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Lecture

CHEM 212 Lecture Notes - Bohr Model, Atom, Actinide


Department
Chemistry
Course Code
CHEM 212
Professor
Richard Oakley

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The theories of atomic and molecular structure depend on quantum mechanics to de-
scribe atoms and molecules in mathematical terms. Although the details of quantum
mechanics require considerable mathematical sophistication, it is possible to under-
stand the principles involved with only a moderate amount of mathematics. This chap-
ter presents the fundamentals needed to explain atomic and molecular structures in
qualitative or semiquantitative terms.
2-1
Although the Greek philosophers Democritus (460-370
BC)
and Epicurus (341-270
HlSTORlCAL
BC)
presented views of nature that included atoms, many hundreds of years passed
DEVELOPMENT OF
before experimental studies could establish the quantitative relationships needed for a
ATOMIC THEORY
coherent atomic theory. In 1808, John Dalton published
A
New System of Chemical
~hiloso~h~,'
in which he proposed that
. . .
the ultimate particles of all homogeneous bodies are perfectly alike in weight, figure,
etc. In other words, every particle of water is like every other particle of water, every parti-
cle
of
hydrogen
is
like every other particle of hydrogen, etce2
and that atoms combine in simple numerical ratios to form compounds. The terminolo-
gy he used has since been modified, but he clearly presented the ideas of atoms and
molecules, described many observations about heat (or caloric, as it was called), and
made quantitative observations of the masses and volumes of substances combining to
form new compounds. Because of confusion about elemental molecules such as
Hz
and
02,
which he assumed to be monatomic
H
and
0,
he did not find the correct formula for
water. Dalton said that
'~ohn Dalton,
A
New System
qf
Chemical Philosophy,
1808;
reprinted wi
der Joseph, Peter Owen Limited, London,
1965.
'lbid.,
p.
113.
. -.-

Only pages 1-3 are available for preview. Some parts have been intentionally blurred.

16
Chapter
2
Atomic Structure
When two measures of hydrogen and one of oxygen gas are mixed, and fired by the elec-
tric spark, the whole is converted into steam, and if the pressure be great, this steam be-
comes water.
It
is most probable then that there is the same number of particles in two
measures of hydrogen as in one of oxygen.3
In fact, he then changed his mind about the number of molecules in equal volumes of
different gases:
At the time
I
formed the theory of mixed gases,
I
had a confused idea, as many have,
I
sup-
pose, at this time, that the particles of elastic fluids are all of the same size; that a given vol-
ume of oxygenous gas contains just as many particles as the same volume of hydrogenous;
or if not, that we had no data from which the question could be solved.
. . .
I
[later] became
convinced.
. .
That every species of pure elastic fluid has its particles globular and all of a
size; but that no two species agree
in
the size of their particles, the pressure and tempera-
ture being the same.
4
Only a few years later, Avogadro used data from Gay-Lussac to argue that equal
volumes of gas at equal temperatures and pressures contain the same number of mole-
cules, but uncertainties about the nature of sulfur, phosphorus, arsenic, and mercury va-
pors delayed acceptance of this idea. Widespread confusion about atomic weights and
molecular formulas contributed to the delay; in 1861, Kekul6 gave
19
different possible
formulas for acetic acid!' In the 1850s, Cannizzaro revived the argument of Avogadro
and argued that everyone should use the same set of atomic weights rather than the
many different sets then being used. At a meeting in Karlsruhe in 1860, he distributed a
pamphlet describing his views.6 His proposal was eventually accepted, and a consistent
set of atomic weights and formulas gradually evolved. In 1869, ~endeleev~ and ~e~er'
independently proposed periodic tables nearly like those used today, and from that time
the development of atomic theory progressed rapidly.
2-1-1
THE PERIODIC TABLE
The idea of arranging the elements into a periodic table had been considered by many
chemists, but either the data to support the idea were insufficient or the classification
schemes were incomplete. Mendeleev and Meyer organized the elements in order of
atomic weight and then identified families of elements with similar properties. By ar-
ranging these families in rows or columns, and by considering similarities in chemical
behavior as well as atomic weight, Mendeleev found vacancies in the table and was able
to predict the properties of several elements (gallium, scandium, germanium, polonium)
that had not yet been discovered. When his predictions proved accurate, the concept of
a periodic table was quickly established (see Figure 1-10). The discovery of additional
elements not known in Mendeleev's time and the synthesis of heavy elements have led
to the more complete modern periodic table, shown inside the front cover of this text.
In the modern periodic table, a horizontal row of elements is called a
period,
and
a vertical column is a
group
or
family.
The traditional designations of groups in the
United States differ from those used in Europe. The International Union of Pure and
Applied Chemistry (IUPAC) has recommended that the groups be numbered I through
18, a recommendation that has generated considerable controversy. In this text, we will
31bid.,
p. 133
4~bid.,
pp.
144-145.
5~.~. Partington,
A Short History of Chemistry,
3rd ed., Macmillan, London,
1957;
reprinted, 1960,
Harper
&
Row, New York, p. 255.
6~bid.,
pp. 256-258.
7~.
I.
Mendeleev,
J.
Russ. Phys. Chem. Soc.,
1869,
i,
60.
8~. Meyer,
Justus Liebigs Ann. Chem.,
1870,
Suppl, vii,
354.

Only pages 1-3 are available for preview. Some parts have been intentionally blurred.

2-1
Historical Development of Atomic Theory
1
7
FIGURE
2-1
Names for Parts of
the Periodic Table.
Groups (American tradition)
IA IIA IIIB IVB VB VIB VIIB VIIIB IB IIB IIIA IVA VA VIA VIIA VIIIA
Groups (European tradition)
IA IIA
IIIA
IVA VA VIA VIIA VIII 1B IIB IIIB IVB VB VIB VIE
0
Groups (IUPAC)
123
*
4
5
6
7 8 9 10 11 12 13 14 15 16 17 18
0
Transition metals
use the IUPAC group numbers, with the traditional American numbers in parentheses.
Some sections of the periodic table have traditional names, as shown
in
Figure
2-1.
*
**
2-1-2
DISCOVERY OF SUBATOMIC
PARTICLES AND THE BOHR ATOM
During the 50 years after the periodic tables of Mendeleev and Meyer were proposed,
experimental advances came rapidly. Some of these discoveries are shown in Table 2-1.
Parallel discoveries in atomic spectra showed that each element emits light of
specific energies when excited by an electric discharge or heat. In 1885, Balmer showed
that the energies of visible light emitted by the hydrogen atom are given by the equation
58
90
TABLE
2-1
Discoveries
in
Atomic
Structlrte
1896
A
H
Becquerel D~scovered
radioactivity
of uranium
1897
J J
Thomson Showed that electrons have a negahve charge, w~th
charge/mass
=
1 76
X
1011
C/kg
1909
R A. Milhkdn Medsured the eleclron~c charge
(1
60
X
lo-''
C); therefore, the mass of
1
the electron
IS
9
11
X
kg,
------
of the mass of the
H
atom
1836
191 1
E.
Rutherford Established the nuclear model of the atom (very small, heavy nucleus
surrounded by mostly empty space)
1913
H.
G.
J.
Moseley Determined nuclear charges by X-ray emission,
establishing
atomic
numbers as more fundamental than atomic masses
I I
Lanthanides
I I
Actinides
71
103
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