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One example of a molecule that contains a dative covalent bond is the ammonium ion () shown in the figure below

The hydrogen ion does not contain any electrons, and therefore the electrons that are in the bond that forms between this ion and the nitrogen atom, come only from the nitrogen

Notice that the hydrogen ion is charged and that this charge is shown on the ammonium ion using square brackets and a plus sign outside the square brackets

We can also show this as: Note that we do not use a line for the dative covalent bond

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

If an atom has an electron pair it will normally not share that pair to form a bond

If an atom has more than one unpaired electron it can form multiple bonds to another atom

A dative covalent bond can be formed between an atom with no electrons and an atom with a lone pair

There are dots around element Y and from our knowledge of Lewis diagrams we know that these represent the valence electrons

X contributes one electron (represented by a cross) to the bond and X has no other electrons

From our knowledge of Lewis diagrams we look at how many cross and dot pairs there are in the molecule and that gives us the number of covalent bonds

These are single bonds since there is only one dot and cross pair between adjacent atoms

Note that Y could also be sulfur and X hydrogen and the molecule would then be hydrogen sulfide (sulfur dihydride)

The types of intermolecular forces that occur in a substance will affect its properties, such as its phase, melting point and boiling point

You should remember from the kinetic theory of matter (see grade ), that the phase of a substance is determined by how strong the forces are between its particles

The weaker the forces, the more likely the substance is to exist as a gas

This is because the particles are able to move far apart since they are not held together very strongly

If the forces are very strong, the particles are held closely together in a solid structure

Remember also that the temperature of a material affects the energy of its particles

The more energy the particles have, the more likely they are to be able to overcome the forces that are holding them together

Note that we are showing two dimensional figures when in reality these are three dimensional

The following five experiments investigate the effect of various physical properties (evaporation, surface tension, solubility, boiling point and capillarity) of substances and determine how these properties relate to intermolecular forces

A formal experiment on the effects of intermolecular forces is included in this chapter

In this experiment learners will investigate how intermolecular forces affect evaporation, surface tension, solubility, boiling points and capillarity

Some of the substances that are used (nail polish remover (mainly acetone if you use the non acetone free variety), methylated spirits (a mixture of methanol and ethanol), oil (a mostly non-polar hydrocarbon), glycerin (a fairly complex organic molecule)) are quite complex substances and learners may not have the skills needed to determine the types of intermolecular forces at work here

You should guide learners in this and tell them the intermolecular forces for these substances

You can help learners work out the strength of the intermolecular forces by telling them that larger molecules have stronger intermolecular forces than smaller molecules

This is often a big factor in determining which substance has the strongest intermolecular forces

Each experiment focuses on a different property and sees how that property relates to intermolecular forces

It will often not be easy for learners to see the small differences between some of the molecules chosen and so they need to use a combination of experimental results and knowledge about the strength of the intermolecular force to try and predict what may happen

Each experiment ends with a conclusion about what should be found to guide learners

It is very important to work in a well ventilated room (one with lots of air flow) particularly when working with methanol and ethanol

Many of the substances used (particularly nail polish remover, ethanol and methylated spirits) are highly flammable and so care must be taken when heating these substances

It is recommended that learners use a hot plate rather than a Bunsen burner to heat these substances as this reduces the risk of fire

When doing chemistry experiments it is also extra important to ensure that your learners do not run around, do not try to drink the chemicals, do not eat and drink in the lab, do not throw chemicals on the other learners and in general do act in a responsible and safe way

The guidelines for safe experimental work can be found in the science skills chapter from grade

From these experiments we can see how intermolecular forces (a microscopic property) affect the macroscopic behaviour of substances

If a substance has weak intermolecular forces then it will evaporate easily

Substances with weak intermolecular forces also have low surface tension and do not rise as far up in narrow tubes as substances with strong intermolecular forces

Substances are more likely to be soluble in liquids with similar intermolecular forces

We will now look at some more properties (molecular size, viscosity, density, melting and boiling points, thermal expansion, thermal conductivity) in detail

The carbon atoms link together to form chains of varying lengths

The boiling point and melting point of these molecules is determined by their molecular structure, and their surface area

The more carbon atoms there are in an alkane, the greater the surface area and therefore the higher the boiling point

The melting point also increases as the number of carbon atoms in the molecule increases

there are few carbon atoms), the organic compounds are gases because the intermolecular forces are weak

As the number of carbon atoms and the molecular mass increases, the compounds are more likely to be liquids or solids because the intermolecular forces are stronger

You should see that the larger a molecule is the stronger the intermolecular forces are between its molecules

This is one of the reasons why methane () is a gas at room temperature while pentane () is a liquid and icosane () is a solid

