Compounds

 

A compound is formed when two or more elements combine together chemically.

 
The word "compound" means made up of several parts.
Fact

There are about 100 known elements

There are hundreds of thousands different compounds


I
II
III
IV
V
VI
VII
VIII
H
He
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Ar
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Cs
Ba
La
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Ti
Pb
Bi
Po
At
Rn
Fr
Ra
Ac
   

Why do elements combine to form compounds?

Many elements are chemically reactive. Some elements are so reactive that they must be stored in a specia way to prevent them from reacting e.g. elements from Group I. Sodium and Potassium are so rective with water or moisture that they must be stored under liquid paraffin. Phosphorous must be stored under water to prevent it from reacting with the oxygen in the air and bursting into flames. Other elements such as gold do not react easily and some elements such a the noble gaes do not react at all and will not combine with anything to form a compound.

 

Many of our everyday substances are compounds:

           
Water
Carbon dioxide
Salt
Sugar
Plastic
Sand

The properties of compounds are usually totally different to the properties of the elements from which they are made of. One example is NaCl illustrated below.
Images courtesy of webelements.com

 

Sodium

Chlorine

SSodium Chloride

image of Na

 

image of Cl

 


Examine the following table for other examples:
 
Element
Element
Compound

Carbon

A black solid

 

Oxygen

A gas needed for burning

Carbon dioxide

A colourless gas - can be used to put out fires

Sodium

 

A shiny very reactive metal that explodes in water

 

Chlorine

 

A green highly poisonous gas

Sodium Chloride

 

A white harmless compound that we use to flavour food

 

Hydrogen

An explosive gas

 

Oxygen

A gas needed for burning

 

Water

A liquid we drink and can be used to put out fires

 

Iron

A metallic grey solid easily attracted to a magnet

 

Sulphur

A bright yellow solid

 

Iron Sulphide

A stoney like grey solid with no magnetic properties


Some elements particularly many of the gases occur naturally as molecules e.g. (Hydrogen, Oxygen, Nitrogen and Chlorine)

Molecules may be made up of atoms of the same element such as Hydrogen gas (H2 - two atoms of Hydrogen), or atoms of different elements such as Sodium Chloride (NaCl).

Both elements and compunds are "pure" substances. Hydrogen gas consists only of Hydrogen molecules and salt consists only of salt molecules.

However many substances are just mixtures of different elements or compounds. Air is just a mixture of different gases. Our breathing system can easily separate out the oxygen from the air we breathe in because the gases are not chemically combined.

A tablespoon of sand and salt is just a mixture. They do not chemically combine - they just mingle together.

Identifying the differences between Mixtures and Compounds:

Compounds

Mixtures

  • Consists of a single substance

  • Consists of a two or more substances

  • The elements in a compound are in a fixed proportion

  • The proportion of substances in a mixture does not matter

  • Properties of a compound are usually different to those of the elements making it up

  • Properties of a mixture are usually the same as those of the substances making it up

  • Heat is usually necessary to combine the elements in a compound

  • No heat is necessary

  • Usually very difficult to separate

  • Usually easy to separate

 

Matter

Elements

Atom

Atomic and Mass numbers

Atomic structure

Electron configurations

Molecules

Bonding

History

Examination questions

Online Quizzes

Teachers Corner