Atoms, Molecules and Compounds
Atom
-an atom is the smallest part of an element
-matter is made up of tiny particles called atoms. This means that everything is
made from atoms. An atom is the smallest particle that can exist.
John Dalton proposed the Atomic theory, which states that
Matter consists of very small particles ( atoms)
Atoms cannot be broken into smaller parts
Atoms of one element are the same. They have same size , mass and
properties
Atoms of different elements have different size, mass and properties
When atoms combine as small whole numbers they form compounds
Atoms are shown in diagrams as small circles. Different colours or sizes of
spheres are used to show different atoms. Fig 1.1 shows Oxygen and Hydrogen
atoms:
Fig 1.1 Oxygen and Hydrogen atoms
Note that each atom of Hydrogen and Oxygen is shown as a single sphere.
Element
 An element is made up of only one type of atom.
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 An example of an element is Copper.
 A piece of pure Copper is made up of only of Copper atoms only.
 There are 118 known elements on Earth and they are all listed in the
Periodic table
 Each element has its own name and symbol.
 Symbols are used to represent elements and each element from the
Periodic table has a symbol. This symbol can be made up
of one or two letters but it always starts with a capital letter. For
example, the symbol for Nitrogen is N and the symbol for Magnesium is
Mg.
Table 1.1 shows the common elements and their symbols
Table 1.1: Common elements and their symbols
Element
Carbon
Nitrogen
Oxygen
Hydrogen
Sulphur
Chlorine
Magnesium
Copper
Iron
Symbol
C
N
O
H
S
Cl
Mg
Cu
Fe
Molecule
 atoms can combine to form molecules
 a molecule is the smallest particle of a compound
 A molecule describes two or more atoms bonded together (all compounds
are molecules
 The atoms of elements like Hydrogen join as pairs, making a molecule.
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 Molecules are shown in diagrams by drawing two or more spheres joined
which are joined together.
 Fig 1.2 and 1.3 shows the molecules of Hydrogen, Nitrogen, Oxygen ,
Fluorine , Chlorine, Bromine and Iodine
Fig 1.2 : Molecules of Elements
Fig 1.3: Molecules of elements
Compound
 Elements can combine in a chemical reaction and form a compound.
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 A compound is a substance made up of two or more atoms
of different elements chemically joined (or bonded) together.
 For example, carbon dioxide gas (CO2) consists of one carbon atom and
two oxygen atoms bonded together.
 Fig 1.4 shows a Water molecule. Water is formed when one atom of
oxygen combines with two atoms of hydrogen
Fig 1.4: Molecule of a Compound (Water)
SUMMARY OF ATOMS, MOLECULES AND COMPOUNDS
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