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Explain 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.

Worksheet Overview

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 dioxide, 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 different 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 polythene, which is the plastic used for shopping bags.

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.

 

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

Ammonia (NH3)

Hydrogen chloride (HCl)

Iron oxide (FeO)

Water (H2O)

Match up the names and formulas of these compounds.

Column A

Column B

water
NH3
ammonia
CH4
methane
CO2
carbon dioxide
H2O

Look at this diagram of the hydrogen molecule. Tick the true statements about this molecule.

Hydrogen shows covalent bonding.

In covalent bonding, electrons move between atoms.

Both atoms need a complete outer shell of electrons

For hydrogen, each atom needs 2 electrons in its outer shell.

There are two electrons in the shared area, so this is a double bond.

One of these diagrams is the correct dot-and-cross diagram for oxygen molecules (O2). Pick the correct diagram, and the correct reasons for the other diagrams being wrong.

A
B
C

 

Column A

Column B

A
correct diagram
B
there are two many electrons
C
there aren't enough electrons in the bond

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

Ammonia is NH3. How many electrons does each atom have in its outermost shell before any bonding happens?

(Remember this answer- it will help you with the next question).

Which of these is the correct dot-and-cross diagram of ammonia?

A
B
C

 

A

B

C

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

A
B
C

 

A

B

C

Fill in the gaps in this paragraph about single and double covalent bonds.

Use these words (you might need to use some more than once).

covalent

double

four

one 

single

two

A

B

C

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

  • Question 1

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

CORRECT ANSWER
Ammonia (NH3)
Hydrogen chloride (HCl)
Water (H2O)
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.

CORRECT ANSWER

Column A

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. Tick the true statements about this molecule.

CORRECT ANSWER
Hydrogen shows covalent bonding.
Both atoms need a complete outer shell of electrons
For hydrogen, each atom needs 2 electrons in its outer shell.
EDDIE SAYS
Since hydrogen only has electrons in the first shell, that means that two electrons are enough to complete the shell. Each hydrogen puts one electron in the shared area, so that is a single bond. It means that each hydrogen atom seems to have two electrons.
  • Question 4

One of these diagrams is the correct dot-and-cross diagram for oxygen molecules (O2). Pick the correct diagram, and the correct reasons for the other diagrams being wrong.

A
B
C

 

CORRECT ANSWER

Column A

Column B

A
there are two many electrons
B
there aren't enough electrons in ...
C
correct diagram
EDDIE SAYS
Oxygen atoms have six electrons in the outermost shell. That means each atom in the molecule has to contribute two electrons to the covalent bond, so there are four electrons in the overlap area.
  • Question 5

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

CORRECT ANSWER
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

Ammonia is NH3. How many electrons does each atom have in its outermost shell before any bonding happens?

(Remember this answer- it will help you with the next question).

CORRECT ANSWER
EDDIE SAYS
This goes back to electron structures and the Periodic Table. Hydrogen has one electron only. Nitrogen is atomic number 7, so its electron structure is 2.5, which is why it is in group 5.
  • Question 7

Which of these is the correct dot-and-cross diagram of ammonia?

A
B
C

 

CORRECT ANSWER
B
EDDIE SAYS
When you draw a covalent dot-and-cross diagram, start with any hydrogen atoms; they can only do one thing. Each hydrogen shares its electron and one electron from the other atom (the nitrogen here). So hydrogen has to form single covalent bonds. That means that three of the five electrons the nitrogen starts get shared, leaving two unshared. Each hydrogen seems to have two electrons, and the nitrogen seems to have eight, so everything is fine.
  • Question 8

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

A
B
C

 

CORRECT ANSWER
C
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 9

Fill in the gaps in this paragraph about single and double covalent bonds.

Use these words (you might need to use some more than once).

covalent

double

four

one 

single

two

CORRECT ANSWER
EDDIE SAYS
The language of single and double covalent bonds can be confusing, because the number of shared electrons is doubled again because two atoms are involved in the bond, and each has to contribute equally to the shared part of the shell. There are also triple bonds (six electrons, three from each atom), written as ≡, but they are much rarer.
  • Question 10

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?

CORRECT ANSWER
(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.
---- OR ----

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