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If we had this number of cold drink cans, then we could cover the surface of the earth to a depth of over

If you could count atoms at a rate of 10 million per second, then it would take you 2 billion years to count the atoms in one mole!The original hypothesis that was proposed by Amadeo Avogadro was that “equal volumes of gases, at the same temperature and pressure, contain the same number of molecules”

His ideas were not accepted by the scientific community and it was only four years after his death, that his original hypothesis was accepted and that it became known as “Avogadro's Law”

In honour of his contribution to science, the number of particles in one mole was named Avogadro's number.We use Avogadro's number and the mole in chemistry to help us quantify what happens in chemical reaction

If we measure of carbon we have one mole or carbon atoms

You will remember that when the mass, in grams, of an element is equal to its relative atomic mass, the sample contains one mole of that element

This mass is called the molar mass of that element.You may sometimes see the molar mass written as

We will use in this book, but you should be aware of the alternate notation.It is worth remembering the following: On the periodic table, the relative atomic mass that is shown can be interpreted in two ways

The mass (in grams) of a single, average atom of that element relative to the mass of an atom of carbon

The average atomic mass of all the isotopes of that element

ElementRelative atomic mass (u)Molar mass ()Mass of one mole of the element (g)MagnesiumLithiumOxygenNitrogenIron Table 19.1: The relationship between relative atomic mass, molar mass and the mass of one mole for a number of elements

Calculate the number of moles of iron (Fe) in an sample

If we look at the periodic table, we see that the molar mass of iron is

This means that 1 mole of iron will have a mass of .If 1 mole of iron has a mass of , then: the number of moles of iron in must be:There are 2 moles of iron in the sample

You need to imagine that the horizontal line is like a division sign and that the vertical line is like a multiplication sign

So, for example, if you want to calculate , then the remaining two letters in the triangle are and and is above with a division sign between them

Your calculation will then be Remember that when you use the equation , the mass is always in grams (g) and molar mass is in grams per mol ()

Always write the units next to any number you use in a formula or sum

Calculate the number of moles of copper there are in a sample that with a mass of

Calculate the number of atoms there are in a sample of aluminium that with a mass of

However, you need to remember that all your calculations will apply to the whole compound

So, when you calculate the molar mass of a covalent compound, you will need to add the molar mass of each atom in that compound

The number of moles will also apply to the whole molecule

For example, if you have one mole of nitric acid () the molar mass is and there are molecules of nitric acid

This is the mass of all the atoms in one formula unit of the compound

For example, one mole of sodium chloride () has a formula mass of and there are molecules of sodium chloride in one formula unit.In a balanced chemical equation, the number that is written in front of the element or compound, shows the mole ratio in which the reactants combine to form a product

If there are no numbers in front of the element symbol, this means the number is '1'.Video: VPezceg In this reaction, 1 mole of nitrogen molecules reacts with 3 moles of hydrogen molecules to produce 2 moles of ammonia molecules

Scalars are physical quantities which have only a number value or a size (magnitude)

A scalar tells you how much of something there is.Scalar A scalar is a physical quantity that has only a magnitude (size)

For example, a person buys a tub of margarine which is labelled with a mass of

The mass of the tub of margarine is a scalar quantity

It only needs one number to describe it, in this case, .Vectors are different because they are physical quantities which have a size and a direction

A vector tells you how much of something there is and which direction it is in.Vector A vector is a physical quantity that has both a magnitude and a direction

For example, a car is travelling east along a freeway at

The car is moving at (this is the magnitude) and we know where it is going – east (this is the direction)

Your weight is proportional to your mass (magnitude) and is always in the direction towards the centre of the earth

This chapter is about how things move along a straight line or, more scientifically, how things move in one dimension

This is useful for learning how to describe the movement (motion) of cars along a straight road or of trains along straight railway tracks

There are three features of motion that we use to describe exactly how an object moves

They are:The jerk is the name we give to how fast the acceleration is changing.Traffic often moves along a straight line

The word energy comes from the Greek word energeia (ένέργεια), meaning activity or operation

Energy is closely linked to mass and cannot be created or destroyed

In this chapter we will consider gravitational potential and kinetic energy.Video: VPgjm

As far as we know, the Earth we live on is the only planet that is able to support life

Amongst other factors, the Earth is just the right distance from the sun to have temperatures that are suitable for life to exist

