Elements, Atoms, Compounds and Mixtures

Elements in the Earth’s crust     Obtaining metals from their ores


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  • There are about 100 elements in total, although  only about 20  are in everyday use.
  • An element is the simplest of all chemical substances.
  • It is made up of only one kind of atom and cannot be decomposed.

Structure of an atom:
An atom is far too small to see, even with a microscope.
Each atom has a central nucleus (made up of protons and neutrons) with electrons moving around it.

Chemical symbols
Every element has a symbol. Each symbol represents one atom.
eg O= oxygen atom, C= Carbon atom, Fe = iron atom.

The symbols always start with a capital letter.
If the symbol contains two letters the second letter is always small.

Element Symbol Element Symbol
Calcium Ca Chlorine Cl
Copper Cu Helium He
Iron Fe Hydrogen H
Magnesium Mg Nitrogen N
Sodium Na Oxygen O
Carbon C Sulfur S
Key
Blue = metal
Pink = non-metal

Properties of an element


Metals and non-metals

One important group of elements are the metals and non-metals.
An element is either a metal or a non-metal according to its properties.

Metals Non-metals
Malleable
(bendable)
Brittle
Usually strong Usually weak
High melting point
(metals are usually solid) 
Low melting point
(non-metals are usually gas or liquid) 
Good conductors of electricity Poor conductors of electricity
(except carbon, which does conduct)
Good conductors of heat
(good thermal conductors)
Poor conductors of heat
poor thermal conductivity
Metals are sometimes magnetic Non-metals are never magnetic
The oxides of a metal, when soluble,
form an alkaline solution.
The oxides of a non-metal usually form an acidic solution

Other things you need to know about metals:
Metals can react with other substance:
Metals react with oxygen to produce oxides (see action of heat)
Metals react with acids to produce hydrogen (see acids and alkalis)
Some metals are more reactive than others (see activity series)

Molecules
A molecule is formed when two or more atoms chemically join together.

An interactive game to teach you more about atoms

 

Compounds

 Two (or more) elements can combine together to form a COMPOUND

Carbon dioxide and water are both compounds because each are contain TWO kinds of atom

Equation showing oxygen and water combining to form water:

Hydrogen+ oxygen → water

A compound must be made of at least two elements chemically joined together.

magnesium + oxygen → magnesium oxide
(element)      (element)           (compound)

 

Naming a compound

Compounds often seem to have confusing names but there are some simple rules to help work out the name of a compound

  • When two elements combine the compound will end in ‘ide
    eg sodium + chloride → sodium chloride
    Copper + oxygen → copper oxide
  • When a compound contains three or more elements, one of which is oxygen, it’s name will end in ‘ate
    eg copper sulfate contains copper, sulfur and oxygen

 

Each compound has it’s own formula which shows the atoms it contains

The formulae of some common compounds
Carbon dioxide          CO2
Copper oxide             CuO
Copper Sulphate        CuSO4
Copper carbonate      CuCO3
Magnesium oxide       MgO
Salt (sodium chloride) NaCl
Sulphur dioxide           SO2
Water                         H2O
Zinc oxide                   ZnO

 

Mixtures

A substance is a mixture if it contains more than one substance  which are not chemically joined.

Which of the boxes below contains a mixture? ANSWER

The main differences between a compound and mixture are summarized in the table below:

Mixture

Compound

A mixture contains two or more substances NOT chemically joined. Contains at least two elements chemically joined together
The different substances may be visible and the appearance may not be uniform Will look uniform in appearance (ie the same all the way through).
The substances can often be easily separated eg by filtration, evaporation, distillation, chromatography, using a magnet or a separating funnel. The elements in a compound cannot be separated without a chemical change. ie a new substance will always be formed.
The substances can be mixed in any proportion. The elements in a compound are always present in a fixed proportion.

 

 

Elements, Mixtures and Compounds: Summary

A substance is an element if it contains only a single kind of atom and can NEVER be decomposed.
eg copper, oxygen, carbon, iron, sulfur, zinc, gold

A substance is a compound if it contains more then one element chemically joined together.

eg water, magnesium oxide, copper sulfate, calcium carbonate, cobalt chloride, petrol

A mixture exists when there are at least two substances present (either elements, compounds or a combination of the two).

These two substances are not chemically joined and can often be easily separated.

eg air, ink, orange juice, wood, salty water