Foundation and higher tiers

Atoms, elements, compounds and molecules

Elements are simple substances composed of only one type of atom. The periodic table lists all the known elements. Each element in the periodic table has its own chemical symbol. It is important that you learn the names and symbols for the first 20 elements in the periodic table as these are the ones you are mostly likely to meet.

Why not do a little research on some of the elements and find out about their properties, common reactions and the people who discovered them? The chemical symbols for some the elements often come from the names of the scientists who discovered them, or from the names of gods or astronomical objects.

Some elements have only a single letter for their chemical symbol, e.g. oxygen is given the symbol O. It is a capital O and not a lowercase o. This is true for all single letter chemical symbols in the periodic table. If the chemical symbol is made up of 2 letter e.g. sodium metal has the chemical symbol Na. The first letter is always capitalized and the second is always lowercase, so the chemical symbol for sodium is never NA or na, always Na. Iron is Fe, NEVER fE or FE.

Compounds

Compounds are formed when two or more elements chemically join together. The atoms present in the compound are held together by chemical bonds. You will learn about three types of chemical bonds in your GCSE chemistry course, these are covalent, ionic and metallic bonds. The images below show 3 common compounds that you will meet in your GCSE course. These three compounds contain only non-metal atoms; compounds like this which contain only non-metal elements are called covalent compounds. The 3 compounds shown below are all molecular. A molecule is simply a small groups of atoms.

This is a molecule of methane. It is a compound formed from the elements hydrogen and carbon. There are 4 hydrogen atoms and 1 atom of carbon in this compound. Its chemical formula is CH4
	This is a molecule of water. It is a compound formed between the elements hydrogen and oxygen. Its chemical or molecular formula is H2O.
This molecule is called ammonia, it is a compound made up of the elements nitrogen and hydrogen. Its chemical or molecular formula is NH3.

Elements, compounds and molecules self-check

Drag the left hand boxes below to show their correct meanings

The properties of compounds

Compounds have very different properties from the elements that make them up; as an example consider the reaction shown in the image below between aluminium metal and the non-metal element iodine. Aluminium is a shiny solid metallic element while iodine is a bluish-black non-metallic solid with a shiny lustre. If aluminium and iodine are mixed on a tin lid and a few drops of water are added a very violent reaction occurs, the product of this reaction is aluminium iodide. Aluminium iodide is a a dull greyish coloured ash which does not resemble either of the starting reactants that make it up.

Iron and sulfur

iron_(s) + sulfur_(s) → iron sulfide_(s)

Ionic compounds

Compounds formed between metals and non-metal elements are called ionic compounds. These compounds are made up of ions. Ions are charged atoms formed when atoms lose or gain electrons. Metals tend to lose electrons when they react and form ions with a positive charge, while non-metal elements tend to gain electron when they react. Ionic compounds are not molecular, that is their structure is not made up of small groups of atoms; instead they have giant structures of ions called ionic lattices. The physical properties of compounds such as melting, boiling points and whether it conducts electricity tend to be determined by the type of structure a compound has.

Particle pictures

The diagram shows a particle picture for the reaction of iron and sulfur to form iron sulfide. You can see that all the iron atoms and all the sulfur atoms are the same; this obviously means that they are elements. However once they react to form the compound iron sulphide the iron and sulfur atoms join with each other. Iron sulfide has a giant ionic lattice structure. Only a few iron and sulphur ions are shown but in reality the structure would consist of billions of ions all joined together.

This is very different from the small molecular structure shown for covalent compounds above. Ionic compounds have very different chemical and physical properties from covalent compounds largely due to differences in their structures and the types of bonds holding the particles together. You will learn more about this when you revise the section on bonding and structures.

Naming compounds

Most compounds end in the letters -ide, this tells us that the compound is made up of only 2 elements. Some compounds end in the letters -ate, this means the compounds have more than 2 elements and one of them is oxygen e.g.

Word equations

When an element reacts to form a compound we can show this chemical reaction as a word equation e.g. hydrogen gas burns in air (oxygen) to form water (hydrogen oxide). A word equation for this reaction is shown below.

hydrogen_(g) + oxygen_(g) → hydrogen oxide_(g)

Here the reactants are hydrogen and oxygen and the compound formed is called the product of the reaction. Compounds are named as described above. Here the last 4 letters of the non-metal oxygen (ygen) are replaced by -ide.

Quick quiz

Test your understanding of the key ideas on this page by completing the paragraph below by choosing the correct word from the drop down menus. The words which appear in the drop down menus are shown in the yellow boxes below.

Key Points

• Elements are simple substances made up of only one type of atom.
• The chemical symbols for elements USE CAPITAL LETTERS for single symbol elements and CAPITAL LETTER and lowercase for elements with a 2 letter chemical symbol e.g. N for nitrogen, not n! H for hydrogen not h, He for helium, not HE or he.
• Compounds contain more than one type of atom chemical bonded together
• Covalent compounds contain only non-metal atoms. Most covalent compounds have a molecular structure.
• Ionic compounds contain metal and non-metal atoms chemically bonded. These compounds have a giant lattice structure.
• The molecular formula for a compound tells you the ratio the particles are present in e.g. H₂O, there are twice as many hydrogen atoms as oxygen atoms present in water.

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