The particles that make up compounds

The particles of a compound always consist of two or more atoms. In Physical Sciences Gr. 10 you will learn that these atoms combine in different ways. In some cases they can form molecules. You may remember that 'molecule'is the word scientists use for a cluster of atoms that stick together in a specific way. Other compounds consist of atoms which are arranged in a regular pattern called a crystal lattice.

The molecules of a compound always consist of two or more different kindsof atoms, like the molecules of water in the following diagram.

Compounds that form crystal lattices consist of many atoms, but they always combine in a fixed ratio. For example, in sodium chloride (table salt), there is one chlorine atom for every sodium atom in the crystal. The smallest 'unit' that is repeated in the crystal consists of one Na and one Cl. The formula NaCl represents one 'formula unit' of NaCl.

From the diagram of the water molecules and the sodium chloride lattice above, we can see that a compound is not simply a mixture of elements. A mixture of the elements hydrogen and oxygen would look like this:

Why are the hydrogen and oxygen atoms paired in the diagram above? Before we answer that question, here is an important reminder: Elements are made up of just one kind of atom.

Some elements exist as diatomic molecules, like the ones in the diagram on the right below and the hydrogen and oxygen molecules in the 'mixture' diagram above. The most important examples of diatomic molecules are H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂. Diatomic means 'consisting of two atoms'.

Can you see that the water molecules in the diagram above are all identical? That brings us to the next point about compounds.

The atoms in molecules and lattices are combined in a fixed ratio

In water, for example, one oxygen atom (O) has combined with two hydrogen atoms (H). All water molecules are exactly the same in this respect.

Any other combination of hydrogen and oxygen atoms would NOT be water. For example, hydrogen peroxide consists of the same elements as water (hydrogen and oxygen) but the ratio is different: two oxygen atoms have combined with two hydrogen atoms.

The next important point about compounds is the following.

Each compound has a unique name and formula

Water can be represented by the formula H₂O. The formula tells us that two hydrogen atoms (H) are combined with one oxygen atom (O) in a molecule of water.

What is the formula of hydrogen peroxide? Can you remember the name of the compound with the formula CO₂? Remember to take notes as you discuss things in class!

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What formula represents one 'formula unit' of the type of iron oxide in the previous diagram?

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The atoms in a compound are held together by chemical bonds

What holds the clusters of atoms that we call molecules together? When atoms combine to form molecules, they do so because they experience an attractive force between them. The forces that hold atoms together are called chemical bonds.

Next, we need to be reminded where compounds come from.

Compounds form during chemical reactions

In all chemical reactions, the atoms in molecules rearrange themselves to form new molecules. This is how compounds form: the atoms in one set of compounds separate as bonds break between them, and they get rearranged into new groups as new bonds form. When this happens, we say a chemical reaction has occurred. Look at the following illustration.

In the example above, the elements to the left of the arrow are called the reactants. They have rearranged to form a new compound. This is called the product and it is shown to the right of the arrow.

Can you describe what happened to the atoms and the bonds in this reaction? Discuss which bond broke, which ones formed, and how the atoms were rearranged during the reaction.

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The final aspect of compounds that we learnt in Gr. 8 is that each compound can be represented by a unique chemical formula:

A compound has a chemical formula

Compare the formula for water with the diagram of the water molecule you saw earlier. Can you make the connection?

The chemical formula of a compound is the same for all the molecules of that compound. When we read the formula, the subscripts tell us how many atoms of a particular element is in one molecule of that compound:

When we write H₂O, we actually mean H₂O₁. According to convention, we do not use 1 as subscript in formulae and so the first formula is the correct one. What it means is that there are 2 hydrogens to every 1 oxygen. This is also a ratio and can be written as 2:1. We will practise writing formulae in the next activity.

Now that we have refreshed our memories, we are going to return to the table that scientists use to organise their knowledge about the elements. Can you remember what it is called?

What information can we find on the Periodic Table?

The information that most commonly appears on each tile of the Periodic Table is the following:

• The chemical symbol; and
• The atomic number

The diagram below shows an example of one of the tiles on the Periodic Table. Can you identify the element it represents? How many protons does it have in its atoms?

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There are different versions of the Periodic Table, which can each contain different information about the elements. Can you identify what information is provided about the elements in the following table?

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The following Periodic Table only shows the symbols for the elements.

Other versions of the Periodic Table may contain additional information, such as:

• The element name; and/or
• The atomic mass, usually indicted at the bottom of each tile for an element.

