Electron Cloud Model
•As a result of continuing research throughout
the 20th century, scientists today realize that
energy levels are not neat, planet-like orbits
around the nucleus of an atom.
•Instead, they are spherical regions of space
around the nucleus in which electrons are most
likely to be found. (like lanes of a track)
• Electrons travel in lanes like a runner on a track.
Electrons can be anywhere in their lane.
• The lanes are called electron clouds.
Electron Cloud Model
•Electrons themselves take up little space
but travel rapidly through the space
surrounding the nucleus.
•These spherical regions where electrons travel
may be depicted as clouds around the nucleus.
•So. . . .The space around the nucleus of an
atom where the atom’s electrons are found
is called the electron cloud.
Electron Cloud Model
Nucleus
No electrons
here.
Electron
clouds
Electrons will be
found here 90% of
the time.
•Each cloud is determined by the
probability of an electron’s
location. An electron will be
found in it’s cloud about 90% of
the time.
Properties of Subatomic
Particles
Particle
Symbol
Relative
Charge
Relative
Mass
(amu*)
Actual
Mass (g)
Electron
e-
-1
1/1840
9.11 x 10-28
Proton
p+
+1
1
1.66 x 10-
Neutron
no
24
0
1
1.66 x 1024
1 amu = 1.66 x 10-24 grams
amu = atomic mass unit
The NUCLEUS
The Nucleus contains protons and neutrons
Atomic Number — the number of protons
in an element.
Hydrogen’s is 1, Neon’s is 10, Gold’s is 79
** As the atomic number changes, so does
the identity of the element.
Mass Number — the total number of particles in
the nucleus (protons + neutrons).
# of Neutrons = Mass # - Atomic #
Using Atomic Number and Mass Numbers to determine the
number of protons, neutrons and electrons
Mass #
39
19
K
Element Symbol
Atomic #
Number of Protons = Atomic number = 19 protons
Number of Neutrons = Mass # - Atomic #
= 39 – 19 = 20 neutrons
Number of electrons = # protons = 19 electrons
(In a neutral atom!)
How Can Atoms of the Same Element be Different?
Isotopes — Atoms of the same element that have
different number of neutrons
Since neutrons do not have a charge, adding
more does not affect the atom.
This only adds to the mass of the atom
(increases mass number).

Each particle in the nucleus has a mass of one.
That means . . . The number of particles in the
nucleus = Mass Number

The number of electrons will still be the same as
the protons.
Ions — Atoms of the same element that
have a different number of electrons
than protons.
•Having more or less electrons creates an imbalance
in the charge of the atom.
•The number in the charge is how many more or less
electrons there are.
•If there are more protons than electrons,
then the charge is positive. (Loss of electrons)
•If there are more electrons than protons,
the charge is negative. (Gain of electrons)
Ions
16
8
Protons
O
-2
Charge
8
Neutrons 8
Electrons 10
(-2 )charge means that there
was a gain of 2 electrons,
so you add 2 to the number
of protons.
Practice – Finding the number of
p+, no and e1.)
3.)
142
56
Ba
56 no___
86 e-___
56
p+ ___
2.)
27
13
+3
Al
13 no___
14 e-___
10
p+ ___
* (+) charge means
subtract from p+
38
17
-1
Cl
* (-) charge
means add to p+
17 no_____
21 e-___
18
p+ ___
4.)
60
27
Co
27 no___
33
p+ ___
e-___
27