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Atoms
• All elements are made up of very small particles called atoms
(look back to previous notes).
• All atoms in an element are the same.
• Atoms aren’t EMPTY SPACE. They are actually made up of
even smaller particles called sub-atomic particles.
• Three sub-atomic particles make up an atom. They are
called:1. PROTONS
2. NEUTRONS
3. ELECTRONS
Sub-Atomic Particles in an Atom (1)
• The structure of an atom is shown below.
The centre of the atom is hard and is
called the NUCLEUS. It contains
PROTONS and NEUTRONS.
The ELECTRONS are very light
and are found MOVING around
the outside of the nucleus.
The simplistic shape of an atom is represented by a sphere (3-D),
or a circle (2-D). However, the diagram above is more accurate.
The outline shape of the atom is actually the flight path of the
electrons.
Sub-Atomic Particles in an Atom (2)
• The 3 sub-atomic particles have different properties.
Particle
Mass (amu)
Charge
Location
Symbol
Proton
1
1+
Nucleus
p+
Neutron
1
0
Nucleus
n0
Electron
0
1-
Moving
outside
nucleus
e-
***amu = atomic mass units***
Sub-Atomic Particles in an Atom (3)
• A simplified diagram of an atom.
The ELECTRONS are
-
NEGATIVELY charged.
The charge on the nucleus is
POSITIVE due to the PROTONS
present.
+
Atoms are said to be ELECTRICALLY NEUTRAL, because the
POSITIVE charges of the PROTONS are EQUAL to the NEGATIVE
charges of the ELECTRONS, and CANCEL OUT.
Important Numbers
• Atoms of different elements vary in size and mass due to them
containing different number of sub-atomic particles.
• How do we know the numbers of sub-atomic particles different
atoms contain??????
• We use two very important numbers called the:» 1. ATOMIC NUMBER
» 2. MASS NUMBER
1. The Atomic Number
• The ATOMIC NUMBER of an atom is equal to the number of
PROTONS in the nucleus of the atom, which is the same as the
POSITIVE charge on the nucleus.
• In a NEUTRAL ATOM, the ATOMIC NUMBER is also the same
as the number of ELECTRONS.
• The ATOMIC NUMBER, is the number an atom has on the
periodic table.
Cl
17
The Atomic Number can be
represented like this.
2. The Mass Number
• The MASS NUMBER of an atom is the same as the TOTAL
number of PROTONS and NEUTRONS in the NUCLEUS of an
atom.
35
Mass number always goes here.
PROTON + NEUTRONS
Cl
17
Atomic number
PROTONS = ELECTRONS
• Mass number - Atomic number = Number of
neutrons
Using this information
Element
Atomic
Number
Carbon
Mass
Number
Calcium
3
20
10
20
80
Argon
Number Number
of
of
neutrons electrons
12
7
Neon
Number
of
protons
18
45
22
2. The Mass Number (cont)
7
Li
This atom has:- 3 protons (3+ charge on nucleus)
3 electrons
4 neutrons
3
39
K
This atom has:- 19 protons (19+ charge on nucleus)
19 electrons
20 neutrons
19
All atoms except 1
1
H
have both protons and neutrons in the nucleus.
Isotopes
• ISOTOPES are atoms of the SAME element, with the SAME
ATOMIC NUMBER but have a DIFFERENT MASS NUMBER (
This means a DIFFERENT NUMBER of NEUTRONS in the
NUCLEUS).
• A machine called a MASS SPECTROMETER is used to show
that most elements contain a mixture of isotopes.
• For example, CHLORINE is made up of 2 isotopes and
HYDROGEN is made up of 3 isotopes.
35
Cl
17
37
and
1
Cl
17
H
1
2
,
H
1
3
and
H
1
Electron Arrangement (how electrons are arranged
in atoms) (1)
• Electrons move around (orbit) the outside of a nucleus.
• The electrons do NOT randomly do this. They orbit in neatly
arranged in ENERGY LEVELS or SHELLS.
Third Energy Level has the
highest energy
Second Energy Level
+
First energy level has the
lowest energy
Nucleus
Electron Arrangement (2)
•
Imagine the energy levels/shells of an atom to be like the layers of an
onion, with each layer only holding a MAXIMUM number of electrons.
The diagram below is called a TARGET PICTURE.
FIRST SHELL - Can hold a
maximum of 2 electrons
+
SECOND SHELL - Can hold
a maximum of 8 electrons
THIRD SHELL - Can hold a
maximum of 8 electrons
Electron Arrangement (3)
• Electrons don’t fill energy levels in a random way.
• They fill the first energy level, then the second and so on, until
all the electrons are inside a level.
• Electrons are put into a shell singly at first (at the 4 points of
a clock face), then paired up. This is shown below.
7+
This atom is nitrogen, because its
atomic number 7 (7 positive
charges).
The shorthand way of writing this
electron arrangement is:2, 5
Electron Arrangement (4)
Element
Lithium
Atomic
Number
3
Number of
electrons
3
Electron
arrangement
2, 1
Oxygen
8
8
2, 6
Magnesium
12
12
2, 8, 2
Chlorine
17
17
2, 8, 7
Potassium
19
19
2, 8, 8, 1
Electron Arrangement (5)
• A notation exists for writing electron arrangement, this is
shown on page 1 of the data book.
This is a SILICON atom, because
the number of + charges is equal to
the ATOMIC NUMBER.
This means the number of - charges
(electrons) are also 14.
+
14
14 electrons are arranged:- 2 in the
first energy level, then 8 in the
second, and lastly 4 in the third (adds
up to 14.)
Shorthand notation :- 2, 8, 4
Electron Arrangement (5)
• Chemists have shown that the electron arrangement of an atom
is related to its CHEMICAL PROPERTIES (how it reacts).
• It has been found that elements in the same group of the
periodic table have very similar properties.
• Looking at the electron arrangement of elements within a group
it can be seen that they ALL have the SAME NUMBER OF
OUTER SHELL ELECTRONS. It is these electrons which are
involved in chemical reactions.
• The number of outer electrons is the SAME as the GROUP
NUMBER of the ELEMENT.
Electron Arrangement (6)
Group
number
1
2
3
4
5
6
7
8 or 0
No. of
1
outer
electrons
2
3
4
5
6
7
8
Example
Calcium Aluminium
2,8,8,2 2,8,3
Sodium
2,8,1
All react violently
with water
Carbon Nitrogen Sulphur Chlorine
2,4
2,5
2,8,6
2,8,7
Very reactive,
poisonous
non-metals
Argon
2,8,8
All unreactive
gases
The Noble Gases
• All the noble gases have 8 outer electrons - a full outer shell
(octet). For neon this is 2.
• Atoms with a full outer shell are unreactive and this explains
the chemical properties of the noble gases.
• Atoms only react to get a full outer shell of electrons, thus
becoming stable.