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Early ideas about atoms

The word atom comes from atomos, an ancient Greek word meaning indivisible. The Greek philosopher Democritus (460-370 BCE) maintained that all matter could be divided and sub-divided into smaller and smaller units, and eventually there would be a tiny particle that could not be divided any further - an atom. This was remarkable because there was no way ancient Greeks could support this theory by observation or experiment.

John Dalton

John Dalton (1766-1844)

Understanding of atoms didn’t progress much beyond Democritus’ theory until the English chemist John Dalton (1766-1844) started to look at it in the 1800s. Dalton did experiments, worked out some atomic weights, and invented symbols for atoms and molecules. His most important conclusions are summarised below.

Dalton's theories about atoms took a long time to be accepted by scientists. Some of his ideas about gases were incorrect, and it was difficult for many years to do the experiments needed to support his theories, because atoms are too small to see.

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Atoms and elements

Although the word 'atom' comes from the Greek for indivisible, we now know that atoms are not the smallest particles of matter. Instead, they have a small central nucleus surrounded by even smaller particles called electrons.

The structure of the atom

All substances are made from atoms. And, as Dalton suggested, any given element is made of atoms of just one particular sort. The atoms of any element are different from the atoms of any other element. So iron contains a different sort of atoms from those of sulphur, and the atoms in carbon are different from those of oxygen.

Chemical symbols the atoms of each element are represented by chemical symbols. These usually consist of one or two different letters, but sometimes three letters are used for newly-discovered elements. The first letter in a chemical symbol is always an UPPERCASE letter, and the other letters are always lowercase. So, the symbol for magnesium is Mg and not mg, MG or mG.

Every element has its own chemical symbol. For example, iron is Fe, sulphur is S, sodium is Na and oxygen is O.

The periodic table

There are more than 100 different elements. The periodic table is a chart showing all the elements arranged in a particular way. The

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vertical columns in the periodic table are called groups. Each group contains elements that have similar properties.

The periodic table

The periodic table has eight main groups. For example, group 1 contains very reactive metals such as sodium - Na - while group 7 contains very reactive non-metals such as chlorine - Cl.

Note that you will never find a compound in the periodic table, because these consist of two or more different elements joined together by chemical bonds.

Reactions and compounds

New substances are formed by chemical reactions. When elements react together to form compounds their atoms join to other atoms using chemical bonds. For example, iron and sulphur (often spelt 'sulphur') react together to form a compound called iron sulphide (often spelt 'sulphide'), and sodium and oxygen react together to form sodium oxide.

Chemical bonds involve electrons from the reacting atoms. Bonds can form when:

• electrons are transferred from one atom to another, so that one atom gives electrons and the other takes electrons, or

• Electrons are shared between two atoms.

Chemical formulae

The chemical formula of a compound shows how many of each type of atom join together to make the units that make the compound up. For example, in iron sulphide every iron atom is joined to one sulphur atom, so we show its formula as FeS. In sodium oxide, there are two sodium atoms for every oxygen atom, so we show its formula as

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,Na2O. Notice that the 2 is written as a subscript, so Na2O would be wrong.

The diagram below shows that one carbon atom and two oxygen atoms combine to make up the units of carbon dioxide - its chemical formula should therefore be written as CO2.

Carbon dioxide units contain one carbon atom and two oxygen atoms

Sometimes you see more complex formulae such as Na2SO4 and Fe(OH)3:

• A unit of Na2SO4 contains two sodium atoms, one sulphur atom and four oxygen atoms joined together.

• A unit of Fe(OH)3 contains one iron atom, three oxygen atoms and three hydrogen atoms (the brackets show that the 3 applies to O and H).

Equations

When elements are joined to cause a chemical reaction, no atoms are made or lost during the process - but at the end of it they are joined differently from the way they were at the start. This means that the mass of the substances at the start - the reactants - is the same as the mass of the substances at the end - the products.

Copper and oxygen reaction - getting a balanced equation

We use balanced equations to show what happens to the different atoms in reactions. For example, copper and oxygen react together to make copper oxide. Take a look at the word equation for the reaction, here:

copper + oxygen copper oxide

You can see that copper and oxygen are the reactants, and copper oxide is the product.

If we just replace the words shown above by the correct chemical formulae, we will get an unbalanced equation, as shown here:

Cu + O2 CuO

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Notice that we have unequal numbers of each type of atom on the left-hand side compared with the right-hand side. To make things equal, we need to adjust the number of units of some of the substances until we get equal numbers of each type of atom on both sides of the arrow.

