1 of 8
© Boardworks Ltd 2008
Protons, neutrons and electrons
The nucleus is:
 made up of protons and neutrons
 positively charged because of the
protons
 dense – it contains nearly all the
mass of the atom in a tiny space.
Electrons are:
 very small and light, and negatively charged
 able to be lost or gained in chemical reactions
 found thinly spread around the outside of the nucleus,
orbiting in layers called shells.
2 of 8
© Boardworks Ltd 2008
Protons, neutrons and electrons
3 of 8
© Boardworks Ltd 2008
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 electron shells are the size
shown, the nucleus would be too small to see.
4 of 8
© Boardworks Ltd 2008
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.
5 of 8
© Boardworks Ltd 2008
Bohr Models
• Used to represent a model of an atom.
• To draw a Bohr model follow these steps:
(We will use Helium as an example)
6 of 8
© Boardworks Ltd 2008
Making a Bohr Model Using Helium
1. Look to the periodic table and determine
how many protons, neutrons and electrons
are in 1 atom of helium.
P=____ N=_____ E=_____
2. Draw a circle and label the # of P and N in
the inside of the circle
P= 2
N= 2
7 of 8
© Boardworks Ltd 2008
Making a Bohr Model Using Helium
3. Draw your 1st electron shell.
P= 2
N= 2
4. Draw up to 2 electrons in the 1st shell.
P= 2
N= 2
5. If you need to add more electrons, you need to
add more electron shells! Remember…2, 8, 8!!!
8 of 8
© Boardworks Ltd 2008
Calculate electron configurations
9 of 8
© Boardworks Ltd 2008
Which element?
10 of 8
© Boardworks Ltd 2008
Valence electrons
Valence Electrons are:
 The electrons in the outermost shell
 Responsible for atomic bonding
 Equal to the last digit of the group number
 How many valence electrons in this atom? What
group would it be in?
11 of 8
© Boardworks Ltd 2008
LEWIS (DOT) SYMBOLS FOR THE ELEMENTS
A Lewis dot structure for an atom consists of
the symbol for the element and one dot for
each valence electron.
12 of 8
© Boardworks Ltd 2008
How to Draw a Lewis Structure
1) Find your element on the periodic table.
2) Determine the number of valence
electrons by looking at the group
(column)
3) This is how many electrons you will draw.
13 of 8
© Boardworks Ltd 2008
Lewis Structures
1) Write the element
symbol.
2) Carbon is in the 14th
group, so it has 4
valence electrons.
3) Starting at the right,
draw 4 electrons, or
dots, counterclockwise around the
element symbol.
14 of 8
© Boardworks Ltd 2008
Lewis Structures
1) Check your work.
2) Using your periodic
table, check that
Carbon is in the 4th
group.
3) You should have 4
total electrons, or
dots, drawn in for
Carbon.
15 of 8
© Boardworks Ltd 2008
Lewis Structures
What would the Lewis Dot
Structure for Phosphorus
look like?
16 of 8
© Boardworks Ltd 2008
17 of 8
© Boardworks Ltd 2008