in all directions.
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