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Fondamenti di Chimica & Educazione Ambientale
Lecture 2
Atoms, Molecules & Ions
“Leucippo è l’iniziatore di quel famigerato sistema
atomistico, che, risuscitato modernamente, è passato come
principio fondamentale dell’indagine razionale sulla natura.
Esaminato questo sistema per sè, lo troviamo bensì assai
povero e tale da offrire poco alla ricerca; ma deve essere
riconosciuto a Leucippo il grande merito di aver distinto,
come si dice comunemente nella nostra fisica, le qualità
generali dei corpi sensibili, o le primarie dalle secondarie, o
le essenziali dalle non essenziali.”
Georg Wilhelm Friedrich Hegel
Jena 1805 - 1806
Georg Wilhelm Friedrich Hegel
“Lezioni di Storia della Filosofia” La Nuova Italia Editrice
Vol. 1, p. 231, 1963
Elements
Pure substances composed of only one type of atom.
These are found and listed on the Periodic Table
Atoms
The smallest particle of an element that
retains all of the properties of the element.
Chemical Element
u A species of atoms; all atoms with the same
number of protons in the atomic nucleus.
u A pure chemical substance composed of atoms
with the same number of protons in the atomic
nucleus. Sometimes this concept is called the
elementary substance as distinct from the chemical
element as defined under 1, but mostly the term
chemical element is used for both concepts
Molecules
A sufficiently stable electrically neutral
group of at least two atoms in a definite
arrangement held together by chemical
bonds.
Compounds
A pure substance consisting of two or more
different elements chemically bonded
together in a fixed proportion by mass.
Dmitrij Ivanovič Mendeleev
08/02/1834 - 20/01/1907
H=1[5]
Li=7
Be=9,4
B=11
C=12
N=14
O=16
F=19
Na=23
Ti=50
Zr=90 ?[2]=180
V=51
Nb=94 Ta=182
Cr=52 Mo=96 W=186
Mn=55 Rh=104,4[3]
Pt=197,4[4]
Fe=56 Ru=104,4
Ir=198
Ni=Co=59
Pd=106,6
Os=199
Cu=63,4 Ag=108 Hg=200
Mg=24 Zn=65,2 Cd=112
Al=27,4 ?[6]=68 Ur=116[7]
Au=197?
Si=28
?[8]=70 Sn=118
P=31
As=75 Sb=122 Bi=210?
S=32
Se=79,4 Te=128?
Cl=35,5 Br=80 J=127[9]
K=39
Rb=85,4 Cs=133 Tl=204
Ca=40 Sr=87,6 Ba=137 Pb=207
?[10]=45 Ce=92[11]
?Er=56 La=94
?Yt=60 Di=95
?In=75,6 Th=118?
Alkaline Earths
Halogens
Main Group
Alkali
Metals
Transition Metals
Main Group
Lanthanides and
Actinides
Calchogens
Pnictogens
Noble Gases
The Periodic table
http://www.meta-synthesis.com/webbook/35_pt/
pt_database.php?Button=post-2000+Formulations
Dalton, John
1766-1844
Lithograph
A given compound always contains
the same proportion of elements, by
mass.
Dalton’s Atomic Theory
1. 
Each element is made up of atoms;
2. 
Atoms of a given element are identical while atoms of different
elements differ;
3. 
Chemical compounds are made up of specific whole number ratios of
atoms;
4. 
Reactions involve reorganization of atoms. Atoms do not change.
Essay on a Manner of Determining
the Relative Masses of the Elementary
Molecules of Bodies, and the
Proportions in Which They Enter Into
These Compound
Lorenzo Romano Amedeo Carlo Avogadro
Journal de Physique, 1811, 73, 58-76
Atomic Masses:
A relative comparison of the average masses of atoms.
Avogadro’s Hypothesis (1811):
Equal volumes of a gas contain the same number
of atoms under the same conditions of T and P.
Atomic Structure
Until the end of the 19th century, not much
was known about the atom.
People knew:
i) 
ii)
There were different elements
Each had a unique mass and reacted in
a special way.
BUT, WHAT WAS THE ATOM?
Introduction
to
Atomic
Theory
The atom was a black box. People knew
there were different substances (different
boxes), each with unique properties, but
“why” was a question.
Sir Joseph John Thomson
(1856 - 1940) was awarded
the 1906 Nobel Prize.
“… in recognition of the great merits of his
theoretical and experimental investigations on
the conduction of electricity by gases.”
