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Final

CHEM112 Lecture 1: Chemistry 112 Fall


Department
Chemistry
Course Code
CHEM 112
Professor
John Carran
Study Guide
Final

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Chemistry 113
Week 2-3 Atoms and Electromagnetic Radiation
Overview
“Atoms” is the first unit covered in our theme of ‘Structure and Bonding’ and is also the
first unit of the course that contains testable material. A discussion of atoms is a natural
starting point for a chemistry course. Molecules are composed of atoms connected in
different ways via bonds, chemical reactions occur when bonds between atoms change
and many chemical phenomena can be understood in terms of atomic properties.
This unit assumes you are familiar with the basic composition of an atom. Specifically, it
is assumed that you know:
1. Atoms are composed of protons, electrons, and neutrons.
2. Protons carry a positive charge, electrons carry a negative charge, and neutrons are
neutral. Protons and electrons have charges of equal magnitude.
3. Protons and neutrons reside in the nucleus and electrons move around the nucleus.
4. Protons and neutrons are nearly equal in mass, and are much heavier than
electrons. As such, the majority of an atom’s mass is in its nucleus.
5. Atoms are neutral, with the number of protons equaling the number of electrons.
The number of protons in the nucleus is termed the atomic number. Note that
atoms can gain or lose electrons to bear charges. These charged entities are called
ions.
If these concepts seem unfamiliar, you may wish to review concepts related to atomic
structure in chapter 2 of your text.
Items ii and iii from the list above are particularly important to chemistry in the context
of structure and bonding. Molecules are collections of positively charged nuclei and
negatively charged electrons held together in a stable configuration. For example, when
one draws water as shown below, it is implied that oxygen and hydrogen nuclei exist at
the positions indicated by their respective atomic symbols.
On their own, the nuclei in a molecule repel each other via Coulomb repulsion because
they have positive charges. Without some attractive force to balance this repulsion, the
molecule would decompose. The necessary stabilizing interaction is provided through the
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attraction of the nuclei to the electrons, which occupy regions around and between nuclei.
In a very simple picture, one can envision electron density residing between two nuclei as
acting like ‘glue’ that holds the nuclei together. That is, while the nuclei may repel each
other, which has a tendency to push them apart, they are also mutually attracted to
electron density between them. At some point, the repulsion between nuclei is balanced
by the attraction of the nuclei to the electrons, yielding a stable ‘bond’. These bonds are
indicated by the lines between atomic symbols in molecular structures like that of the
water molecule shown above. Chemical reactions involve a redistribution of electrons in
a molecule such that some bonds break and other bonds form. Meanwhile, the
arrangement of bonds in a molecule, i.e. how the electron density is distributed, causes
molecules to adopt different structures and shapes.
From the description above, it is apparent that understanding the behaviour of electrons is
critical to understanding chemical bonding and structure. Electrons are small, light
particles whose behaviour cannot be described properly through classical mechanics
(Newton’s equations). Instead, it is necessary to employ quantum mechanics to describe
the behaviour of electrons. As such, we are going to start by examining the origins, basic
features of and supporting evidence for quantum mechanics. This will involve an
examination of the electromagnetic radiation. The concepts learned through the study of
quantization of electromagnetic radiation will then be used to describe the behaviour of
electrons in atoms. Once we can describe the behaviour of electrons in atoms, it will be
possible to describe electron configurations and how these configurations lead to trends
in elemental properties. Later units will build on the concepts introduced in this unit to
explore how electrons are shared between different atoms to yield molecules.
While it may seem like quantum mechanics is a ‘physics’ topic instead of something to
be learned in ‘chemistry’, I would suggest that you forget about these artificially
constructed ‘walls’ between disciplines and recognize that science is a study of natural
phenomena that cannot be rigidly divided into disciplines. This will help you not only in
this course, but as you learn more about scientific topics. Indeed, a large number of
chemical phenomena can be explained in terms of the quantum mechanical behaviour of
electrons in atoms and molecules. A few examples that may interest you where the
quantum mechanics of electrons and atoms plays a key role are available through the
links below.
Biology (quantum mechanical interactions between components of pigment molecules enhance
photosynthesis):http://physicsworld.com/cws/article/news/2010/feb/04/quantum-mechanics-
boosts-photosynthesis
Materials Science (quantum mechanical transfer of electrons between surfaces and experimental
instruments allows imaging of atoms on surfaces):
http://www.nobelprize.org/educational/physics/microscopes/scanning/index.html
Life Science/Medicine (quantum mechanical tunneling increases rates of enzymatic
reactions):http://seedmagazine.com/content/article/the_quantum_shortcut/
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Learning Outcomes
Upon successful completion of this Unit, you will be able to:
Explain properties of electromagnetic radiation and perform related calculations
Calculate properties related to the photoelectric effect
Describe the Bohr model of hydrogen and use the associated equations to predict
atomic spectra
Explain de Broglie waves and use associated equations
Explain standing waves and associated properties
Determine allowed sets of quantum numbers and use them to define atomic
orbitals
Identify orbital shapes
Assign electron configurations in atoms using the aufbau principle
Predict periodic trends in the properties of atoms
Explain connections between electron structure and absorption/emission spectra
Properties of Electromagnetic Radiation
INTRODUCTION
Electromagnetic radiation corresponds to oscillations in electric and magnetic fields that
propagate as travelling waves. These waves include visible light, radio waves,
microwaves and X-rays. An example of such a wave is illustrated in Figure 2-1, with the
wavelength, l, and amplitude, A, indicated. Understanding details of electromagnetic
radiation is fundamental to developing a quantum mechanical description of electrons in
atoms. This is true for multiple reasons, including:
quantum theory originated from an examination of electromagnetic radiation
emitted by heated objects
a lot of evidence for the validity of quantum mechanics stems from examining
electromagnetic radiation absorbed or emitted by atoms
the wavelike properties of electromagnetic radiation are also exhibited by small
particles like electrons that matter in the context of chemistry
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