Atom  The smallest particle of an element.Scientists originally thought that atoms could not be split up, but this is not the case.

Molecule A cluster of non-metal atoms that are chemically bonded together.The atoms in a molecule are joined by covalent bonds.The atoms always join in fixed ratios and molecules have a specific formula, eg H2O or N2.There are molecules of compounds (eg CH4) and molecules of elements (eg O2).

ElementsA pure substance that is listed on the periodic table and only has one type of atom in it.There are over 100 elements.Most are metals, a few are semi-metals, and the rest are non-metals.

Compound A pure substance made from more than one type of element chemically bonded together.Elements bond in fixed ratios and so can be represented by a chemical formula. For example, sodium chloride has the same number of sodium ions and chloride ions, so its formula is NaCl; whereas water is always made from twice the number of hydrogen atoms as oxygen atoms, so it is H2O.

Mixture An impure substance made from different elements or compounds.Mixtures can usually be separated by physical techniques such as filtering and distillation.Air is a mixture that contains the elements nitrogen, oxygen and argon, and also the compound carbon dioxide.

Atomic Structure:The structure of the atom

Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom. The nucleus is very much smaller than the atom as a whole. The electrons are arranged in energy levels around the nucleus.

The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall. Atoms can lose or gain electrons. When they do, they form charged particles called ions:

• if an atom loses one or more electrons, it becomes a positively charged ion
• if an atom gains one or more electrons, it becomes a negatively charged ion

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.

Chemical Symbols

The full symbol for a chlorine atom

If you know the atomic number and mass number you can calculate the number of each sub-atomic particle in an atom. The full chemical symbol for an element shows its mass number at the top, and its atomic number at the bottom.

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 by subtracting the atomic number from the mass number. In this example, it is 35 – 17 = 18 neutrons.

An English scientist called John Newlands put forward his Law of Octaves in 1864. He arranged all the elements known at the time into a table in order of relative atomic mass.

Atomic Weight:

Both Newlands and Mendeleev arranged the elements in order of their atomic weight (now called relative atomic mass).

Both scientists produced tables in which elements with similarproperties were placed at regular intervals. However, Mendeleev did some things with his table that made it more useful than Newlands’ table – for example, he swapped the order of some elements if that fitted their properties better.

Forming ions:

Ions are electrically charged particles formed when atoms lose or gainelectrons. This loss or gain leaves a full outer shell, so the electronic structure of an ion is the same as that of a noble gas (such as helium, neon or argon). Metal atoms and non-metal atoms do different things when they ionise. Metal atoms lose the electron, or electrons, in their highest energy level and become positively charged ions.

Positively charged sodium and aluminium ions:

Non-metal atoms gain an electron, or electrons, to become negatively charged ions.

Negatively charged oxide and chloride ions

Lithium, Li

Lithium is in Group 1. It has one electron in its outer shell. When this electron is lost, a lithium ion, Li+, is formed.

Sodium, Na

Sodium is also in Group 1. It has one electron in its outer shell. When this electron is lost, a sodium ion, Na+, is formed.

Forming ions: 

When metals react with non-metals, electrons are transferred from the metal atoms to the non-metal atoms, forming ions. The resulting compound is called an ionic compound.

Reactions between metals and non-metals include:

• sodium + chlorine → sodium chloride
• magnesium + oxygen → magnesium oxide
• calcium + chlorine → calcium chloride

Word and balanced equations

Chemical equations show what happens in a reaction. In general, we write:

reactants → products

The reactants are the substances that react together. The products are the substances produced in the reaction. Individual substances are separated by a plus sign.

Word and Balanced Equations:

A word equation gives the names of the substances involved in a reaction. For example:

copper + oxygen → copper(II) oxide

Copper and oxygen are the reactants, and copper(II) oxide is the product.

Balanced Equations:       Balanced equations give the symbols and formulas of the substances involved in a reaction. In the example above, if we just replace the words shown above with the correct chemical formulas, we will get an unbalanced equation, as shown here:

Cu + O₂    →    CuO     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 we now have two copper atoms and two oxygen atoms on each side. This matches what happens in the reaction.