The early experiments of J.J. Thompson cast light on
the question. An evacuated tube, containing a small
amount of a gas was attached to a power supply
LVIII. On the Masses of the Ions in Gases at Low Pressures.
By J.J. Thomson, M.A., F.R.S.,
Cavendish Professor of Experimental Physics,
Cambridge Philosophical Magazine
December 1899
Series 5, Vol. 48, No. 295, p. 547-567
The same cathode ray was seen no
matter what the tube was filled with.
-
+
Conclusion:
The cathode ray was a fundamental
unit of all materials.
Through a number of experiments, J.J. was
able to determine the charge (e) to mass (m)
ratio of the cathode ray.
-
+
e/m = some number
e/m =-1.75881962 x 1011 C/Kg.
By similar analysis he was also able to
look at the canal Ray, the positive
piece “left over” after the Cathode ray
had been ejected.
-
+
e/m was different for each gas that
was in the evacuated tube.
Starting with two different atomic “Boxes”
Same
Positive pieces “left
over” are different
+
+
q ν 02 Δy EΔy
=
= 2
m EdL B dL
x = ν 0t
1⎛ q ⎞ 2 1⎛ q ⎞ ⎛ x ⎞
y = ⎜ ⎟ Et = ⎜ ⎟ E ⎜ ⎟
2⎝ m⎠
2 ⎝ m ⎠ ⎝ ν0 ⎠
2
⎛ dy ⎞
⎛ q ⎞ Ed Δy
tan α = ⎜ ⎟ = ⎜ ⎟ 2 =
⎝ dx ⎠ x=d ⎝ m ⎠ ν 0
L
2
q ν 0 Δy
=
m EdL
q ν Δy
=
m EdL
2
0
Eq = qνB
E
ν=
B
q ν 02 Δy EΔy
=
= 2
m EdL B dL
The Plum Pudding Model
At this point J.J was ready to talk about
the constitution of the atom: a large
“sea” of positive charge, imbedded with
electrons.
Millikan’s Oil Drop
Experiment
The e/me ratio
(-1.75881962 x 1011 C/Kg)
What does it tell us?
Zap with radiation
Small droplets of oil, each given a negative charge, are pushed
upward by an electric force while they simultaneously fall
downward under the influence of gravity. Controlled variations of
the electric field show that the negative charge on each drop is
always a multiple of a certain value e.
Millikan’s Oil Drop Experiment
Experimental data indicate that:
e = -1.60217733 x 10-19 C;
me = 9.1093897 x 10-31 kg.
Back to Plum Pudding
A “sea of positive
charge” containing
embedded electrons
The gold foil experiment:
They shot a particles through the foil. This
should be equivalent to shooting a howitzer shell
through paper tissue.
United Kingdom and New Zealand
Victoria University Manchester, U. K.
b. 1871 (in Nelson, New Zealand) d. 1937
Nobel prize in Chemistry 1908
The gold foil experiment:
They shot a particles through the foil.
This should be equivalent to shooting a
howitzer shell through paper tissue.
A solar system model of the atom.
Electrons circle the nucleus.
The Concept of Atomic Number
Z = number of protons
You will notice that most of the
Periodic Table is well-arranged
according to Atomic Mass.
A = atomic mass = #protons + neutrons
Z = atomic number = # protons
A
Z
X
Isotopes:
Atoms with the same number of protons
(Z) but different number of neutrons (AZ)
A
X
Different atomic masses
Protium mass:
1.007825 u
Deuterium mass: 2.01410178 u
Composition of the Atom
Electrons
Nucleus
But, the nucleus is made of
protons and neutrons
2e-
1e-
2e-
Li
Be
2e- 8e-
2e-
2e-
8e-
......
8e-
1e-
2e-
8e-
2e-
Ar
8e8e2e-
Ca
8e8e-
Mg
8e-
K
......
2e-
Ne
2e-
1e-
Na
2e-
......
Law of Definite Proportion
A given compound always contains the
same proportion of elements, by mass.
Law of Multiple
Proportions
When two elements form two different
compounds, the mass ratio of
the
elements in one compound is related
to the the mass ratio of the elements in
the latter compound by a small whole
number.
Mass of Oxygen combining
with 1 gram of Nitrogen
Compound A
Compound B
1.1428 g
2.2857 g
Divide by smallest number to get
ratios:
1.1428/1.1428 = 1
2.2857/1.1428 = 2
We really do not know the real formula
NO
N2O2
NO2
N 2O 4
Lecture 2
End
Lecture 2
End