Download Chapter 2 Atoms, Molecules, and Ions

Lecture Presentation
Chapter 2
Atoms, Molecules,
and Ions
Prepared by
John N. Beauregard
Starting from a presentation by
James F. Kirby
Quinnipiac University
Hamden, CT
© 2015 Pearson Education, Inc.
Atomic Theory of Matter
•  The idea that atoms are the
fundamental building blocks of matter
was originated ~2400 years ago by
the Greek philosopher Democritus.
•  It reemerged in the early 19th century
in a scientific theory championed by
the British chemist John Dalton.
•  Dalton’s atomic theory explains
several scientific laws: the
conservation of mass in chemical
reactions, the constant composition of
chemical compounds, and the law of
multiple proportions.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Postulate #1 of Dalton’s Atomic Theory
Each element is composed of extremely small
particles called atoms. (Or the atom is the
fundamental building block of chemical
matter.)
Note: As far as we know, this is absolutely true.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Postulate #2 of Dalton’s Atomic Theory
All atoms of a given element are identical to
one another in mass and other properties,
but the atoms of one element are different
from the atoms of all other elements.
Note: We now know this is not exactly true, as most elements have
more than 1 isotope. However, all isotopes of a given element have
identical reactivity, and the average atomic mass is the same for any
sample of a given element. So, at the macroscopic level, we can treat
the atoms of an element as though they have all the same mass.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Postulate #3 of Dalton’s Atomic Theory
Atoms of an element are not changed into
atoms of a different element by chemical
reactions (or atoms are neither created nor
destroyed in chemical reactions.)
Note: This is always true for chemical changes. However, we now
know that atoms of one element do sometimes changes into atoms
of other elements. However, this occurs in processes that fall
within the realm of nuclear physics, not chemistry.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Postulate #4 of Dalton’s Atomic Theory
Atoms of more than one element combine to form
compounds; a given compound always has the
same relative numbers and kind of atoms.
Click here to view a YouTube
video tutorial on the atomic
theory of matter. The video
also shows how atomic theory
explains the law of constant
composition of chemical
compounds. This law was
introduced in Chapter 1.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Law of Conservation of Mass
The total mass of substances present at the
end of a chemical process is the same as the
total mass of substances present before the
process took place.
Ø  Dalton’s atomic theory explains this law by saying
that, during a chemical reaction, the atoms composing
the starting chemicals (reactants) are simply rearranged to form new substances (products). Thus,
conservation of mass equates to conservation of
atoms.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Law of Multiple Proportions
If two elements, A and B, form more than one
compound, the masses of B that combine
with a given mass of A are in the ratio of
small whole numbers.
Click here to view a YouTube video
tutorial on the Law of Multiple
Proportions. The law is explained in
the context of the atomic theory of
matter.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Discovery of Subatomic Particles
In Dalton’s original model, the atom was the smallest
particle possible. However, subsequent experimental
discoveries showed that the atom itself was made up of
smaller particles.
Ø  Electrons (cathode ray and oil drop experiments)
Ø  Radioactivity (Note: an important topic but not covered
on the AP Exam.)
Ø  Nucleus (gold foil experiment): composed of protons
and neutrons.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Discovery of The Electron:
J.J. Thomson’s Cathode Ray Experiment (1897)
English physicist J.J. Thomson found that streams of negatively
charged particles (electrons) emanate from cathode tubes, causing
fluorescence. (Note: the negative charge of the particles is evident
from the direction of their deflected path.)
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
• 
Thomson determined the electric field strength needed to straighten
the path of a cathode ray deflected by a magnetic field of known
strength. The allowed him to determine the charge/mass ratio of the
electron: –1.76 x 108 coulombs/gram (C/g).
• 
He obtained the same result no matter which metal was used for the
cathode. This indicated that the electron was common to all types of
atoms.
Click here to view a
YouTube video of a
cathode ray
demonstration. The
significance of
Thomson’s
observations is also
discussed.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Millikan Oil-Drop Experiment:
Determination of the Charge of an Electron
§  In 1909, Robert Millikan determined the charge on the
electron to be 1.602 x 10–19 Coulombs (C).
§  He combined this with Thomson’s result for the charge/mass
ratio of the electron and was able to calculate the mass of an
electron.
