• In some elements, the atoms are joined in
groups of two or more – this is called a
Other elements that contain atoms joined in
molecules are hydrogen, nitrogen, chlorine and
• A compound is the substance produced when
two or more elements combine in a chemical
reaction – they are also molecules.
The Structure of Atoms
• Neutrons = neutral
• Protons = positive charge
• Electrons = negative charge
• At the centre of an atom.
• Made up of protons and neutrons.
• Neutrons are heavier than protons – both are
much heavier than electrons.
• Atomic Number = Number of Protons
• Electrons spin around the nucleus. They don't
stay in defined areas around the nucleus. They
are found in clouds that can have different
• Electrons are negative and attracted to the
• The number of electrons and protons is the
• The electrons spin around the nucleus in
• The closer the shell is to the nucleus the
greater the attraction.
• Closer shells have lower energy levels and the
outermost shells the highest energy level.
Maximum number of electrons = 2n2
Shell Max. Number of Electrons
n = 1 2
n = 2 8
n = 3 18
n = 4 32
• Atoms that are joined = molecules or large
• These atoms are held together by chemical
• All other atoms react, gain, lose or share
electrons – result is a bond.
• The result is particles with full outer shells
that hold eight electrons.
• Ions are atoms that have become charged
because they have had electrons removed –
or because they have removed electrons
from other atoms.
• No longer neutral.
• Positively charged ions (+) have more protons
– lose electrons CATION.
• Negatively charged ions (-) have more
electrons – gain electrons ANION.
• When metallic elements bond with non-
• Metal atoms – weak hold on outer-shell
• Non-metallic atoms – strong hold on outer-
• Non-metallic tend to remove outer-shell
electrons from any metal atoms they are near.
• Results in ions.
• Electrostatic forces pull the +ve and –ve ions
together to form a strong ionic bond.
• Result in a three-dimensional structure called
• Nucleus – protons and neutrons that are
constantly moving – emit electromagnetic
radiation called gamma rays.
• Some nuclei even eject particles at high
• The emission of electromagnetic radiation or
particles is known as a nuclear reaction or
• During nuclear decay atoms may change from
one element to another – transmutation.
• Most atoms contain stable nuclei.
• The unstable atoms could eject particles or
electromagnetic waves from their nucleus at
any moment – nuclear decay.
• The unstable atoms are known as
radioisotopes e.g. Carbon-14
• Isotopes are atoms that have the same
number of protons but a different number of
• Carbons isotopes include –
Types of Nuclear Decay