Bohr Models
Bohr’s Model of the Atom
• Focuses on electrons and their
arrangement.
• Bohr stated that electrons move with
constant speed in fixed orbits around the
nucleus, like planets around a sun.
• Bohr correctly assigned energy levels to
electrons, but electrons do not move in
fixed orbits around the nucleus.
Bohr’s Model of the Atom
Valence Electron
Electron
Nucleus
Electron Cloud Model
• The electron cloud model replaced
Bohr's vision of electrons moving in
predictable paths.
• An electron cloud is a visual model
of the most likely locations for
electrons in an atom.
– probability of finding an electron is higher in the
denser regions of the cloud.
Electron Cloud Model
• An electron cloud is a good
approximation of how electrons
behave in their orbitals.
– An orbital is a region of space around the
nucleus where an electron is likely to be
found.
– The electron cloud represents all the
orbitals in an atom.
Electron Cloud Model
Because we cannot see
electrons in the Electron
Cloud Model, we will
draw Bohr Models to
illustrate the number of
electrons in the atom of
an element.
Before drawing a Bohr Model…
• Horizontal rows on the Periodic Table are called
periods.
– The row number tells you how many energy levels, or
rings, the element has.
– Elements in the same row have the same number of
rings.
– The number of rings increases as you move DOWN
the Periodic Table.
• Example: Sodium (Na) and Magnesium (Mg) are
in the third row so they both will have three rings
in their Bohr Model.
Horizontal Rows=Period=Number of Rings
1
2
3
4
5
6
Will not draw Bohr Models for Transition Metals
Number of Electrons on a Ring
• Each ring has a maximum number of electrons
that can fit on it.
Ring
Maximum Number of
Electrons
1
2
2
8
3
18
4
32
• Each ring CAN and will hold LESS than the
maximum number of electrons.
Examples:
Hydrogen is
on row #1 of
the periodic
table so it has
one energy
level.
Lithium is on
row #2 of the
periodic table
so it has two
energy levels.
Sodium is on
row #3 of the
periodic table
so it has three
energy levels.
Before drawing a Bohr Model…
• Vertical columns on the Periodic Table are called
groups.
– Group numbers tell you how many valence electrons
each element in that column has.
• Valence electrons are electrons that are located on the LAST
energy level.
– The number of valence electrons increases as you
move ACROSS the Periodic Table.
• Example: Sodium and Potassium are both in the
1st column, so they both will have one valence
electron in their last energy level.
– If the group number is higher than nine, so 10-18,
drop the one to determine the number of valence
electrons.
Vertical Columns=Groups=Number of Valence
Electrons
1
2,8
3 4 5 6 7
2
Will not draw Bohr Models for Transition
Metals
Examples: (Each “dot” represents an electron.)
Hydrogen is in
column #1 of
the periodic
table so it has
one valence
electron. Of
course, it only
has one
electron at all!
Lithium has
three electrons.
One valence
electron
because it is
column #1.
Sodium has 11
electrons. One
valence
electron
because it is in
column #1.
Examples: (Each “dot” represents an electron.)
Helium has two
electrons. It is
in column #8 so
one would think
it should have
eight valence
electrons but it
cannot because
it only has two
total electrons.
It has one ring
because it is in
row #1.
Boron has five
electrons.
Three valence
electron
because it is
column #3. It
has two rings
because it is in
row #2.
Argon has 18
electrons. Eight
valence
electron
because it is in
column #8. It
has three rings
because it is in
row #3.
Drawing Bohr Models
1. Determine the number of rings, or energy levels.
(Look at the period, or row, number.)
2. Determine the number of protons, neutrons,
and electrons.
3. Determine the number of valence electrons.
(Look at the group, or column, number.)
4. Draw the correct number of rings.
5. Draw the correct number of electrons on each
ring, or energy level. (Use Energy Levels,
Orbitals, and Electrons table.)