Click here to view a
YouTube video tutorial
on Millikan’s experiment.
The video also reviews
Thomson’s CRT
experiment.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Sample Exercise: Given both Thomson’s experimental result
for the charge to mass ratio for the electron (–1.76 x 108 C/g)
and Millikan’s experimental result for the charge of the
electron (–1.602 x 10–19 C), calculate the mass of a single
electron. How does your result compare to the mass of a
hydrogen atom (1.68 x 10–24 g)?
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Radioactivity
(Note: This material is not covered on the AP Exam.)
•  Radioactivity is the spontaneous emission of high-energy
radiation by an atom.
•  It was first observed by Henri Becquerel (1896). Marie and
Pierre Curie also made important contributions to our early
understanding of this phenomenon.
•  Rutherford (prior to his gold foil experiment) showed that
radioactivity was associated with the transmutation of one
element into another. He also identified three types of radiation:
alpha, beta, and gamma rays.
•  We now understand that radiation corresponds to either particles
(alfa or beta) or photons (gamma) that are ejected from the
nucleus of an atom as it changes from one element to another
(i.e. during a “nuclear reaction”).
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Types of Radioactivity
(Note: This material is not covered on the AP Exam.)
Three types of radiation discovered by Ernest Rutherford:
–  Alpha (α) particle: positively charged. Eventually it was found that it is
a helium nucleus emitted from the nucleus of a heavy atom as it
decays into lighter atom.
–  Beta (β) particle: negatively charged. Eventually it was determined that
it is an electron, but one emitted from a decaying nucleus (NOT an
electron cloud).
–  Gamma (γ) ray: Uncharged. Eventually it was found to be a photon of
high energy light emitted from a decaying nucleus.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Radiation is the Result of Nuclear Decay
(Note: Not covered on the AP Exam)
Example of an Alpha Decay Nuclear Reaction
High energy helium nucleus (2+ charge not shown) ejected from parent
nucleus
Example of an Alpha Decay Nuclear Reaction
High energy electron (–1 charge depicted as a superscript) ejected from
parent nucleus, came from a proton that changed to a neutron
These nuclear reactions involve the conversion of mass into energy, which
manifests itself as high K.E. of the alpha and beta particles and high energy
electromagnetic radiation (gamma rays)
© 2015 Pearson Education, Inc.
Atoms,
Molecules,
and Ions
In Summary:
•  Nuclear Chemistry (Chapter 21 in Brown &
Lemay) is not covered on the AP Exam. So you
won’t be tested on it in this class.
•  Neither will you have to write nuclear reactions.
•  Note: It may be helpful to have some familiarity
with alpha radiation due to its importance in
Rutherford’s gold foil experiment, which is
covered later in this presentation.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
The Plum Pudding Model:
Early Attempt to Explain how the Atom was Constructed
•  Around 1900 J.J. Thomson
proposed his “plum
pudding” model the atom.
•  It pictured the net neutral
atom as a sphere of
positive matter with
negative electrons
embedded in it.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Discovery of the Nucleus by Ernest Rutherford
Around 1910 Ernest Rutherford shot α particles at a thin
sheet of gold foil and observed the pattern of scatter of
the particles.
Click here to view a video tutorial
on Rutherford’s experiment and
how his results led Rutherford to
propose the nuclear model of the
atom to replace of the “plum
pudding” model.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Description of Rutherford’s Results
Ø  Most alpha particles either passed
straight through the gold foil or
were only slightly deflected
Ø  However, around 1 in 20,000
alpha particles were scattered at
large angles form the foil.
Ø  Since even a few particles were
deflected at large angles,
Thomson’s “plum pudding” model
could not be correct.
(What had Rutherford expected
based on the “plum pudding”
model and why?)
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
The Nuclear Model of the Atom
•  Rutherford postulated a very small,
dense nucleus with the electrons
surrounding it.
•  Most of the volume is empty space.
•  Atoms are very small: 1 – 5 Å (or
100 – 500 pm).
•  Later other subatomic particles
(protons and neutrons) were
discovered as parts of the nucleus.
Click here to view a demonstration of Rutherford’s
experiment using detectors based on modern
technology. The video also covers Rutherford’s
original experiment and how his results led to the
nuclear model of the atom.
© 2015 Pearson Education, Inc.
