1.8 Elements compounds and mixtures

compounds, elements or mixtures ?

 Sun and air?

Study the seaside image for a few seconds . Try to describe it:

You might say that the image shows : A calm sea, clear blue sky, bright sunshine and a perfectly smooth beach.

You could describe it in terms of the states of matter visible: 

​SOLID ​Sand
​LIQUID ​Sea
​GAS​Air
​GAS and PLASMA​Sun

But as a chemist you might be asked to classify each of these four components as either mixtures, compounds or elements. This section will help you to do that.


 1.8 Meet the Elements

Three of the Noble gases

 Students should:

1.8 understand how to classify a substance as an element, compound or mixture

The chemical elements are all listed on the periodic table. The elements are listed in order of atomic number.

All substances are made up of atoms. All of the atoms in a sample of an element have the same atomic number.

The simplest element is hydrogen. This is the first element in the periodic table as it has the atomic number 1.  The Sun produces energy as the result of  nuclear fusion of hydrogen atoms. This releases vast amounts of energy and produces heavier elements such as helium. As it is so hot - most of the gas in the sun is actually in the form of plasma ( the fourth state of matter). 

...

Periodic Table – Royal Society of Chemistry

Interactive periodic table with element scarcity (SRI), discovery dates, melting and boiling points, group, block and period information.

 1.8 Mixture or compound?

Particles in a mixture are not combined

 A mixture of herbs

MIXTURE : A mixture is made up of two or more components which although mixed together can be separated by physical means. The components are not chemically combined. 

Air is a mixture of gases. The gases in air are the elements Nitrogen (78%), and oxygen Oxygen (21%), the compound carbon dioxide (~ 0.03%) and the noble gases.

Pure water is a compound with the formula H2O . Seawater contains lots of dissolved salts and lots of minute suspended particles. So seawater is really a mixture too.

Particles in a compound are chemically combined 
Salt is a compound of sodium and chlorine

COMPOUND : A compound is always made up of two or more different elements in definite proportions joined by chemical bonds. The elements cannot be separated by physical means

Common salt is a compound whose chemical name is sodium chloride. Sodium is a metallic element with the atomic number 11. Chlorine is a non-metal with the atomic number 17. 

Sand is made up of tiny particles of rock. The main compound in the rocks which make up sand is silicon dioxide - a compound  with the formula SiO2 .


 1.8 Some elements up close

Giant metallic lattices. More than 70% of the elements in the periodic table have a structure similar to this. The negatively charged electrons are "delocalised" and can move from atom to atom.

A metallic element

Giant covalent lattices. These elements form giant lattices have high melting points and form crystalline structures.

Diamond and Silicon have structures like this. There are no free electrons in this sort of structure so they do not conduct electricity.

Monatomic gases. The particles which make up the the Noble Gases are single atoms; they do not join up with one another. 

The noble gases have stable atoms with a full outer shell of electrons.  This makes them unreactive or inert.



A solid element with small molecules
A gaseous element with small molecules

Simple molecular elements. These are often gases at room temperature but can be liquid or easily melted solids.

The atoms shown here are paired up to form diatomic molecules. The forces between the molecules are weak and so elements with this structure will have low melting points and might even be gases at room temperature. 

The examples shown here represent:

left: the structure of iodine ( I2) and

right: the strucutre of oxygen (O2)



 1.8 Some compounds up close

A compound containing small molecules
A compound which forms a giant covalent lattice
A compound which is made of a giant ionic lattice
A compound made of long chain molecules

 Activity 1. Complete the matching exercise below

1.10 Simple distillation
Compound or mixture?