Also, the Earth's atmosphere has exactly the right type of gases in the right amounts for life to survive

Our planet also has water on its surface, which is something very unique

In fact, Earth is often called the “Blue Planet” because most of it is covered in water

This water is made up of freshwater in rivers and lakes, the saltwater of the oceans and estuaries, groundwater and water vapour

Together, all these water bodies are called the hydrosphere.The EarthVideo: VPbzp

In this chapter learners will explore Newtons three laws of motion and Newtons law of universal gravitation

Learners will also learn more about forces and the different kinds of forces

The following provides a summary of the topics covered in this chapter.Different kinds of forces

The types of forces covered are: normal force, frictional force, applied force and tension.Force diagrams and free body diagrams

In this section learners will see how to take a problem and draw diagrams to show all the forces acting on a body

They will learn what a force diagram is and what a free body diagram is

Each law is covered in detail and practical applications such as rockets, lifts and seat belts are covered

Learners are introduced to the ideas of weight and mass as well as the acceleration due to gravity

In this chapter we will learn how a net force is needed to modify the motion of an object

We will recall what a force is and learn about how force and motion are related

We are also introduced to Newton's three laws and we will learn more about the force of gravity.Ratio and proportion - Physical Sciences, Grade 10, Science skillsEquations - Mathematics, Grade 10, Equations and inequalitiesUnits and unit conversions - Physical Sciences, Grade 10, Science skills

Covalent bonding involves the sharing of electrons to form a chemical bond

The outermost orbitals of the atoms overlap so that unpaired electrons in each of the bonding atoms can be shared

By overlapping orbitals, the outer energy shells of all the bonding atoms are filled

As they move, there is an attraction between these negatively charged electrons and the positively charged nuclei

This attractive force holds the atoms together in a covalent bond

Covalent bond A form of chemical bond where pairs of electrons are shared between atoms

We will look at a few simple cases to deduce some rules about covalent bonds

Remember that it is only the valence electrons that are involved in bonding, and so when diagrams are drawn to show what is happening during bonding, it is only these electrons that are shown

For this case we will look at hydrogen chloride and methane

The electron configuration of hydrogen is and the electron configuration for chlorine is .The hydrogen atom has valence electron and the chlorine atom has valence electrons

The Lewis diagrams for hydrogen and chlorine are: Notice the single unpaired electron (highlighted in blue) on each atom

This does not mean this electron is different, we use highlighting here to help you see the unpaired electron

Notice how the two unpaired electrons (one from each atom) form the covalent bond

The dot and cross in between the two atoms, represent the pair of electrons that are shared in the covalent bond

We can also show this bond using a single line: Note how we still show the other electron pairs around chlorine

From this we can conclude that any electron on its own will try to pair up with another electron

So in practise atoms that have at least one unpaired electron can form bonds with any other atom that also has an unpaired electron

Remember that we said we can place unpaired electrons at any position (top, bottom, left, right) around the elements symbol

Water is made up of one oxygen and two hydrogen atoms

From what we learnt in the first examples we see that the unpaired electrons can pair up

Instead of writing the Lewis diagram for hydrogen twice, we simply write it once and use the in front of it to indicate that two hydrogens are needed for each oxygen

And now we can answer the questions that we asked before the worked example

We see that oxygen forms two bonds, one with each hydrogen atom

Oxygen however keeps its electron pairs and does not share them

If an atom has an electron pair it will normally not share that electron pair

A lone pair stays on the atom that it belongs to

A lone pair can be used to form a dative covalent bond

In the example above the lone pairs on oxygen are highlighted in red

When we draw the bonding pairs using lines it is much easier to see the lone pairs on oxygen

Notice the two electron pairs between the two oxygen atoms (highlighted in blue)

Because these two covalent bonds are between the same two atoms, this is a double bond

This forms a double covalent bond (which is shown by a double line between the two oxygen atoms)

Hydrogen has valence electron, carbon has valence electrons and nitrogen has valence electrons

Notice the three electron pairs (highlighted in red) between the nitrogen and carbon atom

Because these three covalent bonds are between the same two atoms, this is a triple bond

Nitrogen keeps its electron pair and shares its three unpaired electrons with carbon

A dative covalent bond is also known as a coordinate covalent bond

Earlier we said that atoms with a pair of electrons will normally not share that pair to form a bond

But now we will see how an electron pair can be used by atoms to form a covalent bond