8.
PERIODIC TABLE
At first glance, the periodic table looks very complex. In fact it is a large grid of every element that exists. The elements are arranged in order of their atomic number. The atomic number is the number of protons each atom has in its nucleus. By arranging the elements in this way, those with similar properties (characteristics) are grouped together. As with any grid, the periodic table has rows running left to right, and columns running up and down. The rows are called PERIODS and the columns are called GROUPS.
Hydrogen (H) is the first element in the periodic table because it has just one proton in its nucleus. Helium (He) is second, because it has two protons, and so on. The periodic table can be coloured-coded. Often, each group is given a particular colour so that it is easy to pick out all the elements that belong to a particular group.
As well as a name, each element has a symbol, a shorthand way of writing the element in chemical equations. Often this is the first letter or two of the element’s name, but it can come from a Latin name. Each also has an atomic number and a mass number.
GALLIUM
One element that Mendeleyev left a gap for in his periodic table was gallium (element 31). Mendeleyev called it eka-aluminium because he predicted it would have similar properties to aluminium. In 1875, French scientist Lecoq de Boisbaudran discovered gallium. It has the exact properties that Mendeleyev predicted. Gallium is a soft, silvery metal with a melting point of 29.8ºC (85.6ºF).
BIOGRAPHY: DIMITRI MENDELEYEV Russian, 1834-1907
This chemist was convinced there was an order to the elements. He collected information on each one and, in 1869, he published a table of elements on which the modern periodic table is based. He left gaps for elements he predicted would be found, such as gallium, germanium, and scandium.
GROUPS
There are 18 groups (columns) in the periodic table. Group 1 (also known as the alkali metals) is the column on the far left of the table. Elements in the same group have similar, but not identical characteristics. This is because they all have the same number of electrons in their outermost shell. You can tell a lot about an element just by knowing which group it is in.
INCREASING SIZE
As you move down one element in a group, there is a large jump in the number of protons and neutrons in the nucleus, and a new shell of electrons is added. The extra particles make the atom heavier and the extra shell of electrons makes the atom take up more space.
METAL IN SPACE
An astronaut’s visor is gold-plated to reflect sunlight. This shiny, hard-wearing metal does not corrode (rust), making it ideal for use in space, where materials cannot be replaced easily. Gold, copper, and silver belong to group 11. Group 11 metals are also called coinage metals, because they are used to make coins.
PERIODS
The properties of the elements across a period (row) change gradually. The first and last elements are very different. The first is a reactive solid – it catches fire when it mixes with oxygen – and the last is an unreactive gas. However, they have the same number of electron shells. All the elements in the third period, for example, have three shells for their electrons.
DECREASING SIZE
As you go across a period, the atoms get slightly heavier, but they also get smaller. This is because the number of electron shells stays the same across the period, but the number of protons in the nucleus increases. The stronger, attractive force from the positively charged protons sucks the negatively charged electrons tighter into the centre.
PHOSPHORUS MATCH
Phosphorus is a non-metal element. It is a yellowish, waxy, slightly see-through solid. Like magnesium, it is very reactive. Because of this, phosphorus compounds are used on the tips of matches. Phosphorus glows in the dark, an effect called phosphorescence.
UNREACTIVE ARGON
Argon is very unreactive and does not combine with other elements. In arc welding, metals are melted surrounded by argon gas. The argon keeps oxygen out, so that oxygen cannot react with the melted metals.
9.
MOLECULES
Most atoms join up with other atoms through chemical BONDS to form larger particles called molecules. They can join up with atoms of the same element or with atoms of different elements. Substances whose molecules contain different types of atom are called compounds. Chemical reactions can CHANGE MOLECULES and when this happens, new molecules and therefore new compounds are formed.
VARIETY OF MOLECULES
Molecules can be simple or complex. They can even be made up of just one atom. The element argon is a one-atom molecule. Other molecules can consist of two atoms of the same element. The oxygen molecule is made up of two oxygen atoms bonded together. However, in certain circumstances, three oxygen atoms bond together, forming a molecule called ozone.
SIMPLE MOLECULE
Water molecules (H20) are very simple. They are made of two hydrogen (H) atoms bonded to one oxygen (O) atom. All water molecules are the same, but they are different from the molecules of any other substance. A water molecule is the smallest possible piece of water. You can break it up into smaller pieces, but they wouldn’t be water anymore. The symbols that scientists use to represent molecules are called chemical formulae.
COMPLEX MOLECULE
Some molecules, such as the plastic in a snorkel, contain hundreds or even thousands of carbon, hydrogen, and chlorine atoms joined together in long, winding chains. Such complex molecules are called polymers. They are possible because carbon atoms are able to form very stable bonds with other carbon atoms. Most of the molecules that make up living things are made of complex polymers.
