Chapter 2
Atoms, Molecules,
and Ions
Atoms,
Molecules,
and Ions
Dalton’s Postulates
1) Each element is composed of extremely small particles called atoms.
Subatomic particles had not been discovered yet at the time Dalton
proposed his theory.
2) 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. Obviously the first part of the postulate
was later revised when neutrons were discovered and isotopes were
found.
3) Atoms of an element are not changed into atoms of
a different element by chemical reactions; atoms
are neither created nor destroyed in chemical
reactions. Law of conservation of matter.
4) Compounds are formed when atoms of more than
one element combine; a given compound always
has the same relative number and kind of atoms.
Law of constant composition.
Atoms,
Molecules,
and Ions
Law of Constant Composition
Joseph Proust (1754–1826)
• Also known as the law of definite
proportions.
• The elemental composition of a pure
substance never varies.
• MgO is always 60.3% Mg and 39.7% O
by mass, no matter the size of the
sample, as long as the compound is
pure.
Atoms,
Molecules,
and Ions
Law of Multiple Proportions
Two compounds containing nitrogen and
oxygen are:
NO2 and N2O
To find if the law of multiple proportions apply
to these examples we must work with the ratio
of masses:
For NO2: 14 g N / 32 g O
For N2O: 28 g N / 16 g O
Making the mass of O = 1
NO2 : 0.4375 g N / g O
N2O: 1.75 g N / g O
Atoms,
Molecules,
and Ions
Law of Multiple Proportions
NO2 : 0.4375 g N / g O
N2O: 1.75 g N / g O
Once the mass of one of the elements is set
to 1, the other element can be compared:
1.75 / 0.4375 = 4/1
Small number ratios!!!!
Atoms,
Molecules,
and Ions
Law of Conservation of Mass
The total mass of substances present at
the end of a chemical process is the
same as the mass of substances
present before the process took place.
Atoms,
Molecules,
and Ions
The Electron
• Streams of negatively charged particles were
found to emanate from cathode tubes.
• J. J. Thompson is credited with their
Atoms,
discovery (1897).
Molecules,
and Ions
The Electron
Thompson measured the charge/mass ratio
of the electron to be 1.76  108 coulombs/g.
Atoms,
Molecules,
and Ions
Millikan Oil Drop Experiment
Once the charge/mass
ratio of the electron
was known,
determination of either
the charge or the mass
of an electron would
yield the other.
Atoms,
Molecules,
and Ions
Millikan Oil Drop Experiment
Robert Millikan
(University of Chicago)
determined the charge
on the electron in
1909.
See video
Atoms,
Molecules,
and Ions
Radioactivity:
• The spontaneous emission of radiation
by an atom.
• First observed by Henri Becquerel.
• Also studied by Marie and Pierre Curie.
Atoms,
Molecules,
and Ions
Radioactivity
• Three types of radiation were discovered by
Ernest Rutherford:
  particles.
  particles.
  rays
See video
Atoms,
Molecules,
and Ions
The Atom, circa 1900:
• “Plum pudding” model,
put forward by
Thompson.
• Positive sphere of matter
with negative electrons
imbedded in it.
Atoms,
Molecules,
and Ions
Discovery of the Nucleus
Ernest Rutherford
shot  particles at a
thin sheet of gold foil
and observed the
pattern of scatter of
the particles.
See video
Atoms,
Molecules,
and Ions
The Nuclear Atom
Since some particles
were deflected at
large angles,
Thompson’s model
could not be correct.
Atoms,
Molecules,
and Ions
The Nuclear Atom
• Rutherford postulated a very small,
dense nucleus with the electrons
around the outside of the atom.
• Most of the volume of the atom is empty
space.
Atoms,
Molecules,
and Ions
Other Subatomic Particles
• Protons were discovered by Rutherford
in 1919.
• Neutrons were discovered by James
Chadwick in 1932.
Atoms,
Molecules,
and Ions
Subatomic Particles
• Protons and electrons are the only particles that
have a charge.
• Protons and neutrons have essentially the same
mass.
• The mass of an electron is so small we ignore it.
Atoms,
Molecules,
and Ions
Symbols of Elements
Elements are symbolized by one or two
letters.
Atoms,
Molecules,
and Ions
Atomic Number
All atoms of the same element have the same
number of protons:
The atomic number (Z)
Atoms,
Molecules,
and Ions
Atomic Mass
The mass of an atom in atomic mass units
(amu) is the total number of protons and
neutrons in the atom.
Atoms,
Molecules,
and Ions
Isotopes:
• Atoms of the same element with different masses.
• Isotopes have different numbers of neutrons.
11
C
6
12
C
6
13
C
6
14
C
6
Atoms,
Molecules,
and Ions
Problem 1. Complete the following table.
protons
neutrons
electrons
Mass
number
Atomic
number
36
88
38
32S
Co2+
33
35ClAtoms,
Molecules,
and Ions
How can isotopes be detected?
Mass Spectrometry
• Molecules are converted to ions so that they can be moved
about and manipulated by external electric and magnetic
fields. The three essential functions of a mass
spectrometer, and the associated components, are:
The Ion Source: A small sample is ionized, usually to
cations by loss of an electron. M+
The Mass Analyzer: The ions are sorted and separated
according to their mass-to-charge ratio.
The Detector: The separated ions are then measured their
abundance. The results are displayed on a chart.
Atoms,
Molecules,
and Ions
Mass Spec.
