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Chemistry
Basic introduction to atoms, ions
and bonding
Atoms
• All matter is made up of tiny particles.
• Atoms are made up of even smaller particles.
• Diagram of an Atom
Electron (-ve charge)
Nucleus of protons (+ve) and neutrons
Atoms
• The number of PROTONS(+) = the number
of ELECTRONS(-)
• Therefore, atoms are uncharged overall.
• (Atoms of different elements have different
numbers of electrons, protons, and
neutrons.)
Ions
• ‘Formed’ as an atom gains or loses an
electron.
• Ions may have a positive charge or a
negative charge
Ions
ATOM
Sodium
SYMBOL
Na
minus an electron 
IONS
Na+
Chlorine
Cl
plus an electron 
Cl-
Hydrogen
H
minus an electron 
H+
Carbon
C
does not form an ion
(Some symbols are from Roman names e.g. Iron = Ferrum =
Fe)
Molecules
• An atom of Hydrogen cannot exist on its own
• But a molecule of Hydrogen can exist on its own
• Molecules consist of atoms linked together by
chemical bonds caused by the interchange of
electrons.
Hydrogen
molecule
H2
Water
molecule
H2O
Sulphuric acid
molecule
H2SO4
Atomic Structure
• The nucleus contains Protons (+ve)
and Neutrons (neutral)
• Electrons (-ve) occur in “energy levels”
Atomic Structure
Each level can take up to a certain number of
electrons
Energy level 1 - max 2 electrons
Energy level 2 - max 8 electrons
Energy level 3 - max 8 electrons
When an energy level is full,
further electrons occupy the
next energy level until that is
filled and so on
Atom with 17 electrons
Atomic Structure Diagrams
• Diagrams are not to scale (If you draw a
nucleus of 1 cm diameter, electrons in the
first energy level would be 100 metres away
from the nucleus)
• Electrons are not in concentric circles, but
exist in regions, sometimes called orbitals
or shells or clouds of electrons
• Diagrams are useful though to help explain
BONDING
Atomic structure diagram for Chlorine
(Do not really need to show protons
and neutrons in the nucleus as they
do not change)
Atomic structure diagram for Chlorine
An atom of chlorine has 17 electrons altogether
The atomic number = the number of protons
present in that atom
Number of PROTONS(+) = Number of ELECTRONS(-)
The mass number = the number of protons plus the
number of neutrons
E.g.
35
17
Atomic number is 17
Mass number is 35
Bonding
• Why do Na and Cl join together?
• Why always one particle of Na to one
particle of Cl?
• What holds the different particles
together?
Bonding
• Noble gases are stable and unreactive - may be
found as “free atoms”
• Noble gases are: Helium (Atomic number 2) or
Neon (Atomic number 10).
• Reason for this is Fully Filled Outer Shells
• Atoms of other elements can become stable by
Losing, Gaining or Sharing electrons until they
also have Fully Filled Outer Shells
Chemical Reactions
• Involve sharing or exchanging electrons in
order that atoms can obtain a more stable
arrangement of electrons
• They obtain stability by bonding with other
atoms.
• (Protons and neutrons play no part in
chemical reactions)
Chemical Reactions
Basically, there are 2 main types of
bonding:
Ionic or Covalent
Particles (i.e. molecules, compounds) form a
chemical bond linking them together.
How bonds are made 1
Valence:
how easily an atom or molecule with and
unpaired electron (a radical) can combine
with other chemicals.
Valance is based on the number of electrons
that would be added, lost or share if it reacts
with other atoms.
Valencies
• Elements and radicals can be given numbers
which are called their valencies or
combining powers e.g. Na = 1
• The valency of an element is usually the
number of electrons an atom of the element
must lose or gain (either completely or by
sharing) in order to obtain a stable atomic
structure.
Valencies
Valency of 1
Hydrogen
Chlorine
Hydroxide
Hydrogen
Carbonate
Symbol
Ion Formed
H
Cl
OH
HCO3
H+
Cl OHHCO3-
Valencies
Valency of 2
Oxygen
Calcium
Iron
Sulphate
Valency of 3
Phosphate
Valency of 4
Carbon
Symbol
Ion Formed
O
Ca
Fe
SO4
O- Ca++
Fe++
SO4 - -
PO4
PO4 - - -
C
No ion
How Bonds are made 2
• one atom losses an electron
– -it can be attracted to another atom that may have excess
electrons
• This is electrostatic attraction between two
oppositely charged ions.
• bonds between atoms that have lost an ion
(cation)
• Bonds between atoms that have gained
electrons (anion).
Ionic Bonding
The formation of Positive Ions
• Sodium atom (Na)
(2,8,1)
Sodium ion (Na+)
(2,8,)
+
(plus an electron
)
(Less energy is needed to lose one electron than gain seven)
Ionic Bonding
The formation of Negative ions
Chlorine atom
Chlorine ion
(2,8,7)
(2,8,8)
-
(An electron has been
gained)
(Less energy is needed to gain one electron than lose 7)
Combining of Na and Cl
• One electron lost by Na can be gained by Cl
• The ions are held together in the compound
because positively charged Na ions are
attracted to oppositely charged Cl ions
• The force which holds them together is called
Electrostatic Attraction
• Ionic compounds , when in aqueous solution,
are called electrolytes
How Bonds are formed 3
• If the chemical bond involves the sharing
of electron pairs between atoms it is called
– Covalent bonding.
Covalent Bonding
• Sharing, rather than losing or gaining electrons, is called
covalent bonding
• Therefore, no positive or negative ions are formed
Chlorine atoms x 2
Chlorine molecule Cl2
Covalent Bonding
• A pair of electrons (1 electron from each
atom ) bonding 2 atoms together is called a
single covalent bond
• Can also get double covalent bonds (and
also triple covalent bonds)
• Examples of molecules bonded covalently:
H2, O2, H2O, NH3 etc