BONDING
When atoms join together to form molecules, they are held together by chemical bonds. These bonds form as a result of the sharing or exchange of electrons between the atoms. It is only the electrons in the outermost shell that ever get involved in bonding. Different atoms use these electrons to form one of three different types of bond: ionic bonds, covalent bonds, or metallic bonds.
DIFFERENT KINDS OF BONDS BETWEEN ATOMS
IONIC BONDS
In ionic bonds, electrons are transferred from one atom to another. When sodium and chlorine combine to form sodium chloride (salt), sodium loses an electron and becomes positively charged; chlorine takes that electron and becomes negatively charged. Ionic bonds are difficult to break. Ionic compounds are usually solids with high melting points.
COVALENT BONDS
In a covalent bond, electrons are shared between two atoms. When two oxygen atoms bond together to form an oxygen molecule, they share four electrons – two from each oxygen atom. Other examples of covalent bonding are water (H2O), and carbon dioxide (CO2). Covalent compounds are usually liquids or gases with low melting points.
METALLIC BONDS
Metal atoms are bonded to each other through metallic bonding. In this type of bonding, all the atoms lose electrons, which float around in a common pool. The electrons in this pool can move around freely, which is why metals can transfer heat or electricity so well. If one part of the metal is heated, the electrons carry the heat quickly to other parts.
CHANGING MOLECULES
All around you, molecules are changing and rearranging their atoms in chemical reactions to form new molecules and new compounds. When you breathe in oxygen, it goes through a chemical change inside your body and forms a new compound, carbon dioxide, which you breathe out. Catalysts are special types of molecules that speed up chemical reactions, but do not actually change themselves. They are used, for example, in catalytic converters in cars.
SFX REACTION
A special effects explosion is a chemical reaction that releases energy. Pyrotechnic experts want each explosion to be unique, so they use different types and amounts of explosives. In every chemical reaction, some bonds between atoms are broken and new ones are made. Energy is needed to break a bond, but energy is released when a bond is made. Depending on the number and type of bonds broken and made, a reaction may take in or give out energy.
CATALYTIC CONVERTER
When a car engine burns petrol, it releases harmful gases. Cars fitted with a catalytic converter change the harmful gases into safer gases. When they enter the catalytic converter, the gases form temporary bonds with the surface of the catalyst. This brings them into close contact with each other and allows new, safer gases to form.
ENZYMES IN THE KITCHEN
Enzymes are catalysts found in nature. For example, it is the enzymes in yeast that cause bread dough to rise. When yeast is mixed with warm water and sugar it starts to grow and bubbles of carbon dioxide gas are produced. When the yeast mixture is added to flour and water to make a dough, the dough rises. Heating bakes the bread and kills the yeast. Scientists use chemical equations to show how molecules change in a chemical reaction.
10.
CHEMICAL REACTIONS
In a chemical reaction, the molecules of one substance break apart and join together with those of another substance to create a different compound (combination of molecules). Many chemical reactions are NON-REVERSIBLE CHANGES .You cannot turn a baked cake back into its raw ingredients. Some chemical reactions can be reversed, and re-formed into the original substances. These are REVERSIBLE CHANGES.
PHYSICAL CHANGE
A melting ice lolly is an example of a physical change, not a chemical change. The liquid ice lolly is not a new material, just a different form of the old one. Physical changes do not create new substances and no chemical bonds are broken or made. Melting, freezing, tearing, bending, and crushing are all physical changes that alter a substance’s appearance but not its chemical properties.
NON-REVERSIBLE CHANGE
Many chemical reactions are non-reversible changes. This means they are permanent changes that cannot be undone. You cannot turn the new materials made back into the original materials again. Rusting is a non-reversible change. However, if rust is mixed with magnesium powder another chemical reaction occurs and iron can be extracted from the rust.
BURNING
Burning is a non-reversible chemical change. When you burn wood, the carbon in the wood reacts with oxygen in the air to create ash and smoke, and energy in the form of light and heat. This is a permanent change that cannot be undone – you cannot turn ashes back into wood.
REVERSIBLE CHANGE
A few chemical reactions can be reversed – the original materials can be re-created from the new materials. These reactions are called reversible changes. They have a forward reaction and a backward reaction. Both reactions are actually happening at the same time but, depending on the conditions, one will be stronger than the other.
NITROGEN DIOXIDE
When the gas nitrogen dioxide is heated, a forward chemical reaction changes the brown nitrogen dioxide gas into two colourless gases – nitrogen monoxide and oxygen. However, if these colourless gases are cooled, they will re-form into brown nitrogen dioxide gas. This is called a backward chemical reaction.
11.