The diagrams below show examples of how this information is sometimes presented.

How are the elements arranged on the Periodic Table?

We have learnt that the elements on the Periodic Table are arranged in a very specific way.

The elements are arranged in order of increasing atomic number. The element with the smallest atomic number is hydrogen (H: atomic number = 1) is in the top lefthand corner of the table. The elements with the largest atomic number are found at the bottom of the table.

The elements are also arranged in regions and these regions are often presented in different colours. The following Periodic Table shows us where the metals, non-metals and semi-metals can be found.

We can summarise:

• The metals are found on the left of the Periodic Table, reaching across almost the entire table, except the top right hand corner. In the table above, the metals are blue.
• The non-metals are found in a relatively small, triangular region at the top right hand side of the table. In the table above, the non-metals are red.
• A few elements that have metallic and non-metallic properties (called the semi-metals) separate the metals from the non-metals. They occur in a diagonal strip on the right hand side of the table. In the table above, the semi-metals are yellow.

Now that we have revised what we already learnt in previous grades, let's learn some new characteristics of the Periodic Table.

All tables have rows and columns. Can you remember the difference between vertical and horizontal? Draw short lines to show the difference between 'vertical' and 'horizontal' in the following table.

Vertical	Horizontal

Vertical runs 'up and down', and horizontal runs 'from side to side'. In a conventional table the columns run vertically, and the rows run horizontally.

There are special words to describe the columns and rows of the Periodic Table. The following diagram shows what the column and rows are called.

Groups: The vertical columns of the Periodic Table are called groups. The groups on the Periodic Table are numbered in such a way that Group 1 is on the left. How many groups are there?

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The groups are numbered from 1 to 18. On older tables, the groups are numbered in a more complicated way. The colourful Periodic Table from Gr. 7 (shown earlier) is an example of the numbering style that you may find in older textbooks and other science resources.

Periods: The horizontal rows of the Periodic Table are called periods. The first period is at the top of the table. What is the first element in the third period?

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Which element is in Group 14 and in the second period? Write its symbol and its name.

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Names and chemical symbols

In Gr. 7 we learnt that each element has a unique name. We also learnt that each element has a unique symbol. There is a list of simple rules to remember when using chemical symbols:

1. Every element has its own, unique symbol.
2. The symbol is usually (but not always) the first one or two letters of the name of the element.
3. The first letter of the symbol is always a capital letter.
4. If the symbol has two letters, the second letter is always a small letter.
5. Some elements have symbols that come from their Latin names.

As scientists, we are expected to know the names and symbols of all the most important elements. You will not be expected to learn all of them off by heart just yet, but at the end of this chapter you must know the names and chemical symbols of the first 20 elements on the table. To make them a little easier to remember, they have been placed in a table below.

You may wonder why the Periodic Table has exactly 18 groups and not 14 or 10 or any other number. That is a very good question! The actual explanation is quite complex, and has to do with how the electrons inside the atom are distributed. You will learn about this in greater detail if you take Physical Sciences in Gr. 10.

Properties of elements in the same group

Elements from the same group often have similar physical and chemical properties. For now, it is enough to know that the electrons in the atoms of an element determine the chemical properties of that element. And since the 'electron patterns' repeat after every 18th element, there are 18 groups. Since the elements in a group have similar 'electron patterns', they will behave similarly in chemical reactions.

Here is an example to convince you of the fact:

The metals of Group 1 are called the alkali metals. Can you write the name and chemical symbol of the lightest member of the group? You can disregard hydrogen, which is really a non-metal, but is placed with the alkali metals on the Periodic Table because it has a similar electron pattern.

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Lithium, and all the other alkali metals, are soft dull-grey metals. The look very similar and have similar physical properties.

These elements all react in a very peculiar way with water.

For example, when a small piece of lithium is dropped in water, it will immediately start to react with the water. Here is the chemical equation for the reaction:

2 Li + 2 H₂O → 2 LiOH + H₂

The piece of lithium metal will dance around on the surface of the water, because the reaction produces hydrogen gas (H₂), which causes tiny bubbles to stream from under the lithium. Heat is also given off and sometimes the hydrogen gas will start to burn on top of the water. The other product that forms is lithium hydroxide. Can you find its formula in the chemical equation above?

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Write the word equation underneath the chemical equation above.

What are the reactants and the products in the above chemical reactions?

