# Understand Covalent Bonding

In this worksheet, students will learn what covalent bonding is, and how to show it on a dot and cross diagram.

### 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.

(In this diagram, only the outermost shells are shown. It's usually easier to work out the dot and cross diagram by drawing all the shells). 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, it seems to have eight electrons, and if you look round each carbon, they seem to have eight electrons.

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.

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)

What type of bonding involves electrons moving from one atom to another?

Which type of bonding involves pairs of atoms sharing electrons?

Match up the names and formulas of these compounds.

## Column B

Water
NH3
Ammonia
H2O
Methane
CO2
Carbon dioxide
CH4

Look at this diagram of the H2 molecule. How many electrons are there in the shared part of the shells?

Is this dot-and-cross diagram ionic or covalent?

Ionic

Covalent

Is this dot-and-cross diagram ionic or covalent?

Ionic

Covalent

Oxygen forms molecules of O2 (2 oxygen atoms). Oxygen is a non-metal, so the bonding is covalent.

The electron configuration for oxygen is 2.6. How many more electrons does an oxygen atom need to have a stable outer shell?

1

2

3

4

Carry on thinking about O2 molecules. How many electrons must each oxygen atom put in the shared part of the shell?

1

2

3

4

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

 A B C

A

B

C

• Question 1

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

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

What type of bonding involves electrons moving from one atom to another?

Ionic
EDDIE SAYS
Although this activity is mostly about covalent bonding, it's really important to be clear about the difference between ionic and covalent bonding. In exams, people often draw the wrong sort of dot and cross diagram, and it's a shame to lose marks because of that.
• Question 3

Which type of bonding involves pairs of atoms sharing electrons?

Covalent
EDDIE SAYS
Again- make sure you're clear about the difference between ionic and covalent; it makes the whole topic less confusing.
• Question 4

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 5

Look at this diagram of the H2 molecule. How many electrons are there in the shared part of the shells?

Two
2
EDDIE SAYS
Each hydrogen starts with one electron. When H2 forms, each hydrogen puts that electron into the shared part. Then it seems as if each hydrogen atom has two electrons around it. That's enough to fill the first shell.
• Question 6

Is this dot-and-cross diagram ionic or covalent?

Ionic
EDDIE SAYS
If you see an electron being moved from one atom to another- that's ionic. (Notice that one of the electrons around the Cl is a dot; that shows it originally came from the lithium). If atoms overlap so that electrons can be shared- that's covalent.
• Question 7

Is this dot-and-cross diagram ionic or covalent?

Covalent
EDDIE SAYS
This question uses the same idea as the last one; covalent bonding is when atoms overlap, so that some electrons can be shared.
• Question 8

Oxygen forms molecules of O2 (2 oxygen atoms). Oxygen is a non-metal, so the bonding is covalent.

The electron configuration for oxygen is 2.6. How many more electrons does an oxygen atom need to have a stable outer shell?

2
EDDIE SAYS
Oxygen is in Group 6, so it has 6 electrons in its outer shell, and it needs 8.
• Question 9

Carry on thinking about O2 molecules. How many electrons must each oxygen atom put in the shared part of the shell?

2
EDDIE SAYS
Each oxygen has six of its own electrons, so it also needs to have 2 electrons from the other atom in the shared part. That means there are four electrons in the shared part (2 from each atom).
• Question 10

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

 A B C

C
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. For a covalent bond, there have to be an even number of electrons in the shared part.
---- OR ----

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