Atoms,
Molecules,
and Ions
Summary of Subatomic Particles
•  Protons (+1) and electrons (–1) have equal but opposite charges.
Neutrons are have no charge.
•  Protons and neutrons have essentially the same mass (about 1 amu
each). The mass of an electron (about 1/1800 amu) is negligible in
comparison.
•  Protons and neutrons, which carry most of the mass, are found in the
nucleus. Protons give the nucleus a positive charge.
•  Electrons travel around the nucleus and account for all of the negative
charge and almost al of the volume of the atom.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
The Atomic Mass Unit (amu)
•  Atoms have extremely small masses; the heaviest
known atoms have a mass of only 4 × 10–22 g.
•  Atomic Mass Unit (amu):
–  a convenient unit for expressing the masses of atoms and
molecules.
–  By definition, 1 amu = 1/12 mass of a C-12 atom. (Note:
A C-12 atom contains 12 protons, 12 neutrons, and 12
electrons.)
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Isotopes
•  atomic number (Z): the number of protons in the nucleus of an atom. Each
atom of a given element contains the same number of protons (or same
atomic number). For instance, all carbon atoms have Z = 6.
•  Isotopes: atoms of the same element with different masses but identical
chemical reactivity. Each isotope of a given element has the same number
of protons (and electrons) but a different number of neutrons (N).
•  mass number (A): defined A = Z + N. Both protons and neutrons have
masses of about 1 amu; the electron mass is negligible in comparison.
Thus, the mass number gives the approximate the mass of an atom in amu.
For example, the mass of a carbon-14 atom is approximately 14 amu.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Isotopic Symbols of Elements
Shown below is the isotopic symbol for the isotope carbon–12:
Ø  The atomic number (Z) is written as a subscript BEFORE the element
symbol.
Ø  The mass number (A) is is written as a superscript BEFORE the symbol.
Ø  For charged atoms (ions), the charge is written as a superscript AFTER
the symbol. (See the following video for examples.)
Click here to view a tutorial video on
writing isotopic symbols. The video
covers examples for both neutral
atoms and ions.
© 2015 Pearson Education, Inc.
Atoms,
Molecules,
and Ions
Average Atomic Mass (or Atomic Weight)
•  Each sample of a given element contains exactly the same isotopes in the
same relative abundances.
•  In the real world we use large amounts of atoms and molecules. So we
use average masses in calculations, since the average atomic mass is the
same for any sample of a given element.
•  An average mass (or atomic weight) is found using all isotopes of an
element weighted by their relative abundances:
Atomic Weight = Ʃ [(isotope mass) × (fractional natural abundance)]
Click here to view a
tutorial on
calculating average
atomic masses.
Click here to view a
second tutorial with
additional examples
of average atomic
mass calculations.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Mass Spectrometry:
Experimental Technique Used to Obtain Atomic Mass Data
•  Common experimental technique for determining the masses of atoms and
molecules.
•  We will focus on how it is used to determine both the number of isotopes for
a given element and the precise mass of the atoms of each isotope.
•  The schematic below depicts a mass spectroscopy experiment on the
element chlorine, which has two isotopes: 35Cl and 37Cl.
Schematic Diagram of a Mass Spectrometer
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
The Mass Spectrum of Atomic Chlorine
Obtained from the Experiment Depicted on the Previous Slide
Provides the following information:
1.  Precise mass of each isotope
of the element (relative to
that of carbon 12).
2.  Abundance of each isotope
of relative to the other
isotopes of the same
element.
Click here to view a tutorial on mass spectrometry.
The video shows how to analyze the mass spectrum of
an element and use the data to determine the average
atomic mass of that element.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Sample Problem: Do the following calculations based on the mass
spectrum for chlorine shown on the previous slide.
1)  Determine the %-abundance of each isotope of chlorine
2)  Find the (average) atomic mass of chlorine
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
The Periodic Table
•  a systematic organization of the elements: in order of atomic number.
•  the atomic number of an element is at the TOP of its corresponding box.
•  the atomic weight of an element usually appears at the BOTTOM of the box.
(although not shown on this version of the Periodic Table.)