Atoms,
Molecules,
and Ions
The mass spectrum
• A mass spectrum will usually be presented as a vertical bar graph, in
which each bar represents an ion having a specific mass-to-charge
ratio (m/z) and the length of the bar indicates the relative abundance
of the ion.
• The most intense ion is assigned an abundance of 100, and it is
referred to as the base peak.
• Most of the ions formed in a mass spectrometer have a single charge,
so the m/z value is equivalent to mass itself.
• Modern mass spectrometers easily distinguish (resolve) ions differing
by only a single atomic mass unit (amu), and thus provide completely
accurate values for the molecular mass of a compound.
• The highest-mass ion in a spectrum is normally considered to be the
molecular ion (and therefore the molecular mass), and lower-mass
ions are fragments from the molecular ion, assuming the sample is a
single pure compound.
Atoms,
Molecules,
and Ions
The mass spectrum of bromine
The most abundant peak has a
mass of 160 indicating 79Br-81Br.
The less common 158 and 162
will be formed by the same
isotopes. (79Br-79Br and 81Br81Br respectively)
The fragmentation of 160
generates almost equal
amounts of 79Br and 81Br.
This suggests that both isotopes
are almost equally abundant.
Atoms,
Molecules,
and Ions
The mass spectrum of chloro
ethene or vinyl chloride
MW: 62.5 amu • The peak at 62 represents
the compound with the
isotope 35Cl, while the
peak at 64 represents the
compound with 37Cl.
• Notice that the %
abundance (peak’s height)
is about 75% for 35Cl and
about 25% for 37Cl.
• Infer the fragment
responsible for peak at 27.
Atoms,
Molecules,
and Ions
Actual % abundances.
Chlorine: 75.77% 35Cl and 24.23% 37Cl
Bromine: 50.50% 79Br and 49.50% 81Br
Atoms,
Molecules,
and Ions
Average Mass
• Because in the real world we use large
amounts of atoms and molecules, we use
average masses in calculations.
• Average mass is calculated from the
isotopes of an element weighted by their
relative abundances.
• Average atomic mass = ∑(atomic mass x % abundance) for each isotope
100
Atoms,
Molecules,
and Ions
Problem 2. Determine the atomic mass and the identity
of an element with the following mass spectrum data.
Isotope Atomic mass
M-89
M-90
M-91
M-92
75.523
88.896 amu
89.945 amu
90.573 amu
91.747 amu
19.896
3.569
1.012
89
90
91
92
Atoms,
Molecules,
and Ions
Mass percent
Mass % element = (# of atoms of element) ( element’s atomic mass) x 100
molar mass of compound
Problem 3. Calculate the mass % of elements in (NH4)2SO4.
Atoms,
Molecules,
and Ions
Law of multiple proportion
H2O
H2O2
Mass O
16.00 amu
2(16.00 amu) = 32.00 amu
Mass H
2(1.01 amu) = 2.02 amu
2(1.01 amu) = 2.02 amu
Mass O/mass H
16.00 amu/2.02 amu
32.00 amu/2.02 amu
32.00/2.02
16.00/2.02
=2
This tells us that H2O2 has twice as many oxygen
atoms than H2O. This is too easy!!
Atoms,
Molecules,
and Ions
Problem 4: Use the Law of multiple proportions to prove
the formulas for NO2 and N2O
Atoms,
Molecules,
and Ions
Mass ratio of elements in a
pure sample of a compound
• The ratio of isotopes of a certain element in
any pure sample of a given compound is
constant.
• The ratio 12C, 13C, 14C in pure CO2 is
constant.
• As the ratio of the different atoms of each
element that form a given compound is
constant, the ratio of the masses of the
constituent elements in a pure sample of that
compound will also be constant.
Atoms,
Molecules,
and Ions
Example: Two separate pure samples of carbon dioxide were
analyzed. Both samples were found to contain 17.29%
carbon by mass. Justify these findings on the basis of atomic
molecular theory.
All carbon dioxide molecules contain exactly one carbon
and two oxygen atoms. Although different isotopes of
carbon and different isotopes of oxygen have different
masses, the ratio of isotopes of carbon and the ratio of
isotopes of oxygen in any pure sample of carbon dioxide is
constant. If we could calculate the average mass of all the
atoms of an element in a pure sample, we would obtain the
average atomic mass of that element, which is given in the
periodic table. Since the ratio of carbon atoms to oxygen
atoms in both pure samples is the same, the ratio of the
masses of carbon atoms to oxygen atoms in that pure sample
Atoms,
Molecules,
will also be the same.
and Ions
Nuclear Chemistry
• Nuclear decay: An unstable atom (because of too many subatomic
particles in the tiny nucleus) tends to lose particles from the nucleus.
 Alpha decay (α): An atom loses 2 protons and 2 neutrons and its
nuclear mass decreases by 4 (similar to the mass of He). It usually
happens to heavy isotopes.
• 238U  234Th + 4He
 Beta decay (β): A neutron in the nucleus changes into a proton, thus
increasing the atomic number but not the mass and turning into a
more stable isotope of another element. It looks like an electron is
lost. It usually happens to light isotopes.
• 14C  14N + -1e
 Gamma decay (γ): The unstable atom loses energy but retains its
mass and charge.
• 97*Tc  97Tc + γ
Atoms,
Molecules,
and Ions
Nuclear equations
Problem 5. Complete the following nuclear reactions
•
3H
•
232 U
•
144Ce

________ +


228 Th
144Pr
0
e
+ _______
+ _________
• 7Be*  7Be + ____________
Atoms,
Molecules,
and Ions