Here is the balanced symbol equation:

2Cu + O2 2CuO

You can see that now we have two copper atoms and two oxygen atoms on each side. This matches what happens in the reaction.

Two atoms of copper react with two atoms of oxygen to form two units of

copper oxide

Fuels from crude oil

Crude oil is a mixture of compounds called hydrocarbons. Many useful materials can be produced from crude oil. It can be separated into different fractions using fractional distillation, and some of these can be used as fuels. Unfortunately, there are environmental consequences when fossil fuels such as crude oil and its products are used.

Hydrocarbons and alkanes

Hydrocarbons Most of the compounds in crude oil are hydrocarbons. This means that they only contain hydrogen and carbon atoms, joined together by chemical bonds. There are different types of hydrocarbon, but most of the ones in crude oil are alkanes.

Alkanes The alkanes are a family of hydrocarbons that share the same general formula. This is:

CnH2n+2

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The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, methane is CH4 and ethane is C2H6. Alkane molecules can be represented by displayed formulae in which each atom is shown as its symbol (C or H) and the chemical bonds between them by a straight line.

Notice that the molecular models on the right show that the bonds are

not really at 90°

Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from their reaction with oxygen in the air, which we call burning or combustion.

Boiling point and state at room temperature Hydrocarbons have different boiling points, and can be either solid, liquid or gas at room temperature:

• Small hydrocarbons with only a few carbon atoms have low boiling points and are gases.

• Hydrocarbons with between five and 12 carbon atoms are usually liquids.

• Large hydrocarbons with many carbon atoms have high boiling points and are solids.

Distillation

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Distillation is a process that can be used to separate a pure liquid from a mixture of liquids. It works when the liquids have different boiling points. Distillation is commonly used to separate ethanol (the alcohol in alcoholic drinks) from water.

The mixture is heated in a flask. Ethanol has a lower boiling point than water so it evaporates first. The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid. The thermometer shows the boiling point of the pure ethanol liquid. When all the ethanol has evaporated from the solution, the temperature rises and the water evaporates.

This is the sequence of events in distillation:

heating evaporating cooling condensing

Fractional distillation

Fractional distillation differs from distillation only in that it separates a mixture into a number of different parts, called fractions. A tall column is fitted above the mixture, with several condensers coming off at different heights. The column is hot at the bottom and cool at the top. Substances with high boiling points condense at the bottom and substances with low boiling points condense at the top. Like distillation, fractional distillation works because the different substances in the mixture have different boiling points.

Fractional distillation of crude oil Because they have different boiling points, the substances in crude oil can be separated using fractional distillation. The crude oil is evaporated and its vapours allowed to condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.

Oil fractions The diagram below summarises the main fractions from crude oil and their uses, and the trends in properties. Note that the gases condense at the top of the column, the liquids in the middle and the solids stay at the bottom.

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The main fractions include refinery gases, gasoline (petrol), naphtha, kerosene, diesel oil, fuel oil, and a residue that contains bitumen. These fractions are mainly used as fuels, although they do have other uses too. Hydrocarbons with small molecules make better fuels than hydrocarbons with large molecules because they are volatile, flow easily and are easily ignited.

Combustion of fuels

Complete combustion Fuels burn when they react with oxygen in the air. The hydrogen in hydrocarbons is oxidised to water (remember that water, H2O, is an oxide of hydrogen). If there is plenty of air, we get complete combustion and the carbon in hydrocarbons is oxidised to carbon dioxide:

hydrocarbon + oxygen water + carbon dioxide

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Incomplete combustion if there is insufficient air for complete combustion, we get incomplete combustion instead. The hydrogen is still oxidised to water, but instead of carbon dioxide we get carbon monoxide. Particles of carbon, seen as soot or smoke, are also released.

Sulphur Most hydrocarbon fuels naturally contain some sulphur compounds. When the fuel burns, the sulphur it contains is oxidised to sulphur dioxide.

Summary the combustion of a fuel may release several gases into the atmosphere, including:

• water vapour • carbon dioxide • carbon monoxide • particles • sulphur dioxide

These products may be harmful to the environment.

Clouds of smoke and other combustion products are emitted from chimneys

Sulphur dioxide

Sulphur dioxide is produced when fuels that contain sulphur compounds burn. It is a gas with a sharp, choking smell. When sulphur dioxide dissolves in water droplets in clouds, it makes the rain more acidic than normal. This is called acid rain.

