Electron_structure

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Lesson 2
Arranging the electrons and
patterns in electron
configuration
The 2, 8, 8 rule
For the GCSE exam you will be required to arrange the
electrons for the first 20 elements in the Periodic Table.
The arrangement of electrons in an atom is called the
Electron Configuration.
Shell or orbit (starting with the one
closest to the nucleus)
1st
2nd
3rd
4th
Maximum number of electrons that
can be placed in the shell or orbit
2
8
8
Any left over
Drawing an electron configuration
Step 1. Draw a blob for the
nucleus.
Na
Step 2. Draw the first shell. Add 2
electrons.
Step 3. Draw the second shell. Add
8 electrons.
Step 4. Draw the third shell. Add
the last remaining electron.
K
K
2, 8, 8, 1
Group
Number of
electrons in
outer shell
1
2
3
4
5
6
7
0 or 8
1
2
3
4
5
6
7
full
When is an atom not an atom?
Atoms react with other atoms by losing or gaining electrons.
An atom that has lost or gained electrons is called and ION.
An atom that has lost electrons (lost negative charges) is
called a positive ion and an atom that has gained electrons
(gained negative charges) is called a negative ion.
In other words an ion is a charged particle.
How many electrons are lost or gained depends upon in which
Group the element belongs.
The aim is to achieve a full outer shell of electrons (in most
cases this is 8 electrons).
Group
Number of
electrons in
outer shell
Number of
electrons lost
or gained
Charge of ion
1
2
3
4
5
6
7
0 or 8
1
2
3
4
5
6
7
full
Group 0 or 8
He
2
Ne
2, 8
Ar
2, 8, 8
Kr
2, 8, 18, 8
Xe
2, 8, 18, 18, 8
Rn
2, 8, 18, 32, 18, 8
The atoms of these elements
already have a full outer
shell. This is why these
elements don’t react. Hence
they are called the Inert or
Noble gases.
Note the 2, 8, 8 rule only works for the first 20 elements
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