Marc Daniels
Chemistry 591
Lesson Plan (50-45 minutes):
Learning About the Atom and Atomic Structure
Goals: (written on the corner of board 5 minutes)
 To learn a brief history of how the atom was discovered
 To learn about electrons, protons, and neutrons
 To learn about atomic number, mass number, and their notation
 Being able to deduce information about the number of protons an element has or its mass number
given particular pieces of information.
Brief History Introduction (25 minutes):
(Questions of what an atom is: what is it made of and how do atoms in an element differ?)
Discoveries of the Atom
 This was done near the end into the beginning of the 20 th century
 The discovery of the atom is a fairly recent discovery
English Physicist J.J Thomson and The Cathode Ray Tube
 Led to the discovery of the electron through studying electrical discharges in partially evacuated
tubes
 The experiment consisted of applying high voltage to a tube. The voltage created a “ray” or what
is called a cathode ray (emanated from the negative electrode/cathode)
 Since the ray was produced at the negative electrode and was repelled by the negative pole of an
applied electric field….it was suggested that the ray contained negatively charged particles or
what we know as electrons
(During this process, drawings will be done on board to show the design/set up of a cathode ray
tube and the deflections caused by negatively charged particles)
 From experiments, Thomson determined the charge to mass ratio of an electron (e/m = -1.76 x
108 C/g, where C is in Coulombs and represents charge, and m represents the electron mass in
grams)
(If students ask: it is not required to memorize)
 Thomson reasoned that since electrons could be produced from electrodes made of different
types of metals, than all atoms must contain electrons…..however atoms were known to be
electrically neutral….so what would account for the negative charge?….there must be a positive
charge.
 Thomson proposed the “plum pudding model” for an atom (Draw on board)
The Millikan Experiment
 Done in 1909, involving charged oil drops
 This led to the determination of the magnitude of the electrical charge as well as the mass of the
electron (9.11 x 10-31kg)
The Discovery of the Nucleus/ the Nuclear Atom
 The nucleus of the atom was discovered in 1911 by Ernest Rutherford
 Was done to test the “plum pudding model” as suggested by Thomson
 It was suggested that if the PPM model was true than alpha particles will pass through atoms
with little or no deflections……however..?
(Alpha particles are particles that have a 2+ charge….positive charge)
 Most particles passed through without deflections, but many were deflected at very large angles,
and some were even reflected.
 Large deflections could only be caused by a central concentration of positive charges…..the
discovery of the nucleus containing positive charge and electrons moving around the nucleus at a
large distance relative to the nuclear radius
Modern Atomic Structure: (5 minutes)
 The Nucleus of an atom contains both protons and neutrons.
 Protons have a positive charge of 1+, while a neutron has no charge, but both have relatively the
same mass. An electron, on the other hand, has a charge of 1- , and have a mass much lower
than that of a proton or neutron (will give masses but are not required to be memorized….the only
importance is that the realize that protons and neutrons are much more massive than electrons)
 The nucleus is small in comparison to the size of the atom (why so dense? Large mass in a small
volume)
Atomic Number, Mass number, and Their Notation: (15 minutes-remainder of class)
 Why do elements have different properties? This is due to the arrangement and number of the
electrons. Elements that have a different number of electrons and protons show chemically
different behaviors.
 If an element is listed of having no net charge then this means that there are the same number of
protons and electrons.
 Isotopes are atoms with the same number of protons, but different numbers of neutrons.
 AX The superscript here refers to the Mass Number or A which tells the total number of protons
and neutrons in an atom, and X refers to the element.
 ZX The subscript here refers to the Atomic Number or Z and tells us the number of protons in an
element
 An element may have a different number of neutrons, but if it has a different number of protons
then it is no longer the same element refer back to isotopes)
(Will also have to refer to periodic table to show which the atomic number is, and what the atomic
mass is and how to locate them)
Example Problems
What is the notation for Na which contains 11 protons and 12 neutrons? What about the notation of
Na that has 11 protons and 11 neutrons? Though these are the same element, they contain a different
number of neutrons. What are these elements called?
If an element contains 25 protons and 27 neutrons, what is its’ mass number? What is its’ atomic
number?
Using the periodic table: What element has an atomic number of 8 and an atomic mass of 16? How
many protons and neutrons are in this element?
If an element has an overall net charge of 1- (suggests there is one more electron than proton), and
contains 8 protons and 7 neutrons. How many electrons are in this atom? What is its’ atomic mass
and mass number?