ACIDS
The sour taste of food such as lemons is due to acids. Acids in food are weak, but they can sting if they touch a cut on your skin. Strong acids, such as sulphuric acid in car batteries, are much more dangerous as they can burn through materials. Acid compounds all contain hydrogen. They dissolve in water to produce particles called hydrogen ions. The more hydrogen ions an acid contains, the stronger an acid it is.
Lemons and other citrus fruits taste sour because they contain citric acid. Citric acid is used to add a tangy taste to food and soft drinks. Citrus fruits also contain another acid, called ascorbic acid or Vitamin C, which we need for healthy skin and gums.
Strong acids and bases are extremely poisonous, corrosive, and cause bad burns, so their containers are labelled with hazard symbols. Some give information about how to handle the chemicals safely. The symbols are also displayed on the tankers that transport acids and bases, so emergency services know how to handle the substances in the case of an accident or spillage.
12.
BASES
Many cleaning products, such as soap and oven cleaner, are bases. Bases neutralize (cancel out) acids. Alkalis are bases that dissolve in water. Strong bases, such bleach, are corrosive and burn skin. Bases contain particles called hydroxide ions. The more hydroxide ions a base contains, the stronger it is.
LIMESTONE
Limestone is an important base that is dug from the earth in quarries. It comes from calcium carbonate, which formed millions of years ago from the compressed remains of sea-shells and other marine life. Once quarried, limestone is crushed and used to make cement, fertilizers, paints, and ceramics.
13.
METALS
Almost three-quarters of all elements are metals, such as gold and silver. There are also some elements we may not think of as metals, such as the calcium in our bones, and the sodium in table salt (sodium chloride). Metals are defined by their METALLIC PROPERTIES, such as high melting points. Mixtures of metals are called ALLOYS. Solder is an alloy that is used to join metals in plumbing and electrical wiring. It is mainly tin with lead or silver.
METAL GROUPS
Metals are classified according to where they are found in the periodic table. Each group has a set of properties that make the metals useful for different purposes.
ALKALI METALS
These include potassium and sodium, and form Group I of the periodic table. They are extremely reactive metals: they react strongly with water to form strong alkalis.
ALKALINE-EARTH METALS
These elements make up Group II of the periodic table. They combine with many elements in the Earth’s crust. Their oxides react with water to form alkalis.
TRANSITION METALS
This group includes copper, silver, and gold. They are hard and shiny, have high melting points, and are good conductors of heat and electricity.
OTHER METALS
Also called poor metals, these metals are fairly soft and melt easily. They include tin and aluminium and are often used in alloys. Bronze is an alloy of tin and copper.
GOLD IN QUARTZ
Some metals, such as gold, are found naturally as pure metals in rocks. Gold is unreactive, so it does not combine with other elements. Most metals are more reactive and are found combined with other elements in rocks. Iron, for example, is usually combined with oxygen. The rocks in which metals are found are called ores.
EXTRACTING GOLD
To extract gold from its ore, huge grinders crush the ore to a fine powder. The powder is mixed with a solution of cyanide. Only the gold from the ore dissolves in the solution. Powdered zinc is added to bring the gold out of the solution. The gold is melted down and poured into moulds.
METALLIC PROPERTIES
Metals are usually shiny solids with high melting points and are very good conductors of heat and electricity. They are malleable, so they can be beaten into sheets, and ductile, which means they can be drawn into wires. Most are strong and cannot be broken easily. Of course, there are exceptions: mercury, for example, is a metal that has a low boiling point and is liquid at room temperature.
ELECTRICAL CONDUCTORS
The transmission lines (electric cables) that bring electricity to our homes, schools, and offices all rely on copper. Copper is a red-orange metal that is one of the best electrical conductors. Metals conduct electricity well because when metal atoms bond (join together), the electrons in their outer shells move freely. If electricity passes through one part of the metal, the electrons carry the electricity quickly to other parts.
ALLOYS
Alloys are mixtures of metals with properties that make them more useful than pure metals. A mixture of chromium and iron resists rusting much better than iron on its own. Most alloys are made of two or more metals, but some contain a non-metal. Steel is an alloy of iron and carbon. Alloys are made by melting the different materials together. Changing the proportions of the materials can change the properties of the alloy.
14.
NON-METAL ELEMENTS
The metal elements in the periodic table have easily defined properties. The remaining elements, however, have very different properties. They consist of a group of unreactive gases called the NOBLE GASES, a group of reactive elements known as the HALOGENS, and a set of elements referred to as non-metals. In addition, a few elements have properties that place them in between metals and non-metals. They are called the SEMI-METALS.
SULPHUR CRYSTALS
Deposits of the non-metal sulphur are found as deep as 300 m (1,000 ft) below ground. Combined with other elements, sulphur is also found in rocks and minerals, such as gypsum.
