Chemistry Unit 1 Topic 1

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.

Atoms and elements

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. These are shown in the periodic table, which is a chart with all the elements arranged in a particular way. The horizontal rows in the periodic table are called periods and the vertical columns are called groups. The elements in a group have similar properties to each other.

Metals and non-metals

The metals are shown on the left of the periodic table, and the non-metals are shown on the right. The dividing line between metals and non-metals is shown in red on the table below. You can see that most of the elements are metals.

The periodic table divided into non-metals and metals

Chemical symbols

The atoms of each element are represented by a chemical symbol. This usually consists of one or two different letters, but sometimes three letters are used for newly discovered elements. For example, O represents an oxygen atom, and Na represents a sodium atom.

The first letter in a chemical symbol is always an UPPERCASE letter, and the other letters are always lowercase. So, the symbol for a magnesium atom is Mg and not mg, MG or mG.

Atomic structure

Structure of the atom

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.

Protons and electrons have an electrical charge. Both have the same size of electrical charge, but the proton is positive and the electron negative. Neutrons are neutral.

A summary of the electrical charges in sub-atomic particles

Name of particle	Electrical charge
proton	+1
neutron	0
electron	-1

The number of electrons in an atom is equal to the number of protons in its nucleus. This means atoms have no overall electrical charge.

Atomic number and mass number

The atomic number of an atom is the number of protons it contains. All the atoms of a particular element have the same atomic number (number of protons). The atoms of different elements have different numbers of protons. For example, all oxygen atoms have 8 protons and all sodium atoms have 11 protons.

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.

Full chemical symbols

You need to be able to calculate the number of each sub-atomic particle in an atom if you are given its atomic number and its mass number. The full chemical symbol for an element shows its mass number at the top, and its atomic number at the bottom.

The full symbol for a chlorine atom

This symbol tells you that the chlorine atom has 17 protons. It will also have 17 electrons, because the number of protons and electrons in an atom is the same.

The symbol also tells you that the total number of protons and neutrons in the chlorine atom is 35. Note that you can work out the number of neutrons from the mass number and atomic number. In this example, it is 35 – 17 = 18 neutrons.

Electronic structure

The electrons in an atom occupy energy levels. These are also called shells. Each electron in an atom is found in a particular energy level. The lowest energy level (innermost shell) fills with electrons first. Each energy level can only hold a certain number of electrons before it becomes full. The first energy level can hold a maximum of two electrons, the second energy level a maximum of eight, and so on.

Electrons in the first three energy levels for the elements with atomic numbers 1 to 20

Energy level or shell	Maximum number of electrons
first	2
second	8
third	8

Writing an electronic structure

The electronic structure of an atom is written using numbers to represent the electrons in each energy level. For example, for sodium this is 2,8,1 – showing that there are:

• 2 electrons in the first energy level
• 8 electrons in the second energy level
• 1 electron in the third energy level.

You can work out the electronic structure of an atom from its atomic number or its position in the periodic table. Start at hydrogen, H, and count the elements needed to reach the element you are interested in. For sodium, it takes:

• 2 elements to reach the end of the first period (row)
• 8 elements to reach the end of the second period
• 1 element to reach sodium in the third period.

The diagram of the periodic table shows how this works.

Periodic table related to electronic structure

You need to be able to write the electronic structure of any of the first twenty elements (hydrogen to calcium).

Electronic structure diagrams

You need to be able to draw the electronic structure of any of the first twenty elements (hydrogen to calcium). In these drawings:

• the nucleus is shown as a black spot
• each energy level is shown as a circle around the nucleus
• each electron is shown by a dot or a cross.

The electronic structure of some elements

Element	Symbol	Electronic structure (written)	Electronic structure (drawn)
lithium	Li	2,1
fluorine	F	2,7
chlorine	Cl	2,8,7
calcium	Ca	2,8,8,2

Do not worry in the exam about colouring in the electrons. Just make them clear and ensure they are in the right place. You may be asked to use a cross rather than a dot for each electron.

Working out an element's electronic structure

Here is how to use the periodic table to work out an electronic structure:

1. Find the element in the periodic table. Work out which period (row) it is in, and draw that number of circles around the nucleus.
2. Work out which group the element is in and draw that number of electrons in the outer circle – with eight for Group 0 elements – except helium.
3. Fill the other circles with as many electrons as needed. Remember – two in the first circle, and eight in the second and third circles.
4. Finally, check that the number of electrons is the same as the atomic number.

Elements in the same group in the periodic table have similar chemical properties. This is because their atoms have the same number of electrons in the highest occupied energy level. Group 1 elements are reactive metals called the alkali metals. Group 0 elements are unreactive non-metals called the noble gases.

Groups

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 modern periodic table

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.

Group 1

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.

Group 1 of the periodic table

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.

Group 1 elements

Element	Symbol	Electronic structure (written)	Electronic structure (drawn)
lithium	Li	2,1
sodium	Na	2,8,1
potassium	K	2,8,8,1

Reactions of group 1 elements with water

Potassium reacts with a lilac flame

Lithium, sodium and potassium all react vigorously with water to form a metal hydroxide and hydrogen:

metal + water → metal hydroxide + hydrogen

The metal hydroxides are strong alkalis.

The group 1 elements need to be stored under oil to prevent them reacting with oxygen and water vapour in the air.

Reactions of group 1 elements with oxygen

Lithium, sodium and potassium are easily cut with a blade. The freshly cut surfaces are silvery and shiny, but quickly turn dull as the metal reacts with oxygen in the air. The group 1 metals react vigorously with oxygen to form metal oxides. Lithium burns with a red flame, sodium with a yellow-orange flame, and potassium burns with a lilac flame.

Group 0

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.

The noble gases - Group 0

The highest occupied energy levels (outermost shells) of their atoms are full:

• helium atoms have two electrons in their outer energy level
• atoms of the other noble gases have eight electrons in their outer energy level.

This is a neon atom it has a stable arrangement of electrons, and makes the group 0 elements 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.

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 sulfur react together to form a compound called iron sulfide.

Compounds usually have different properties from the elements they contain.

Ionic bonds

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.

Covalent bonds

Compounds formed from non-metals consist of molecules. The atoms in a molecule are joined together by covalent bonds. These bonds form when atoms share pairs of electrons.

Chemical formulas

The chemical formula of a compound shows how many of each type of atom join together to make the units which make up the compound. For example, in iron sulfide every iron atom is joined to one sulfur 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 Na₂O. Notice that the 2 is written as a subscript, so Na2O would be wrong.

This diagram shows that one carbon atom and two oxygen atoms combine to make up the units of carbon dioxide. Its chemical formula is 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 sulfur 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

Chemical equations

You should be able to write word equations for the reactions you study in GCSE Science or GCSE Chemistry. If you are taking the Higher Tier, you should also be able to write and balance symbol equations.

Copper and oxygen reaction: getting a balanced equation

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

You can see that now there are 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 molecules of copper oxide

Remember: never change a formula to balance an equation.

Conservation of mass

No atoms are lost or made during a chemical reaction. This means that the mass is always conserved. In other words, the total mass of products after the reaction is the same as the total mass of the reactants at the start.

This fact allows you to work out the mass of one substance in a reaction if the masses of the other substances are known. For example:

Carbon reacts with oxygen to form carbon dioxide:

C + O₂ → CO₂

12 g of carbon will react to form 44 g of carbon dioxide. It must react with 44 – 12 = 32 g of oxygen to do this. The animation below shows two more examples of conservation of mass.