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CHEMICAL CHANGES and
STRUCTURE
Atomic Structure – Summary Notes
ELEMENTS and the PERIODIC TABLE
 There are 92 naturally occurring elements and
around about another 25 which have been made by
scientists.
 These elements are arranged by their atomic
number and their chemical properties on the
Periodic Table.
 All the elements have their own individual symbol.
 The chemist mainly responsible for the way the
elements are organised was Dmitri Mendeleev.
PERIODIC TABLE INFORMATION
 ALKALI METALS – These are the metals making
up Group 1 e.g. Sodium, Potassium etc. They
are very reactive.
 HALOGENS – These are the reactive non-metals
making up Group 7 e.g. Fluorine, Chlorine etc.
 NOBLE GASES – These are very unreactive
gases making up Group 8 e.g. Helium, Neon etc.
 TRANSITION METALS – These are metals found
between Group 2 and Group 3 e.g. Iron, Copper
and Platinum. They have many uses and many
of them and their compounds are used as
catalysts in industrial processes.
Atoms
All elements are made of very small particles called
ATOMS.
At the centre of every atom is the NUCLEUS
containing both PROTONS and NEUTRONS. The
nucleus is surrounded by fast moving ELECTRONS.
These diagrams show 2 ways of drawing atoms -
More about atoms –
The following table shows important differences
between the 3 particles –
PARTICLE
Proton
MASS
1
CHARGE
+1
LOCATION
Nucleus
Neutron
Electron
1
0
0
-1
Nucleus
Outside
the
nucleus
In an atom there are always the same number of
positive protons as negative electrons. This means
that they have no overall charge i.e. ATOMS ARE
NEUTRAL.
ELECTRON ARRANGEMENTS
Electrons are arranged in ENERGY LEVELS or SHELLS
surrounding the nucleus –
The electron arrangements of the elements are
given in the data book
E.g. Sodium’s electron arrangement is 2,8,1 which means
Sodium has 2 electrons in the 1st energy level, 8 in the second
and 1 in the 3rd. Electrons in the energy level furthest from the
nucleus are called OUTER ELECTRONS. Sodium has 1 outer
electron.
TARGET DIAGRAMS
Electron arrangements can be shown as target
diagrams
E.g. Lithium 2, 1
Phosphorus 2, 8, 5
The outer electrons are very important. Elements in
the same group of the Periodic Table have the
same number of outer electrons and for this
reason have similar chemical properties.
IMPORTANT NUMBERS
1. ATOMIC NUMBER
Each element has its own Atomic Number and they are
arranged on the Periodic Table in order of increasing Atomic
Number. The Atomic Number gives the number of PROTONS
present in an atom of the element.
E.g.
Element number 11 is Sodium
Sodium has Atomic Number 11
Each sodium atom contains 11 protons and its
nucleus has a charge of +11
Since the atom is neutral, it must also have 11
electrons.
2. MASS NUMBER
This is the total number of PROTONS + NEUTRONS in an
atom
E.g. a Sodium atom with Mass Number 23 will contain –
11 protons
11 electrons
12 neutrons
NUCLIDE NOTATION
Chemists use nuclide notation to show the numbers of
sub-atomic particles in an atom or ion.
Number of protons = 10
Number of protons = Number of electrons = 10
Number of neutrons = 22 – 10 = 12
ISOTOPES
Isotopes are atoms with the same Atomic Number
but different Mass Number. This means they are
atoms of the same element but they have different
numbers of neutrons.
Eg. Chlorine has 2 isotopes 35Cl and 37Cl
Number of protons
Number of electrons
Number of neutrons
35Cl
37Cl
17
17
18
17
17
20
Eg. Hydrogen has 3 isotopes, 1H, 2H and 3H
Number of protons
Number of electrons
Number of neutrons
1H
2H
3H
1
1
0
1
1
1
1
1
2
RELATIVE ATOMIC MASS
The Relative Atomic Mass of an element is the AVERAGE
MASS OF ALL THE ISOTOPES present taking into account
the relative abundance of each.
Eg. Chlorine
Chlorine has 2 isotopes - 35Cl and 37Cl
In Chlorine 75% of the atoms have Mass Number 35 and
25% have Mass number 37.
Average mass = (75 X 35) + (25 X 37)
100
=
2625 + 925
100
=
35.5
 The Relative Atomic Mass will always be closest to
the mass of the most abundant isotope.
 Relative Atomic Masses are rarely whole numbers.
They are used in calculations later in the course.