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Matter is Made up of Atoms
Chapter 2
Atoms and Their Structure
Section 2.1
HISTORY OF THE ATOM
460 BC
Democritus develops the idea of atoms
He pounded up materials in his mortar
and pestle until he had reduced them
to smaller and smaller particles which
he called
ATOMA
(greek for indivisible)
HISTORY OF THE ATOM
460 BC
Democritus develops the idea of atoms
The idea that matter is made up of
fundamental particles called atoms is
known as the atomic theory of
matter.
The well-known
philosophers of the
time, Aristotle and
Plato, had a more
respected, (and
ultimately wrong)
theory.
Aristotle and Plato favored the earth, fire, air
and water approach to the nature of matter.
Their ideas held sway because of their
eminence as philosophers. The atomos idea
was buried for approximately 2000 years.
Development of the Modern Atomic Theory
• In 1782, a French chemist, Antoine Lavoisier
(1743-1794), made measurements of
chemical change in a sealed container.
• He observed that the mass of reactants in
the container before a chemical reaction
was equal to the mass of the products after
the reaction.
Development of the Modern Atomic Theory
• Lavoisier concluded that when a chemical
reaction occurs, mass is neither created nor
destroyed but only changed.
• Lavoisier’s
conclusion
became known as
the law of
conservation of
mass.
Development of the Modern Atomic Theory
• In 1799, another French chemist, Joseph
Proust, observed that the composition of
water is always 11 percent hydrogen and 89
percent oxygen by mass.
• Regardless of the
source of the water, it
always contains these
same percentages of
hydrogen and oxygen.
Development of the Modern Atomic Theory
• Proust studied many other compounds and
observed that the elements that composed
the compounds were always in a certain
proportion by mass. This principle is now
referred to as the law of definite proportions
Dalton’s Atomic Theory
• John Dalton (17661844), an English
schoolteacher and
chemist, studied the
results of
experiments by
Lavoisier, Proust,
and many other
scientists.
Dalton’s Atomic Theory
The following statements are the main points of
Dalton’s atomic theory.
1. All matter is made up of atoms.
2. Atoms are indestructible and cannot be divided
into smaller particles (Atoms are indivisible).
3. All atoms of one element are exactly alike, but
are different from atoms of other elements.
Dalton’s Atomic Theory
• Proposed his atomic theory of
matter in 1803.
• Although his theory has been
modified slightly to accommodate
new discoveries, Dalton’s theory
was so insightful that it has
remained essentially intact up to
the present time.
HISTORY OF THE ATOM
1897
Joseph John Thompson
British physicist, J.J. Thomson,
discovered that Dalton’s solid-ball
model was not accurate.
Thomson’s experiments used a vacuum
tube.
Cathode-Ray Tube
• A vacuum tube has
had all gases pumped
out of it.
• At each end of the tube is a metal piece
called an electrode, which is connected
through the glass to a metal terminal outside
the tube.
• These electrodes become electrically charged
when they are connected to a high-voltage
electrical source.
Cathode-Ray Tube
• When the
electrodes are
charged, rays
travel in the tube
from the negative
electrode, which
is the cathode, to
the positive
electrode, the
anode.
• Because these
rays originate at
the cathode, they
are called
cathode rays.
Cathode-Ray Tube
• Thomson found that the rays bent toward a
positively charged plate and away from a
negatively charged plate.
• Thomson concluded
that cathode rays
are made up of
invisible,
negatively charged
particles referred
to as electrons.
1904
His idea was that an atom was made up of electrons
scattered unevenly within an elastic sphere
surrounded by a soup of positive charge to balance the
electron's charge like plums surrounded by pudding.
PLUM PUDDING
MODEL
Cathode-Ray Tube
• From Thomson’s experiments, scientists had
to conclude that atoms were not just neutral
spheres, but somehow were composed of
electrically charged particles.
• If atoms contained extremely light,
negatively charged particles, then they must
also contain positively charged particles.
• In 1886, scientists discovered that a
cathode-ray tube emitted rays not only from
the cathode but also from the positively
charged anode.
• Thomson was able to show that these rays
had a positive electrical charge.
• Years later, scientists determined that the
rays were composed of positively charged
subatomic particles called protons.
• At this point, it seemed that atoms were
made up of equal numbers of protons and
electrons.
• However, in 1910, Thomson discovered that
neon consisted
of atoms of
two different
masses.
• Atoms of an element that are chemically
alike but differ in mass are called isotopes
of the element.
• Today, chemists know that neon consists of
three naturally occurring isotopes.
• Because of the discovery of isotopes,
scientists hypothesized that atoms
contained still a third type of particle that
explained these differences in mass.
• The existence of this neutral particle, called
a neutron, was confirmed in the early 1930s.
Rutherford’s Gold Foil Experiment
• In 1909, a team of
scientists led by Ernest
Rutherford in England
carried out the first of
several important
experiments that
revealed an arrangement
far different from the
cookie-dough model of
the atom.
Rutherford’s Gold Foil Experiment
Conducted an experiment
to see if alpha particles
would be deflected as
they passed through a
thin foil of gold.
Rutherford was aware of
Thomson’s “plum pudding”
model and expected only
minor deflections of
alpha particles.
Rutherford’s Model
• To explain the results of the experiment,
Rutherford’s team proposed a new model
of the atom.
• Because most of
the particles
passed through
the foil, they
concluded that the
atom is nearly all
empty space.
Rutherford’s Model
He suggested that
the positive charge
was all in the central
nucleus. With this
holding the electrons
in place by electrical
attraction
However, this was not the end of the story…
HISTORY OF THE ATOM
1913
Niels Bohr
Studied under Rutherford at the Victoria
University in Manchester.
Bohr refined Rutherford's idea by
adding that the electrons were in
orbits. Rather like planets orbiting the
sun. With each orbit only able to
contain a set number of electrons.
Bohr’s
Atom
nucleus
electrons in orbits
ATOMIC STRUCTURE
Particle
Charge
Mass
proton
+ charge
1
neutron
No charge
1
electron
- charge
0
Atomic Numbers
• The atomic number of an element is the
number of protons in the nucleus of an atom
of that element.
• It is the number of
protons that
determines the
identity of an
element, as well as
many of its chemical
and physical
properties.
Atomic Numbers
• Because atoms have no overall electrical
charge, an atom must have as many electrons
as there are protons in its nucleus.
• Therefore, the
atomic number of an
element also tells the
number of electrons
in a neutral atom of
that element.
Masses
• The mass of a neutron is almost the same as
the mass of a proton.
• The sum of the protons and neutrons in the
nucleus is the mass number of that
particular atom.
Masses
• Isotopes of an element have different mass
numbers because they have different
numbers of neutrons, but they all have the
same atomic number.
Atomic Mass
• In order to have a simpler way of comparing
the masses of individual atoms, chemists
have devised a different unit of mass called
an atomic mass unit, which is given the
symbol u.
• An atom of the carbon-12 isotope contains
six protons and six neutrons and has a mass
number of 12.
Atomic Mass
• Chemists have defined the carbon-12 atom
as having a mass of 12 atomic mass units.
• Therefore, 1 u = 1/12 the mass of a carbon12 atom.
• 1 u is approximately the mass of a single
proton or neutron.
Information in the Periodic Table
• The number
at the bottom
of each box is
the average
atomic mass
of that
element.
• This number is the weighted average mass
of all the naturally occurring isotopes of
that element.
Atomic
Mass
p+
n0
e–
Ca
20
40
20
20
20
Ar
18
40
18
22
18
Br
35
80
35
45
35