Transcript File - singhscience

Electron Configuration
Apply the rules of electron shells to predict the
configuration of the first 20 elements (D/C)
Describe the connection between the number
of outer electrons and the position of an
element in the periodic table (B/A)
Explain the significance of electrons shells in the
way that substances react.
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.
Electrons in an atom are arranged in
electron shells, or energy levels around
the nucleus.
•
•
•
•
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Electrons are shown using dots or crosses
Circles represent the shells.
1st (inner shell) holds 2 electrons.
2nd shells holds 8 electrons.
3rd shell holds 8 electrons.
Task- Complete the worksheet.
Apply the rules of electron shells to predict the
configuration of the first 20 elements (D/C)
The groups and periods of the
periodic table
• The group numbers are the same as the
number of electrons in the outer shells.
• Apart from group 0, where the other shells are
full.
• The number of electron shells is the same as
the period number.
Why are group 1 metals so reactive?
• All the elements in group 1
have 1 electron on their outer
shells.
• Atoms want to have full outer
shells, this makes them stable.
• Sodium is a reactive metal
because it has only 1 electron
o nits outer shell, the atom is
desperate to lose this electron
and therefore reacts
vigorously with things like
water.
As you do down group 1 they become
more reactive, why is this?
• The nucleus of a an
atom has a positive
charge and the electrons
are negative.
• Opposites attract! This
keeps the electrons in
orbit.
As you do down group 1 they become
more reactive, why is this?
• Francium is also in group 1,
but it is more reactive then
sodium. This is because its
outer electron is further from
the nucleus, therefore there
is less attraction, making it
easier to remove.
• There is also more shielding
from the other electron
shells.
• This makes francium highly
reactive!
The electronic configuration of lithium is 2.1.
The electronic configuration of sodium is 2.8.1.
(1) Explain, in terms of their electronic
configurations, why lithium and sodium are both
in group 1 of the periodic table.
(2 marks)
(2) Explain, in terms of their electronic
configurations, why lithium is placed in period 2
and sodium is placed in period 3 of the periodic
table.
(2 marks)
Answer
Acceptable answers
Mark
An explanation linking

(both have) one electron (1)

in the outer
shell/orbit/energy level (1)do not award first
mark if proton/neutron/atom (in outer shell)
both have one outer electron
(2) both need to lose 1
electron to have a full outer
shell (2) fully correct diagrams
of lithium and sodium showing
electronic configurations (1)
have the same number of
electrons in the outer shell (1)
If no marks awarded from
'answer' column, allow any
one from
sodium is more reactive than
lithium ORA (1)
sodium has more electrons
than lithium ORA (1)
sodium has more shells than
lithium ORA (1)
ignore reactivity increases
down the group
(2)
An explanation linking

lithium has 2
shells/orbits/energy levels (1)

sodium has 3
shells/orbits/energy levels (1) max 1 mark
if outer/full shells
max 1 mark if rings/circles/layers
(2)