MAKING SULPHURIC ACID
Sulphur crystals are ground to a powder at sulphur processing plants. The powder is sprayed into a furnace where it reacts with oxygen, forming sulphur dioxide. More oxygen is reacted with the sulphur dioxide to make sulphur trioxide, which is dissolved in water to make oleum.
TRANSPORTING SULPHURIC ACID
Oleum is concentrated sulphuric acid. It is transported to manufacturing plants in tankers. Here, water is added to the oleum in precise measures to make the correct concentration of sulphuric acid. Sulphuric acid is used in the manufacture of detergents, paints, medicines, plastics, and synthetic fabrics.
SEMI-METALS
The elements known as semi-metals have some of the properties of metals and some of the properties of non-metals. Arsenic, for example, has the shininess of a metal but does not conduct heat or electricity very well. Other semi-metals, such as silicon and germanium, are semi-conductors. This means that they can conduct electricity, but only under special conditions. This property makes them very useful in solar panels and computers.
HALOGEN
At first sight, the halogens don’t seem very alike. For example, fluorine is a yellow gas and iodine is a shiny, black solid. However, they are all highly reactive and are quick to combine with other elements to form salts, such as table salt (sodium chloride). They also have important uses. Chlorine is used to disinfect water, and compounds of fluorine – fluorides – are added to toothpaste to prevent tooth decay.
Bromine is the only liquid non-metal element. It is a reddish-brown colour and evaporates quickly to form a choking, poisonous gas. Bromine is found in seawater and mineral springs in the form of salts, called bromides. Bromine compounds are used in photography, as mild sedatives, and in the manufacture of flameproof coatings and dyes.
SILVER BROMIDE IN X-RAYS
In X-ray photography, a plastic film is coated with a paste of a bromine compound called silver bromide. When X-ray light strikes the film, the silver bromide breaks apart and pure silver atoms are left on the film. The more intense the light, the more silver atoms are formed and the darker that part of the image becomes.
NOBLE GASES
Group 18 of the periodic table contains the noble gases. These six unreactive gases do not combine with other elements, so they are usually found on their own. Nearly 1 per cent of air is argon. Traces of neon, helium, krypton, radon, and xenon are also found in air. Argon is used in light bulbs, xenon is used in lighthouse arc lamps, and helium is used to fill airships and hot-air balloons.
NEON LIGHTING
A neon light is a tube containing a noble gas, but not always neon. When electricity is passed through the tube, the atoms of the noble gas emit (give out) light of different colours. Helium emits a yellow light, neon a red light, argon a blue light, and krypton a purple light. Other colours are created by giving the glass tube different coloured coatings.
15.
HYDROGEN
You cannot see, taste, or smell hydrogen, yet this element makes up over 90 per cent of matter. The Sun and stars are made of hydrogen gas. On Earth, hydrogen forms compounds (mixture of elements), and is found in almost every living thing. Hydrogen gas is used to make chemicals such as ammonia, which is needed to make fertilizers. Hydrogen is also used to increase the amount of petrol produced from crude oil.
HYDROGEN IN STARS
Stars are fuelled by hydrogen. At incredibly high temperatures inside stars, hydrogen atoms smash into one another and fuse (join) together to create helium atoms. These reactions give out a huge amount of energy as light and heat. Hydrogen atoms were probably the first atoms to form in the Universe and fuse together to create other, heavier atoms.
SPACE SHUTTLE
The space shuttle uses liquid hydrogen fuel because hydrogen gives out a lot of power for very little weight. Hydrogen, like all fuel, needs oxygen to burn, so the shuttle has a tank of liquid hydrogen and a tank of liquid oxygen. A fine mist of the two liquids is sprayed into the engines and ignited (set alight). The hydrogen explodes, sending steam out of the nozzles and helping to thrust the shuttle into space.
HYDROGENATION
Margarine is made by passing bubbles of hydrogen gas through hot vegetable oil. Extra hydrogen atoms bond (join) with the oil molecules, and the oil changes from a liquid to a more solid form. This process is called hydrogenation. If oil is fully hydrogenated, it becomes completely solid; by stopping part way, it becomes a semi-solid.
HYDROGEN-FUELLED CAR
Scientists are developing hydrogen-powered cars. The cars contain tanks of hydrogen that combine with oxygen from the air to drive them. Hydrogen-fuelled cars produce water instead of polluting exhaust gases. They are not mass-produced, because scientists have not developed a compact and lightweight method for storing hydrogen yet.
BIOGRAPHY: ANTOINE LAVOISIER French, 1743-1794
This chemist is often known as the father of modern chemistry. He studied the “inflammable air” that was discovered by English scientist Henry Cavendish (1731–1810). Lavoisier discovered that this gas combines with oxygen to make water. He named the gas hydrogen, which is Greek for water-former.