Effects of acid rain Acid rain reacts with metals and rocks such as limestone. Buildings and statues are damaged as a result. Acid rain damages the waxy layer on the leaves of trees and makes it more difficult for trees to absorb the minerals they need for healthy growth. They may die as a

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result. Acid rain also makes rivers and lakes too acidic for some aquatic life to survive.

Reducing acid rain Sulphur dioxide can be removed from waste gases after combustion of the fuel. This happens in power stations. The sulphur dioxide is treated with powdered limestone to form calcium sulphate. This can be used to make plasterboard for lining interior walls, so turning a harmful product into a useful one.

Sulphur can be removed from fuels at the oil refinery. This makes the fuel more expensive to produce, but it prevents sulphur dioxide being produced. You may have noticed ‘low sulphur’ petrol and diesel on sale at filling stations.

Global warming

Carbon dioxide from burning fuels causes global warming, a process capable of changing the world’s climate significantly.

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As you can see from the graphs, the amount of carbon dioxide in the atmosphere has increased steadily over the past 150 years, and so has the average global temperature.

Carbon dioxide is a greenhouse gas. It absorbs heat energy and prevents it escaping from the Earth’s surface into space. The greater the amount of carbon dioxide in the atmosphere, the more heat energy is absorbed and the hotter the Earth becomes.

Effects of global warming A rise of just a few degrees in world temperatures will have a dramatic impact on the climate:

• Global weather patterns will change, causing drought in some places and flooding in others.

• Melting of polar ice caps will raise sea levels, causing increased coastal erosion and flooding of low-lying land – including land where major cities lie.

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Global dimming

Tiny particles that are released when fuels are burned cause global dimming. Like global warming, this process may change rainfall patterns around the world.

The amount of sunlight reaching the Earth’s surface has decreased by about 2 per cent every ten years, because more sunlight is being reflected back into space. The particles from burning fuels reflect sunlight, and they also cause more water droplets to form in the clouds. This makes the clouds better at reflecting sunlight back into space.

It is likely that global dimming has hidden some of the effects of global warming, by stopping some of the Sun’s energy reaching the Earth’s surface in the first place. Governments around the world are introducing controls on pollution. There is the possibility that as the air becomes less polluted by smoke and soot, global dimming will decrease, causing the effects of global warming to become more obvious.

Limestone

Limestone is mainly calcium carbonate, CaCO3. When it is heated, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide reacts with water to produce calcium hydroxide. Limestone and its products have many uses, including being used to make mortar, cement, concrete and glass

Thermal decomposition

Metal carbonates such as calcium carbonate break down when heated strongly. This is called thermal decomposition. Here are the equations for the thermal decomposition of calcium carbonate:

calcium carbonate heat

calcium oxide + carbon dioxide

CaCO3 heat

CaO + CO2

Other metal carbonates decompose in the same way. Here are the equations for the thermal decomposition of copper carbonate:

copper carbonate heat copper oxide + carbon dioxide

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CuCO3 heat

CuO + CO2

Notice that in both examples the products are a metal oxide and carbon dioxide. The carbon dioxide gas can be detected using limewater. Limewater turns cloudy white when carbon dioxide is bubbled through it.

Metals high up in the reactivity series - such as calcium - have carbonates that need a lot of energy to decompose them. Metals low down in the reactivity series - such as copper - have carbonates that are easily decomposed. This is why copper carbonate is often used at school to show these reactions. It is easily decomposed, and its colour change, from green copper carbonate to black copper oxide, is easy to see

Copper carbonate decomposes to form copper oxide and carbon

dioxide when heated

Quicklime and slaked lime

For your exam, you need to know how quicklime and slaked lime are obtained from limestone.

Making quicklime If limestone is heated strongly, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide is also called quicklime. It is yellow when hot, but white when cold.

Here are the equations for this reaction:

calcium carbonate heat calcium oxide + carbon dioxide

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CaCO3 heat

CaO + CO2

This is a thermal decomposition reaction.

Quicklime (calcium oxide)

Making slaked lime Calcium oxide reacts with water to form calcium hydroxide, also called slaked lime.

Here are the equations for this reaction:

calcium oxide + water calcium hydroxide CaO + H2O Ca(OH)2

A lot of heat is produced in the reaction, which may even cause the water to boil.

Slaked lime (calcium hydroxide)

Summary Using common names instead of chemical names, this is what happens:

limestone heat quicklime + carbon dioxide

quicklime + water slaked lime

Uses of limestone

Limestone, quicklime and slaked lime are all used to neutralise excess acidity - which may be caused by acid rain - in lakes and in soils.

