Teachers Guide
Electrons in Atoms and Molecules
Overview
Students learn that the distributions of electrons in atoms and molecules affect how they
interact and form various structures and states. Understanding the properties of electrons is
therefore a key to understanding chemistry. Students experiment with how an electron cloud
interacts with a nucleus, a neutron, and a negatively charged ion. Students create covalent bonds
and explore chemical polarity by adjusting the atoms’ electronegativities and arranging their
locations. Finally, students apply an electric field to a model and observe the behavior of the
atom.
Learning Objectives
Students will be able to:
 Draw a connection between distribution of an electron cloud and its interaction
with other charged and neutral particles.
 Explain how a covalent bond is formed between two atoms.
 Explain how the electronegativity of atoms and the geometry of molecules is
related to polarity.
 Express how applying an electric field affects an electron cloud.
Student Prerequisite Knowledge
Students should already have a basic understanding of:
 The structure of an atom
Background and resources
Here is a simplistic model of an electron cloud:
http://regentsprep.org/Regents/physics/phys05/catomodel/cloud.htm
This site takes a little time but has some great animations and it builds the concept of an
electron cloud slowly and methodically:
http://www.chemistryland.com/ElementarySchool/BuildingBlocks/BuildingElements.ht
ml#
This site gives a good primer on chemical bonds with animations, a self-assessment
quiz and additional resources:
http://www.visionlearning.com/library/module_viewer.php?mid=55
This page explains what electronegativity is, and how and why it varies across the
Periodic Table:
http://www.chemguide.co.uk/atoms/bonding/electroneg.html
This story describes the first microscopic image of atomic orbitals:
http://www.insidescience.org/research/first_detailed_photos_of_atoms
Here is an excellent introduction to atomic orbitals and wave theory:
http://winter.group.shef.ac.uk/orbitron/
Approximate time for lesson completion: 45 minutes
Activity Answer Guide
Page 1:
1. Compare clouds in the sky with electron
clouds. What are the similarities and what
are the differences?
Similarities: Both are diffuse. Where there are
more clouds, they are more water vapor.
Similarly, where there are more electron clouds,
there are more electrons.
Differences: Clouds in the sky have random
shapes whereas electron clouds have very
specific geometric shapes. Clouds in the sky
represent real water vapor—there is always
water vapor in any place within a cloud. But
electron clouds represent probability distribution.
We cannot say there is always an electron in an
electron cloud—we can only say how probably it
is to find an electron in it.
2. Can you think of another example in which
the notion of a probability distribution may
be useful in describing the state of a
system?
Any type of target shooting by an expert
marksman at various distances. Emission of
radioactive particles from the nucleus of a
radioisotope – both the timing and the direction
of the particle’s travel.
Page 2:
1. Take a snapshot of the electron cloud that
represents a bound state in an atom.
2. Based on your observation of the
behaviors of the electron cloud, explain why
an electron cannot form an atom with only a
neutron.
The electron cloud disperses in the presence of
a neutron and there is nothing to keep it from
moving away. The electron needs a positively
charged nucleus to hold it in place and to give
the electron cloud its shape.
Page 3:
1. Which of the following is closest to the
maximum distance between the two nuclei
for a covalent bond to form?
(b)
2. In the above model, what type of bond is
formed when the difference of
electronegativity between the two atoms is
greater than 1?
(b)
3. When the three nuclei are in a line, is the
molecule polar? Why?
No. There is no uneven distribution of charges.
4. Take a snapshot of an H2O-like molecule.
Annotate the snapshot to indicate where the
center of the electron cloud is located.
Explain how this is related to the polarity of
the molecule.
2. Considering the charges of the electrons
in the cloud and the nuclei, why do two
atoms that have formed a covalent bond stay
bound together?
The positive nucleus from each atom involved in
the bond is attracted not only to its own
electrons, but the electrons of the other atom.
And similarly, the electrons are attracted to the
nucleus of the opposite atom.
Page 4:
1. Take a snapshot that shows the diatomic
molecule has more negative charge in the
right part.
The center of the electron cloud is not at the
same location with that of the nuclear charges.
This uneven distribution of negative and positive
charges creates a molecule with polarity.
Page 5:
1. Apply an electric field with a weak
intensity to the atom (less than 10 in the
"Field intensity" slider reading). Let the
model run for a while. Observe what
happens. Take a snapshot and annotate your
observations.
4. Explain the behavior of the electron cloud
under a strong electric field.
The strength of the electric field exceeds the
ability of the positive nucleus to hold onto it.
Page 6:
1. In the model to the left, which atom has
higher electronegativity?
(a)
(The atom on the left has greater
electronegativity as evidenced by the larger
electron cloud surrounding that nucleus.)
2. Explain the behavior of the electron cloud
under a weak electric field.
The cloud moves in response to the electric
field. However, the nucleus is holding onto the
cloud. If the electric field is strong enough it can
tear the electron from the atom (ionization).
3. Apply an electric field with a strong
intensity to the atom (greater than 15 in the
"Field intensity" slider reading). Let the
model run for a while. Observe what
happens. Take a snapshot and annotate your
observations.
2. Carbon dioxide (CO2: O=C=O) is a
molecule whose three atoms are aligned
linearly. Recall the experiment you did with
the triatomic molecule on Page 4. Which of
the following is responsible for the nonpolarity of carbon dioxide?
(d)
(The molecule is symmetric. The distribution of
charges is even thus there is no polarity.)
3. A positron is a sub-atomic particle that is
identical to an electron, in all properties
except for its charge. A positron carries a +1
charge while an electron carries a -1 charge.
Why don't atoms have positron clouds?
They would not be held in place by the positively
charged nucleus. In fact they would be repelled
by the nucleus.