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Now, the interesting thing is that all the other alkali metals behave in a similar way. Sodium is more reactive than lithium, so it not only bobs around on the surface of the water, but immediately bursts into flame. The chemical reaction is almost identical, though:

2 Na + 2 H₂O → 2 NaOH + H₂

Can you see how similar it is to the reaction between lithium and water?

Potassium is even more reactive than sodium, so it explodes when it hits the surface of the water:

2 K + 2 H₂O → 2 KOH + H₂

What you should notice is that these elements, all from the same group, react in the same way when they come into contact with water. That is what is meant when we say that elements from the same group have similar chemical properties.

Each compound has a unique name

It is just as important for each compound to have a unique name. The following example will help you see why:

The two compounds CO and CO₂ consist of the same two elements, carbon and oxygen. If we named them both 'carbon oxide' (since they are both made of carbon and oxygen), we could easily confuse them. Under certain circumstances that could create problems, because CO is much more poisonous to humans and animals than CO₂. So it is easy to see why each compound needs a unique name.

When we write the chemical formulae for compounds, they are always a combination of the symbols of the elements in the compound. For example, when we see the formula NaCl, we know that this compound consists of Na and Cl.

When we name compounds, the names of the elements in the compound are combined and sometimes changed slightly, to make a name for the compound.

When we hear the name sodium chloride, for instance, it is quite obvious that the compound being described must consist of sodium and chlorine. But, why is it chloride and not chlorine? Well, as you will see shortly, when we join up the names of the elements, the one that is named last is changed.

All the above may sound very complicated and for this reason a system has been developed for naming compounds. The system was developed by the International Union of Pure and Applied Chemistry (IUPAC). The system is designed in such a way that the name of a compound describes the elements it contains and how they are combined.

The IUPAC system for naming compounds is very complex, but we do not need to learn all its rules. At this point we only need to learn how to name compounds consisting of two elements.

At this level we have to distinguish between two types of compounds, because the type of compound determines how it should be named.

Type 1: Compounds that contain a metal and a non-metal

For compounds of this type, the rule is simple. The metal comes first and the non-metal second. The name of the non-metal changes slightly: the suffix -ide replaces the ending of the name.

All compounds of this type form crystal lattices rather than molecules. What do we call the repeating 'units' in a crystal lattice?

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Here are some examples:

Formula	Consists of	Name	Picture of one formula unit of the compound
NaCl	Sodium and chlorine	Sodium chloride
FeS	Iron and sulfur	Iron sulfide
MgO	Magnesium and oxygen	Magnesium oxide
LiF	Lithium and fluorine	Lithium fluoride

Type 2: Compounds that contain only non-metals

This type of compound is slightly more complicated to name. There are three rules that you have to follow. They are as follows:

Rule 1:

The name of the element further to the left on the Periodic Table comes first, followed by the name of the element further to the right on the table. The name of the second element changes slightly: the suffix -ide replaces the ending of the name.

For example:

• oxygen becomes oxide
• fluorine becomes fluoride
• chlorine become chloride
• nitrogen becomes nitride

Rule 2:

When two or more compounds have different numbers of the same elements (like CO and CO₂ in our example above), we must add prefixesto avoid confusion.

The first four prefixes are listed in the table below:

Number of atoms	Prefix
1	mono-
2	di-
3	tri-
4	tetra-
5	penta-

Here are some examples of how this rule should be applied:

Compounds of carbon and oxygen:

• CO - carbon monoxide (notice that it is not mono-oxide, but monoxide)
• CO₂ - carbon dioxide

Compounds of nitrogen and oxygen:

• NO₂ - nitrogen dioxide
• N₂O₄ - dinitrogen tetroxide (did you notice how tetraoxide becomes tetroxide?)

Compounds of sulfur and oxygen:

• SO₂ - sulfur dioxide
• SO₃ - sulfur trioxide

We are going to practice what we have learnt so far in the next two short activities. First, we will write names from formulae.

Next, we will write formulae from the names of some compounds.

There is one additional rule - an easy one to remember!

Rule 3:

Many compounds are not usually referred to by their systematic names. Instead, they have common names that are more widely known. For example, we use the name water for H₂O, ammonia for NH₃, and methane for CH₄.

In this chapter we reviewed all the information about compounds and about the Periodic Table, that we have learnt in previous years. We added some new information to both of these topics. We also learnt an important new skill, namely to write and interpret the names and formulae of compounds.