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Limestone is used as a building material, and to purify iron in blast furnaces. It's also used in the manufacture of glass, and of cement (one of the components of concrete).

The flow chart below summarises the main uses of limestone and its products.

Glass Glass is made by melting sand and then cooling it. Flat sheets of glass for windows are made by floating molten glass on a layer of molten tin.

Glass manufacturers add sodium carbonate to sand during the manufacturing process, to reduce the melting temperature of the sand and so save energy. The sodium carbonate decomposes in the heat to form sodium oxide and carbon dioxide, but this makes the glass soluble in water. Calcium carbonate (limestone) is therefore also added, to stop the glass dissolving in water. The calcium carbonate decomposes in the heat to form calcium oxide and carbon dioxide. About 90 per cent of glass is soda-lime glass, or bottle glass.

Environmental, social and economic considerations

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The limestone industry You need to be able to evaluate some of the effects of the limestone industry. Here are the main ones:

Benefits Disadvantages

Limestone is a valuable natural resource, used to make things such as glass and concrete.

Limestone quarries are visible from long distances and may permanently disfigure the local environment.

Limestone quarrying provides employment opportunities that support the local economy in towns around the quarry.

Quarrying is a heavy industry that creates noise and heavy traffic, which damages people's quality of life.

Advantages and disadvantages of various building materials Limestone, cement and mortar slowly react with carbon dioxide dissolved in rainwater, and wear away. This damages walls made from limestone, and it leaves gaps between bricks in buildings. These gaps must be filled in or “pointed”. Pollution from burning fossil fuels makes the rain more acidic than it should be, and this acid rain makes these problems worse.

Concrete is easily formed into different shapes before it sets hard. It is strong when squashed, but weak when bent or stretched. However, concrete can be made much stronger by reinforcing it with steel. Some people think that concrete buildings and bridges are unattractive.

Glass is usually brittle and easily shattered, but toughened glass can be used for windows. While glass is transparent and so lets light into a building, buildings with lots of glass can be too hot in the summer.

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Metals

Metals are very useful. Ores are naturally occurring rocks that contain metal or metal compounds in sufficient amounts to make it worthwhile extracting them. For example, iron ore is used to make iron and steel. Copper is easily extracted, but ores rich in copper are becoming more difficult to find. Aluminium and titanium are metals with useful properties, but they are expensive to extract. Most everyday metals are mixtures called alloys.

Methods of extracting metals

The Earth's crust contains metals and metal compounds such as gold, iron oxide and aluminium oxide, but when found in the Earth these are often mixed with other substances. To be useful, the metals have to be extracted from whatever they are mixed with. A metal ore is a rock containing a metal, or a metal compound, in a high enough concentration to make it economic to extract the metal.

The method used to extract metals from the ore in which they are found depends on their reactivity. For example, reactive metals such as aluminium are extracted by electrolysis, while a less-reactive metal such as iron may be extracted by reduction with carbon or carbon monoxide.

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Thus the method of extraction of a metal from its ore depends on the metal's position in the reactivity series:

potassium sodium calcium magnesium aluminium

extract by electrolysis

carbon

zinc iron tin lead

extract by reaction with carbon or carbon monoxide

hydrogen

copper silver gold platinum

extracted by various chemical reactions

Reactivity and extraction method

Note that gold, because it is so unreactive, is found as the native metal and not as a compound, so it does not need to be chemically separated. However, chemical reactions may be needed to remove other elements that might contaminate the metal. Making iron

In the blast furnace Iron is extracted from iron ore in a huge container called a blast furnace. Iron ores such as haematite contain iron oxide. The oxygen must be removed from the iron oxide to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions.

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Blast furnace in a modern steel works

Carbon is more reactive than iron, so it can push out or displace the iron from iron oxide. Here are the equations for the reaction:

iron oxide + carbon iron + carbon dioxide 2Fe2O3 + 3C 4Fe + 3CO2

In this reaction, the iron oxide is reduced to iron, and the carbon is oxidised to carbon dioxide.

In the blast furnace, it is so hot that carbon monoxide can be used to reduce the iron oxide in place of carbon:

iron oxide + carbon monoxide iron + carbon dioxide Fe2O3 + 3CO 2Fe + 3CO2

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Raw materials for the reaction The table shows the raw materials for extracting iron and their function in the process.