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
The Periodic Law
When the elements are arranged in order of
increasing atomic number, a repeating
pattern is observed in their properties.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Periodic Table: Periods and Groups
Periods
§  Horizontal rows on the
periodic table
§  Each corresponds to
one full cycle of the
repeating properties of
the elements
Groups (or Families)
o  Vertical columns in the
periodic table
o  Elements in the same
group have similar
chemical properties.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
These Five Groups Are
Known by Special Names:
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Periodic Table: Metals, Nonmetals, and Metalloids
Metals
•  Located on the left side
of the periodic table.
•  Metallic properties
include
Ø  shiny luster.
Ø  conductive of heat and
electricity.
Ø  Malleable and ductile
Ø  Solid state at room
temperature (except
mercury).
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Periodic Table: Metals, Nonmetals, and Metalloids
Nonmetals
•  located on the right side
of the periodic table
(except for H).
•  Nonmetallic properties
include:
Ø  Poor conductors of heat and
electricity
Ø  can be solid (e.g. C), liquid
(e.g. Br), or gas (e.g. Ne) at
room temperature.
Ø  Brittle when solid
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Periodic Table: Metals, Nonmetals, and Metalloids
Metalloids
•  Located on the steplike strip dividing the
metals and nonmetals
(except Al, Po, and
At).
•  Their properties are
sometimes like
metals and
sometimes like
nonmetals.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Chemical Formulas
•  The subscript to the right of an
element symbol tells the number
of atoms of that element in one
molecule (or formula unit) of the
substance.
•  Molecular substances:
Ø  composed of neutral molecules
and typically contain only
nonmetals (or sometimes a
nonmetal and a metalloid).
Ø chemical formula gives the number
of each type of atom contained in
one molecule of the substance.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Molecular Elements
Some elements can exist in molecular form (i.e.
molecules with just one type of atom)
Ø 
Ø 
Ø 
Ø 
P4 is a molecular form of phosphorus
S8 is a molecular form of sulfur
C60 is a molecular form of carbon
The most stable form of each of the following
seven elements is a diatomic molecule
o 
o 
o 
o 
o 
o 
o 
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Hydrogen (H2)
Nitrogen (N2)
Oxygen (O2)
Fluorine (F2)
Chlorine(Cl2)
Bromine(Br2)
Iodine(I2)
Atoms,
Molecules,
and Ions
Types of Formulas
•  Empirical formula: gives the lowest wholenumber ratio of atoms of each element in a
compound.
•  Molecular formula: gives the exact number of
atoms of each element in a compound.
•  If we know the molecular formula of a compound,
we can determine its empirical formula. (This is
covered in Chapter 3.) The converse is not true!
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Types of Formulas
•  Structural formula: shows the
order in which atoms in a
molecule are attached but does
NOT depict the three-dimensional
shape of molecules.
•  Perspective (aka wedge and
stick) drawings: show both the
order of attachment of the atoms
and their three-dimensional
arrangement in space. This can
also be demonstrated using
molecular models.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Ions: Charged Particles
•  Monatomic Ion: a single atom with a net charge.
•  Cations: formed when one or more electrons is lost. Metals form
monatomic cations.
•  Anions: formed when one or more electron is gained.
Nonmetals form monatomic anions.
•  Note: The charges on the ions of main group elements (groups
1A–8A) can be predicted based on their group number.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Some Common Cations
Recall: Sometimes a
Roman numeral is
used to specify the
charge of a metal
cation. This is done
whenever the metal
for that cation can
form cations with
more than one
possible charge.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Some Common Anions
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Ionic Compounds
v  Generally formed by reacting metals with nonmetals (e.g. NaCl)
v  Electrons are transferred from the metal to the nonmetal. The
oppositely charged ions attract each other and form a crystal lattice.
(Does not exist as individual molecules.)
v  Can also be formed with polyatomic ions (e.g. NH4Cl)
v  Chemical formula gives the lowest whole-number combination of
cations an anions that is net neutral. (i.e. one formula unit of the
compound).
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Ionic Compounds:
Simple Approach for Determining Formulas
Ø  First, determine the respective charges of the cation and anion.
Ø  Then, since compounds are electrically neutral, one can
determine the formula of a compound this way:
•  The charge on the cation becomes the subscript on the anion.
•  The charge on the anion becomes the subscript on the cation.
•  If these subscripts are not in the lowest whole-number ratio,
divide them by the greatest common factor.
