Elements, Compounds and Mixtures

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Elements, Compounds and Mixtures

Chemistry

Elements

The different types of atom are arranged in terms of their size in a table called the Periodic Table of elements.

Elements are substances which are made up of only one type of atom

Molecules

Some atoms form a very strong attachment (‘bond’) to another atom or atoms and as a result they always go around in groups of two or more atoms. We call these guys ‘molecules’.

It is very hard to break these ‘bonds’ and we say that in this case the atoms are ‘chemically combined’. We will look at this bonding in detail in another chapter later.

Basically a molecule is like a very small group of atoms that go around together (they are still too small to see).

Examples:

For the element oxygen the atoms go around in pairs of oxygen atoms.

These are called oxygen molecules and that is why oxygen is often represented as O₂.

For the compound water the atoms go around in ‘gangs’ consisting of two hydrogen atoms and one oxygen atom. These ‘gangs’ are called water molecules and that is why water is often represented as H₂O.

Compounds

A compound is formed when two or more atoms of different elements combine together chemically

My head hurts; what’s the difference between a compound and a molecule?

All compounds are molecules, but not all molecules are compounds (e.g. H₂ is a molecule because it is composed of two atoms chemically combined, but because they are both hydrogen atoms the molecule is not a compound).

Remember when we said that atoms are so small that you can’t see them?

Well molecules are made up of small groups of atoms so you won’t be able to see them either.

A compound however might be something like table-salt; you can hold it in your hand. It is a compound because it is made up of two different types of atom – in this case sodium and chlorine. The chemical name for table-salt is NaCl.

The interesting thing is that both sodium and chlorine can be fatal if ingested (swallowed) but when the two go together to form table salt the result is perfectly safe (once you don’t eat too much!).

We can summarise this as follows:

When elements combine to form compounds they may lose their individual properties.

Other examples of compounds and their constituent elements

Compound	Symbol	State of matter (at room temp)	Elements in the compound (and state at room temp)
Water	H₂O	liquid	Hydrogen (gas) oxygen (gas)
Carbon dioxide	CO₂	gas	Carbon (solid) oxygen (gas)
Magnesium oxide	MgO	solid	Magnesium (solid) oxygen (gas)
Iron sulphide	FeS	solid	Iron (solid) Sulphur (solid)

Mixtures

If a substance is made up of different components but they are just mingled together rather than combined at an atomic level then we call this a mixture.

A mixture contains two or more different substances mingled together but not chemically combined

Mixtures can be (fairly) easily separated, whereas compounds cannot be easily separated.

For example if you pour sulphur power over iron filings and mix them together you get a mixture, and to separate them simply use a magnet which will attract the iron filings and leave the sulphur powder behind.

However if you heated the mixture to a very high temperature their chemical compositions will change and the two elements will ‘bond’ together chemically to become a new substance called iron sulphide. This cannot now be separated and the result is called a compound.

Source : http://www.thephysicsteacher.ie/JC%20Science/JC%20Chemistry/Student%20Notes/2.%20Elements,%20Compounds%20and%20Mixtures.doc

Web site link: http://www.thephysicsteacher.ie

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