Lesson 2.3 Atomic Structure
Suggested Reading:
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Zumdahl: Chapter 2 Section 2.5
Essential Question: How are atoms, ions & isotopes described in
terms of the relative numbers and positions of subatomic particles?
Learning Objectives:
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State the positions and number of protons, neutrons and
electrons for atoms, ions & isotopes.
State the relative masses and relative charges of protons,
neutrons & electrons.
Define the terms mass number (A), atomic number (Z) and
isotopes of an element.
Deduce the symbol for an isotope given its mass number and
atomic number.
Calculate the number of protons, neutrons and electrons is atoms
and ions from the mass number, atomic number and charge.
Calculate the average atomic mass of an atom from isotopic
data.
In this lesson we will briefly review the structure of the atom according to the
nuclear model. We will do this by taking each learning objective one at a time
until we have hit all six. Much of this material may seem pretty basic, at it is,
but I find students often struggle with the basic concepts on exams. The
tendency is to overcomplicate the basic chemistry concepts. Make sure you
really understand the vocabulary. This will help you to avoid this pitfall.
Students often confuse terms pertaining to nuclear structure, so please pay
close attention as you work through the lesson watching out for mistakes and
misconceptions. Don't give away exam points on basic concepts!
Objective 1: State the positions and number of protons, neutrons
and electrons for atoms, ions & isotopes.
Look at the diagram of the atom below. Please make a note of the locations
of the protons, neutrons and electrons. Protons and neutrons are located in
the nucleus, which is the dense positively charged center of the atom. The
electrons are in orbit around the nucleus. In a simple diagram like the one
below, one can simply count the number of subatomic particles. When the
numbers can't readily be counted, we can determine the number of protons
and neutrons from the mass number and atomic number. The number of
electrons will equal the number of protons except in the case of ions.
Elements are identified by the number of protons, but the electrons are
largely responsible for the chemical properties of an elements.
Objective 2: State the relative masses and relative charges of
protons, neutrons & electrons.
Memorize the charges given in the table below.
Objective 3: Define the terms mass number (A), atomic number (Z) and
isotopes of an element.
Please make sure you can distinguish between the following terms.
Mass number (A): The total number of protons and neutrons in a nucleus.
The number of electrons is not included in the mass number because the
mass of an electron is too small to matter.
Atomic number (Z):The number of protons in a nucleus. Elements are defined
by the number of protons. (I think of this as "atom number" because it helps
me to remember that elements are defined by the number of protons. For
example, atom #1 is hydrogen. Silly, I know.)
Isotopes: Atoms of the same element (same number of protons) with different
numbers of neutrons. They have identical atomic numbers but different mass
numbers. Most elements exist as a mixture of isotopes. The atomic mass
given on most period tables in the weighted average of the naturally
occurring isotopes of an element.
Nuclide Symbols
The symbol above is called a nuclide symbol. Nuclide is the general term
applied to each unique atom. Nuclide symbols give the mass number and
atomic number as shown and are helpful in distinguishing isotopes. Nuclide
symbols are commonly used in nuclear chemistry, which we will study later
this year.
Despite differences in composition, each particle above is still
hydrogen. Why is this?
Objective 4: Deduce the symbol for an isotope given its mass
number and atomic number.
It is very easy to deduce the symbol for an isotope given the mass number
and atomic number. One only needs to look at the atomic number and then
find the matching element on the periodic table. The atomic number is usually
written above the element symbol on the periodic table.
To deduce the symbol from the mass number you can either subtract the
number of neutrons from the mass number or compare the mass number to
the atomic weight on the period table.
Mass number - # of neutrons = atomic number
Objective 5: Calculate the number of protons, neutrons and
electrons in atoms and ions from the mass number, atomic number and
charge.
Calculating the number of protons, neutrons and electron is easy if you
remember the following.
# of protons = atomic #
# of electrons = # of protons
# of neutrons = mass # - atomic #
The following link takes you to a webpage that provides a nice explanation
and example.
Objective 6: Calculate the average atomic mass of an atom from
isotopic data.
This type of calculation is what chemist use to determine the atomic weights
that are given on the periodic table. This will definitely be asked on my
exams and on the AP exam. Make sure you can do these types of
calculations!
Work through the tutorial How to Calculate an Average Atomic Weight.
HOMEWORK ASSIGNMENT: Book questions pg. 70-71 questions 19, 35-39, 43,
45, 47-51 and page 72 questions 53, 55, 57, 58, 61, & 63