Raw material Contains Function

iron ore (haematite) iron oxide a compound that contains iron

coke carbon burns in air to produce heat, and reacts to form carbon monoxide (needed to reduce the iron oxide)

limestone calcium carbonate

helps to remove acidic impurities from the iron by reacting with them to form molten slag

air oxygen allows the coke to burn, and so produces heat and carbon monoxide

Steel

Iron Pure iron is soft and easily shaped. This is because its atoms are arranged in a regular way that lets layers of atoms slide over each other. Pure iron is too soft for many uses.

Layers of atoms slide over each other when metals are bent or

stretched

Iron from the blast furnace is an alloy of about 96 per cent iron with carbon and some other impurities. It is hard, but too brittle for most

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uses. So, most iron from the blast furnace is converted into steel by removing some of the carbon.

Steel Carbon is removed by blowing oxygen into the molten metal. It reacts with the carbon producing carbon monoxide and carbon dioxide. These escape from the molten metal. Enough oxygen is used to achieve steel with the desired carbon content. Other metals are often added, such as vanadium and chromium.

There are many different types of steel, depending on the other elements mixed with the iron. The table summarises the properties of some different steels.

Type of steel Iron alloyed with: Properties Typical use

low carbon steel

about 0.25 per cent carbon easily shaped car body

panels

high carbon steel

up to 2.5 per cent carbon hard cutting tools

stainless steel chromium and nickel resistant to corrosion

cutlery and sinks

Alloys

The properties of a metal are changed by including other elements, such as carbon. A mixture of two or more elements, where at least one element is a metal, is called an alloy. Alloys contain atoms of different sizes, which distort the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so alloys are harder than the pure metal.

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It is more difficult for layers of atoms to slide over each other in alloys

Copper, gold and aluminium are too soft for many uses. They are mixed with other metals to make them harder for everyday use. For example:

• Brass, used in electrical fittings, is 70 per cent copper and 30 per cent zinc.

• 18 carat gold, used in jewellery, is 75 per cent gold and 25 per cent copper and other metals.

• Duralumin, used in aircraft manufacture, is 96 per cent aluminium and 4 per cent copper and other metals

The transition metals

You need to know where to find the transition metals in the periodic table. The transition metals are found in the large block between Groups 2 and 3 in the periodic table. Most metals are placed here, including iron, titanium, copper and nickel.

The transition metals (in blue)

Common properties The transition metals have these properties in common:

• They are metals. • They form coloured compounds. • They are good conductors of heat and electricity. • They can be hammered or bent into shape easily. • They are less reactive than alkali metals such as sodium, they

have higher melting points (but mercury is a liquid at room temperature) and they are hard and tough.

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• They have high densities.

Smart alloys can return to their original shape after being bent. They are useful for spectacle frames and dental braces.

Copper

Copper is a transition metal. It is soft, easily bent and it is a good conductor of electricity. This makes copper useful for electrical wiring. Copper does not react with water, which makes it useful for plumbing.

Copper is purified by electrolysis. Electricity is passed through solutions containing copper compounds, such as copper sulphate (sometimes spelt sulphate). Pure copper forms on the negative electrode.

Problems we are running out of ores rich in copper. Research is being carried out to find new ways to extract copper from the remaining ores, without harming the environment too much. This research is very important, as traditional mining produces huge open-cast mines, and the remaining ores are low-grade, which means that they contain relatively little copper and produce a lot of waste rock.

Aluminium and titanium

Aluminium and titanium are two metals with a low density. This means that they are lightweight for their size. They also have a very thin layer of their oxides on the surface, which stops air and water getting to the metal, so aluminium and titanium resist corrosion. These properties make the two metals very useful.

A cross section of aluminium that shows the outer layer of oxide

Aluminium is used for aircraft, trains, overhead power cables, saucepans and cooking foil. Titanium is used for fighter aircraft, artificial hip joints and pipes in nuclear power stations.

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Extraction Unlike iron, aluminium and titanium cannot be extracted from their oxides by reduction with carbon:

• Aluminium is more reactive than carbon, so the reaction does not work.

• Titanium forms titanium carbide with carbon, which makes the metal brittle.

Aluminium extraction is expensive because the process needs a lot of electrical energy. Titanium extraction is expensive because the process involves several stages and a lot of energy. This especially limits the uses of titanium.

Recycling Aluminium is extensively recycled because less energy is needed to produce recycled aluminium than to extract aluminium from its ore. Recycling preserves limited resources and requires less energy, so it causes less damage to the environment.