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Chemistry Chapter 4
Atoms:
The Building
Blocks of Matter
Law of Conservation of Mass
Mass is neither
created nor destroyed
during chemical or
physical reactions.
Total mass of reactants
=
Total mass of products
Antoine Lavoisier
http://www.gre
atscientists.net
/antoinelavoisier/
Proust’s Law of Definite Proportions
• In 1770s a French apothecary discovered the
Law of Constant Composition:
A given compound always contains
the same proportions (by mass)
of elements regardless of the
source
For example water H2O must
Always have 8 g oxygen
Combining with 1 g Hydrogen
Hydrogen Peroxide H2O2 must
Have 16 g Oxygen combining with
1 g Hydrogen
Dalton’s Atomic Theory (1808)
 All matter is composed of extremely
small particles called atoms
 Atoms of a given element are
identical in size, mass, and other
properties; atoms of different
John Dalton
elements differ in size, mass, and
other properties
 Atoms cannot be subdivided, created, or destroyed
 Atoms of different elements combine in simple
whole-number ratios to form chemical compounds
 In chemical reactions, atoms are combined,
separated, or rearranged
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube
to deduce that the electrons are the subatomic
negatively charged particles carrying electricity in
a cathode ray tube.
Thompson measured
the mass of cathode
rays, showing they
were made of particles
that were around 1800
times lighter than the
lightest atom,
Hydrogen
Cathode ray tubes pass electricity
through a gas that is contained at a
very low pressure.
Some Modern
Cathode Ray Tubes
Conclusions from the Study of
the Electron
 Cathode rays have identical properties regardless
of the element used to produce them. All elements
must contain identically charged electrons.
Atoms are neutral, so there must be positive
particles in the atom to balance the negative
charge of the electrons
 Electrons have so little mass that atoms must
contain other particles that account for most of
the mass
Thomson’s Atomic Model
Thomson believed that the electrons were like plums
embedded in a positively charged “pudding,” thus it was
called the “plum pudding” model.
Rutherford’s Gold Foil Experiment
 Alpha particles are helium nuclei
 Particles were fired at a thin sheet of gold foil
 Particle hits on the detecting screen (film) are
recorded
http://videos.howstuffworks.com/hsw/27126matter-and-energy-ernest-rutherfordsexperiment-video.htm
Try it Yourself!
In the following pictures, there is a target hidden by
a cloud. To figure out the shape of the target, we
shot some beams into the cloud and recorded where
the beams came out. Can you figure out the shape of
the target?
The Answers
Target #1
Target #2
Rutherford’s Findings
 Most of the particles passed right through
 A few particles were deflected
 VERY FEW were greatly deflected
“Like bullets bouncing off of tissue
paper!”
Conclusions:
 The nucleus is small
 The nucleus is dense
 The nucleus is positively charged
The Atomic
Scale
 Most of the mass of the
atom is in the nucleus
(protons and neutrons)
 Electrons are found
outside of the nucleus (the
electron cloud)
 Most of the volume of
the atom is empty space
“q” is a particle called a “quark”
Atomic Mass Scale
1 atomic mass unit (u) has a mass of 1/12 of the
mass of carbon-12 atom or 1.66054 *10-24g
(1g= 6.02214*1023 u)
Particle
Charge
Mass (u)
Proton
Positive
(1+)
1.0073
Neutron
None
(neutral)
1.0087
Electron
Negative
(1-)
5.486*10-4
Atomic Number
Atomic number (Z) of an element is the
number of protons in the nucleus of each atom
of that element.
Element
# of protons
Atomic # (Z)
6
6
Phosphorus
15
15
Gold
79
79
Carbon
Mass Number
Mass number is the number of protons and
neutrons in the nucleus of an isotope.
Mass # = p+ + n0
Nuclide
p+
n0
e-
Mass #
Oxygen - 18
8
10
8
18
Arsenic - 75
33
42
33
75
Phosphorus - 31
15
16
15
31
Isotopes
Elements occur in
nature as mixtures
of isotopes.
Isotopes are atoms of
the same element that
differ in the number
of neutrons
• THERE ARE TWO WAYS
TO REPRESENT THE
• DIFFERENT TYPES OF
ISOTOPES
Mass number
Atomic number
1
1
H
0
Or
H-1
Mass number
Charge
(is included for ions
Charged atoms and is not
Included for neutral atoms
• An atom becomes an ion
when it loses or gains
electrons. Now the
electrons and protons are
note balanced.
27
13
Al
13p
13p
13e
27
13
Al
3
-3e13p
10e
Modern Atomic Theory
Several changes have been made to Dalton’s theory.
Dalton said:
Atoms of a given element are identical in
size, mass, and other properties; atoms of
different elements differ in size, mass, and
other properties
Modern theory states:
Atoms of an element have a characteristic
average mass which is unique to that
element.
Average Atomic Mass (Atomic
Weight)
• Average atomic mass is the average of all the
naturally isotopes of that element
Atomic mass= ∑ (mass number ∙ % abundance)
Example
Find the average atomic mass of Carbon.
Isotope
Symbol
Composition of
the nucleus
% in nature
Carbon-12
12C
6 protons
6 neutrons
98.89%
Carbon-13
13C
6 protons
7 neutrons
1.11%
Carbon-14
14C
6 protons
8 neutrons
<0.01%
AAM= (.9889*12)+ (.0111*13)+ (.0001*14)=
AAM (Carbon)= 12.011
Modern Atomic Theory #2
Dalton said:
Atoms cannot be subdivided, created, or destroyed
Modern theory states:
Atoms cannot be subdivided, created, or destroyed
in ordinary chemical reactions. However, these
changes CAN occur in nuclear reactions!
Mass spectrometer
• The MS works by ionizing chemical compounds
to generate charged molecules or molecule
fragments and measurement of their mass to
charge ratios