AQA Chemistry GCSE 1A

Atoms consist of electrons surrounding a nucleus that contains protons and neutrons.

 Neutrons are neutral, but protons and electrons are electrically charged: protons have a relative charge of +1 and electrons have a relative charge of -1.All substances are made of tiny particles called atoms An element is a substance that is made of only one sort of atom. There are about 100 different elements.

All substances are made from tiny particles called atoms. An atom has a small central nucleus made up of smaller sub-atomic particles called protons and neutrons. The nucleus is surrounded by even smaller sub-atomic particles called electrons.

Name of particle            Electrical charge

 proton                    +1

neutron                    0

electron                   -1

The atomic number of an atom is the number of protons it contains

The mass number of an atom is the total number of protons and neutrons it contains. The mass number of an atom is never smaller than the atomic number. It can be the same, but is usually bigger.

The first energy level can hold a maximum of two electrons, the second energy level a maximum of eight, and so on.

Energy level or shell    Maximum number of electrons

first                              2

second                        8

third                             8

A vertical column of elements in the periodic table is a group. The elements in a group have similar chemical properties to each other. For example, group 1 contains sodium and other very reactive metals, while group 7 contains chlorine and other very reactive non-metals. Group 0 (also known as group 8 or group 18) contains helium and other very unreactive non-metals.

The group 1 elements are found on the left hand side of the periodic table. They are called the alkali metals because they form alkaline compounds.

Their atoms all have one electron in their highest occupied energy level (outermost shell). This gives the group 1 elements similar chemical properties to each other.

The group 0 elements are found on the right hand side of the periodic table. They are called the noble gases because they are very unreactive.

When elements react, their atoms join with other atoms to form compounds. Chemical bonds form when this happens, which involves atoms transferring or sharing electrons.

Chemical bonds involve electrons from the reacting atoms. Compounds formed from metals and non-metals consist of ions. Ions are charged particles that form when atoms (or clusters of atoms) lose or gain electrons:

• metal atoms lose electrons to form positively charged ions
• non-metal atoms gain electrons to form negatively charged ions

The ionic bond is the force of attraction between the oppositely charged ions. This animation shows how ions form when sodium atoms react with chlorine atoms to form sodium chloride.

Balanced symbol equations show what happens to the different atoms in reactions. For example, copper and oxygen react together to make copper oxide. Take a look at this word equation for the reaction: copper + oxygen → copper oxide

Copper and oxygen are the reactants because they are on the left of the arrow. Copper oxide is the product because it is on the right of the arrow. If we just replace the words shown above by the correct chemical formulas, we will get an unbalanced equation, as shown here:

Cu + O₂ → CuO  Notice that there are unequal numbers of each type of atom on the left-hand side compared with the right-hand side. To make things equal, you need to adjust the number of units of some of the substances until you get equal numbers of each type of atom on both sides.

Here is the balanced symbol equation: 2Cu + O₂    →    2CuO

Limestone is mainly calcium carbonate, CaCO₃, which when heated 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 cement, mortar and concrete.

calcium carbonate calcium oxide + carbon dioxide

CaCO₃CaO + CO₂

For example, here are the equations for the thermal decomposition of copper carbonate:

copper carbonate copper oxide + carbon dioxide

CuCO₃

CuO + CO₂

If calcium carbonate is heated strongly, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide is yellow when hot, but white when cold.

Here are the equations for this reaction:

calcium carbonate calcium oxide + carbon dioxide

CaCO₃ CaO + CO₂

This is a thermal decomposition reaction.

Calcium oxide reacts with water to form calcium hydroxide, which is an alkali. 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.

Limestone is a type of rock, mainly composed of calcium carbonate. Limestone is quarried (dug out of the ground) and used as a building material. It is also used in the manufacture of cement, mortar and concrete.

Carbonates react with acids to produce carbon dioxide, a salt and water. For example:

calcium carbonate + hydrochloric acid → carbon dioxide + calcium chloride + water

CaCO₃ + 2HCl → CO₂ + CaCl₂ + H₂O  When limestone is heated strongly, the calcium carbonate it contains decomposes to form calcium oxide. This reacts with water to form calcium hydroxide, which is an alkali. Calcium hydroxide is used to neutralise excess acidity, for example, in lakes and soils affected by acid rain. Cement is made by heating powdered limestone with clay. Cement is an ingredient in mortar and concrete:concrete is made by mixing cement with sand, water and aggregate (crushed rock)

• mortar, used to join bricks together, is made by mixing cement with sand and water

Limestone, cement and mortar slowly react with carbon dioxide dissolved in rainwater and wear away. This damages walls made from limestone, and 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

The main advantages and disadvantages of the limestone industry

Advantages 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. think that concrete buildings and bridges are unattractive.