Click here to view a tutorial on writing
formulas for ionic compounds.
Several examples are given.
© 2015 Pearson Education, Inc.
Atoms,
Molecules,
and Ions
Oxyanion: polyatomic anion with one or more
oxygen atoms around a central atom
•  Central atoms on the second row have a bond to, at
most, three oxygens; those on the third row contain
up to four.
•  Charges increase from right to left.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Patterns in Oxyanion Nomenclature
•  When there are two oxyanions involving the same
element
–  the one with fewer oxygens ends in -ite.
–  the one with more oxygens ends in -ate.
•  Example 1: nitrite (NO2−) and nitrate (NO3−)
•  Example 2: sulfite (SO32−) and sulfate (SO42−)
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Patterns in Oxyanion Nomenclature:
What if the Central Atom Has More Than Two
Possible Oxyanions?
•  The one with the second fewest oxygens ends in -ite: ClO2− is
chlorite.
•  The one with the second most oxygens ends in -ate: ClO3− is
chlorate.
•  The one with the fewest oxygens has the prefix hypo- and ends in ite: ClO− is hypochlorite.
•  The one with the most oxygens has the prefix per- and ends in -ate:
ClO4− is perchlorate.
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Nomenclature for Ionic Compounds
The name for any ionic compound should be written as follows
•  Give the cation name followed by the anion name (separated
by a space).
•  If the cation can have more than one possible charge, write
the charge as a Roman numeral in parentheses.
•  If the anion is an element, change its ending to –ide. If the
anion is a polyatomic ion, simply write the name of the
polyatomic ion.
Click here to view a tutorial on
naming ionic compounds.
Lots of examples are given.
© 2015 Pearson Education, Inc.
Atoms,
Molecules,
and Ions
Acid Nomenclature
1.  If the anion in the acid ends in
-ide, change the ending to -ic
acid and add the prefix
hydro-.
Ø HCl: hydrochloric acid
Ø HBr: hydrobromic acid
Ø HI: hydroiodic acid
2.  If the anion ends in -ite,
change the ending to -ous
acid.
Click here to view a video
tutorial on writing the
Ø HClO: hypochlorous acid
names and formulas of
Ø HClO2: chlorous acid
acids. Several examples
3.  If the anion ends in -ate,
change the ending to -ic acid. are given.
Ø HClO3: chloric acid
Ø HClO4: perchloric acid
© 2015 Pearson Education, Inc.
Atoms,
Molecules,
and Ions
Nomenclature of Binary Molecular Compounds
•  The name of the element
farther to the left in the
periodic table (closer to the
metals) or lower in the
same group is usually
written first.
•  A prefix is used to denote
the number of atoms of
each element in the
compound (mono- is not
used on the first element
listed, however).
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Examples: Naming Binary Molecular Compounds
•  The ending on the second element is changed to -ide.
Ø CO2: carbon dioxide
Ø CCl4: carbon tetrachloride
•  If the prefix ends with a or o and the name of the
element begins with a vowel, the two successive vowels
are often elided into one.
Ø N2O5: dinitrogen pentoxide
Click here to view a
tutorial video on writing
the names of binary
molecular compounds.
Lots of examples are
given.
© 2015 Pearson Education, Inc.
Click here to view a second
tutorial on writing the names
and formulas of ionic
compound. Many more
examples are covered.
Atoms,
Molecules,
and Ions
Nomenclature of Organic Compounds
(Note: Not Covered on the AP Exam)
•  Organic chemistry: the chemistry of carbon-based
compounds.
•  Organic chemistry has its own system of nomenclature.
•  Alkanes: the simplest hydrocarbons (compounds containing
only carbon and hydrogen and all single bonds.
•  The first part of the names just listed correspond to the
number of carbons (meth- = 1, eth- = 2, prop- = 3, etc.).
Atoms,
Molecules,
and Ions
© 2015 Pearson Education, Inc.
Nomenclature of Organic Compounds
(Note: Not Covered on the AP Exam)
•  When a hydrogen in an alkane is replaced with
something else (a functional group, like -OH in
the compounds above), the name is derived from
the name of the alkane.
•  The ending denotes the type of compound.
–  An alcohol ends in -ol.
© 2015 Pearson Education, Inc.
Atoms,
Molecules,
and Ions