Starter: Write
some information
about the atom
The structure of the atom
11/03/2016
I did some experiments in 1808 that proved this
and called these particles ATOMS. Most of an
atom is empty space and the nucleus is actually
very small so this diagram is wrong:
Dalton
NEUTRON –
neutral, same
mass as
proton (“1”)
PROTON –
positive, same
mass as
neutron (“1”)
ELECTRON –
negative, mass
nearly nothing
What particles are atoms made
of?
For some time, people thought that atoms were the
smallest particles and could not be broken into anything
smaller.
Scientists now know that atoms are actually made from
even smaller particles. There are three types:
proton
neutron
electron
How are these particles arranged inside the atom?
What is the structure of
an atom?
Protons, neutrons and electrons are not evenly
distributed in an atom.
The protons and neutrons
exist in a dense core at the
centre of the atom. This is
called the nucleus.
The electrons are
spread out around the
edge of the atom. They
orbit the nucleus in
layers called shells.
Mass and atomic number
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Particle
Relative Mass
Relative Charge
Proton
1
+1
Neutron
1
0
Electron
Very small
-1
MASS NUMBER = number of
protons + number of neutrons
SYMBOL
PROTON NUMBER = number of
protons (obviously)
Atomic mass in more detail
11/03/2016
RELATIVE ATOMIC MASS, Ar
(“Mass number”) = number of
protons + number of neutrons
SYMBOL
PROTON NUMBER = number of
protons (obviously) – this number is
always the same for an element
Mass and atomic number
11/03/2016
How many protons, neutrons and electrons?
Atomic structure worksheet
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Proton
Mass
Number of Number of Number of
Element
Symbol
number
number
protons
neutrons
electrons
Use a periodic table to help you complete the table below.
carbon
C
12
aluminium
13
chlorine
18
33
caesium
133
36
titanium
How are electrons arranged?
Electrons are not evenly spread but exist in layers
called shells. (The shells can also be called energy
levels).
The arrangement of electrons in these shells is
often called the electron configuration.
1st shell
2nd shell
3rd shell
Note that this diagram is not drawn to scale – the
atom is mostly empty space. If the electrons are the
size shown, the nucleus would be too small to see.
How many electrons per shell?
Each shell has a maximum number of electrons that it
can hold. Electrons will fill the shells nearest the
nucleus first.
1st shell holds
a maximum of
2 electrons
2nd shell holds
a maximum of
8 electrons
3rd shell holds
a maximum of
8 electrons
This electron arrangement is written as
2,8,8.
Electron structure
11/03/2016
Consider an atom of Potassium:
Potassium has 19 electrons.
These electrons occupy
specific energy levels
“shells”…
Nucleus
The inner shell has __ electrons
The next shell has __ electrons
The next shell has __ electrons
The next shell has the remaining __ electron
Electron structure
= 2,8,8,1
Electron structure
11/03/2016
Draw the electronic structure of the following atoms:
Nucleus
Nucleus
Nucleus
Electron structure
Electron structure
Electron structure
= 2,5
= 2,8,2
= 2,8,8,2
Electron shells worksheet
11/03/2016
Isotopes
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An isotope is an atom with a different number of neutrons:
Notice that the mass number is different. How many
neutrons does each isotope have?
Each isotope has 8 protons – if it didn’t then it just
wouldn’t be oxygen any more.
Strange atomic masses
11/03/2016
When you look at a periodic table sometimes the
atomic mass is not a whole number. Consider
chlorine, for example:
How can an atom have a decimal for its mass?
This is because out of every four naturally
occurring chlorine atoms, 3 have a mass of 35
and 1 has a mass of 37 so the average atomic
mass is:
(3 x 35 + 1 x 37) / 4 = 35.5
Q. Magnesium is often found as 24Mg or 26Mg. If 79% of
magnesium is 24Mg what is the average atomic mass?
(79 x 24 + 21 x 26) / 100 = 24.4
Calculating
relative atomic
mass
Where are r.a.m. values
found?
The values of relative atomic mass (r.a.m.) are usually
given in a data book or found in the periodic table, so you
don’t have to work them out or remember them all!
relative atomic mass
symbol
atomic number
When looking up r.a.m in the periodic table, remember that it
is always the larger of the two numbers given.
Why isn’t r.a.m. always a whole
number?
Relative atomic mass (r.a.m.) is not always a whole number.
For example, the r.a.m. of chlorine is 35.5.
The standard r.a.m. value of each
element is actually the average relative
atomic mass, which takes all the
isotopes of each element into account.
Chlorine has two isotopes:
chlorine-35 (75%) and chlorine-37 (25%).
average r.a.m. of chlorine
= (35 × 75%) + (37 × 25%)
= (35 × 0.75) + (37 × 0.25)
= 26.25 + 9.25
= 35.5
What 2 pieces of information do you need in
order to calculate the r.a.m.?
1. The abundance of the isotopes (%)
2. The relative mass of the isotope.
average r.a.m. of chlorine
= (35 × 75%) + (37 × 25%)
= (35 × 0.75) + (37 × 0.25)
= 26.25 + 9.25
= 35.5
Calculating average r.a.m. from
isotopes
To calculate the average r.a.m. of a mixture of isotopes,
multiply the percentage of each isotope by its relative
atomic mass and then add these together.
Naturally-occurring bromine is composed of two isotopes:
bromine-79 (50.5%) and bromine-81 (49.5%).
What is the average r.a.m. of naturally-occurring bromine?
average r.a.m.
= (79 × 50.5%) + (81 × 49.5%)
= (79 × 0.505) + (81 × 0.495)
= 39.895 + 40.095
= 79.99
This figure can be rounded up.
Past paper questions…
Plenary…
11/03/2016
• What are isotopes?
• Tell me something you have learnt about electron
shells