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Chapter 4
Atomic Structure
Before the atom….

Many cultures believed that all things were
composed of the classical elements:
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Earth, Wind, Water, and Fire
Much of their understanding came from things
that they could see – they could not see atoms.
4.1 Atoms

The discovery and study of atoms (the
fundamental unit of all matter) has been done
through indirect observations, logic, and
scientific deduction.

These particles can only be viewed with the most
powerful telescopes.
4.1 Atoms

Atoms were first proposed to exist by a Greek
philosopher, Democritus, more than 2000 years
ago.
Democritus thought that atoms were invisible,
indestructible, fundamental units of matter.
 His ideas agreed with later scientific theories, but
lacked experimental support because scientific
experiments were unknown in his time.

4.1 Atoms
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2200 years after Democritus, John Dalton (17661844), proposed an atomic theory.

Dalton, who studied chemistry very differently than
Democritus, performed experiments to arrive at his
atomic theory.
4.1 Daltons Atomic Theory
1. All elements are composed of submicroscopic,
indivisible particles called atoms.
2. Atoms of the same element are identical.

The atoms of any one element are different from
those of any other element.
4.1 Dalton’s Atomic Theory
3. Atoms of different elements can physically mix
together or can chemically combine with one
another in simple whole-number ratios to form
compounds.
4. Chemical reactions occur when atoms are
separated, joined, or rearranged.

However, atoms of one element are never changed
into atoms of another element as a result of a
chemical reaction.
4.1 Atoms

atom – the smallest particle of an element that
retains the properties of that element.

Example: Pure copper “penny” would contain
approximately 2.4 x 1022 copper atoms – that is 3.5
trillion times the number of people on this planet.
4.1 Atoms

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Even though atoms are so small, we can see
images from them with the right technology.
Scanning Tunneling Microscopes (STMs) and
Atomic Force Microscopes (AFMs) can be used
to visualize individual atoms.
TEAM Microscope
TEAM Published Article
4.1 Concept Practice
1. Democritus and Dalton both proposed that
matter consists of atoms. Explain how their
approaches to reaching the same conclusion
differed?
A: Democritus approached the subject logically,
but without experimental evidence; Dalton
based his ideas on an analysis of data obtained
by experiments.
4.1 Concept Practice
2. Which of these statements would Dalton have agreed
with (use Dalton’s atomic theory)?
a. Atoms are the smallest particles of matter
b. The mass of an iron atom is different from the mass of
a copper atom
c. Every atom of silver is identical to every other atom of
silver
d. A compound is composed of atoms of two or more
different elements
A: Dalton would have agreed with all of them.
4.2 Electrons, Protons, Neutrons


Most of Dalton’s Atomic Theory is accepted
today – one important revision is that atoms are
not indivisible (they can be broken down)
Protons, neutrons, and electrons are three
subatomic particles that make up an atom.

There are dozens of subatomic particles, but we will
only be studying these three subatomic particles in
chemistry.
4.2 Electrons
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The electron is the smallest subatomic particle
with a mass 1/1840 the size of the proton.
An electron has a negative charge (exactly 1-).
English Physicist Sir J.J. Thompson discovered
the electron in 1897 using the cathode ray
tube. (see p. 86-87)
4.2 Protons

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The proton is a subatomic particle that has a
positive charge (exactly 1+).
A proton is a much more massive subatomic
particle, when compared to the electron, but it is
still very small.
E. Goldstein discovered the proton in 1886
using a cathode ray tube and canal rays (a beam
of protons.
4.2 Neutron

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The neutron is a subatomic particles that have
no charge (neutral).
Neutrons have a mass that is about the same as a
proton.
An English physicist, Sir James Chadwick,
confirmed the existence of the neutron in 1932.
4.2 Electrons, Protons, Neutrons

The fundamental building blocks of atoms are the
electron, the proton, and the neutron.
Table 4.1 Properties of Subatomic Particles
Particle
Symbol Relative
Electrical
Charge
Electron e1Proton
p+
1+
Neutron n0
0
*1 amu = 1.66x10-24 g
Approx.
Relative
Mass (amu*)
1/1840
1
1
Actual
Mass (g)
9.11x10-28
1.67x10-24
1.67x10-24
4.2 Simple Rules about Matter and
Electric Charges
1. Atoms have no electric charge – they are
electrically neutral.
2. Electric charges are properties of particles.

Electric charges are carried by particles of matter.
3. Electric charges exist in a single unit or
multiples of a single unit.

The are no fractions of charges.
4. Electric charges cancel when equal number of
positively charged particles are balanced by
negatively charged particles.
4.2 Concept Practice
3. Since all atoms have negatively charged electrons,
shouldn’t every sample of matter have a negative
charge? Explain.
A: No. The (-) charge is cancelled by the (+) charge.
4. What experimental evidence did Thompson have
for the following ideas?
a. Electrons have a negative charge.
b. Atoms of all elements contain electrons.
4.3 The Structure of the Nuclear Atom

At first (before the discovery of the neutron)
scientists thought that protons and neutrons
were evenly distributed throughout the atom.
4.3 The Structure of the Nuclear Atom
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In 1911, Ernest Rutherford and his co-workers
tested this theory of atomic structure by firing a
beam of alpha particles at a thin gold sheet.

From this experiment Rutherford proposed that
almost all of the mass and all of the positive charge
are concentrated at a small region at the center of an
atom – he called this region the nucleus.
4.3 The Structure of the Nuclear Atom
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nucleus – the central core of an atom,
composed of protons and neutrons.
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Almost all of mass in an atom is contained in a tiny
nucleus which is extremely dense – if it was the size
of a pea it would have a mass of 250 tons!
The nucleus has a positive charge and occupies a
very small volume of the atom – the rest of the
atom is more or less empty space in which the
negatively charged electrons are found.
4.3 Concept Practice
5. How did the results of Rutherford’s gold foil
experiment differ from his expectations?
A: He thought α particles would pass directly
through, however some were deflected.
6. What is the charge, positive or negative, on the
nucleus of every atom?
A: Positive