It is partly the stronger intermolecular forces that explain why petrol (mainly octane ()) is a liquid, while candle wax () is a solid

If these intermolecular forces did not increase with increasing molecular size we would not be able to put liquid fuel into our cars or use solid candles

Compare how easy it is to pour water and syrup or honey

You can see this if you take a cylinder filled with water and a cylinder filled with glycerin

Drop a small metal ball into each cylinder and note how easy it is for the ball to fall to the bottom

In the glycerin the ball falls slowly, while in the water it falls faster

Substances with stronger intermolecular forces are more viscous than substances with weaker intermolecular forces

The solid phase is often the most dense phase (water is one noteworthy exception to this)

This can be explained by the strong intermolecular forces found in a solid

These forces pull the molecules together which results in more molecules in one unit volume than in the liquid or gas phases

The more molecules in a unit volume the denser that substance will be

Substances with weak intermolecular forces will have low melting and boiling points while those with strong intermolecular forces will have high melting and boiling points

In the experiment on intermolecular forces you investigated the boiling points of several substances, and should have seen that molecules with weaker intermolecular forces have a lower boiling point than molecules with stronger intermolecular forces

One further point to note is that covalent network structures (recall from grade that these are covalent compounds that form large networks and an example is diamond) will have high melting and boiling points due to the fact that some bonds (i.e

the strong forces between atoms) have to break before the substance can melt

Covalent molecular substances (eg water, sugar) often have lower melting and boiling points, because of the presence of the weaker intermolecular forces holding these molecules together

As the alcohol (or mercury) is heated it expands and rises up the tube

This is why when you tile a floor you have to leave gaps between the tiles to allow for expansion

It is also why power lines sag slightly and bridges have slight gaps for expansion

Heat is transferred through a substance from the point being heated to the other end

This is why the bottom of a pot gets hot first (assuming you are heating the pot on a stove plate)

In metals there are some free, delocalised electrons which can help transfer the heat energy through the metal

In covalent molecular compounds there are no free, delocalised electrons and the heat does not travel as easily through the material

Explain why the melting point of oxygen () is much lower than the melting point of hydrogen chloride

So if a substance has strong intermolecular forces, then that substance will have a high melting point

We know that stronger intermolecular forces lead to higher melting points

We also know that oxygen has weaker intermolecular forces than hydrogen chloride (induced dipole versus dipole-dipole forces)

Therefore oxygen will have a lower melting point than hydrogen chloride since oxygen has weaker intermolecular forces

Induced dipole forces are the weakest intermolecular forces and hydrogen bonding is the strongest

In order for a liquid to boil the intermolecular forces must be broken and if the intermolecular forces are very strong then it will take a lot of energy to overcome these forces and so the boiling point will be higher

Water has strong intermolecular forces (hydrogen bonds) while carbon tetrachloride only has weaker induced dipole forces

Substances with stronger intermolecular forces take longer to evaporate than substances with weaker intermolecular forces

The type of intermolecular force that can exist when sodium chloride dissolves in methanol is ion-dipole forces

The formation of these forces helps to disrupt the ionic bonds in sodium chloride and so sodium chloride can dissolve in methanol

Tumi and Jason are helping their dad tile the bathroom floor

Their dad tells them to leave small gaps between the tiles

Materials (such as tiles) expand on heating and so small gaps need to be left between the tiles to allow for this expansion

If Tumi and Jason did not leave these gaps between the tiles, the tiles would soon lift up

A beam of sunlight through a window lights up a section of the floor

You might draw a series of parallel lines showing the path of the sunlight from the window to the floor

This is not exactly accurate — no matter how hard you look, you will not find unique lines of light in the sunbeam

However, this is a good way to draw light and to model light geometrically, as we will see in this chapter.We call these narrow, imaginary lines of light light rays

Recall that light can behave like a wave and so you can think of a light ray as the path of a point on the crest of a wave.We can use light rays to model the behaviour of light relative to mirrors, lenses, telescopes, microscopes, and prisms

The study of how light interacts with materials is called optics

When dealing with light rays, we are usually interested in the shape of a material and the angles at which light rays hit it

From these angles, we can determine, for example, the distance between an object and its reflection

You have learnt about the basic properties of waves before, specifically about reflection and refraction

In this chapter, you will learn about phenomena that arise with waves in two and three dimensions: diffraction

We will also build on interference which you have learnt about previously but now in more than one dimension.

The kinetic theory of matter says that all matter is composed of particles which have a certain amount of energy which allows them to move at different speeds depending on the temperature (energy)

There are spaces between the particles and also attractive forces between particles when they come close together.Now we will look at applying the same ideas to gases.The main assumptions of the kinetic theory of gases are as follows: Gases are made up of particles (eg atoms or molecules)