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The
I.Early Models of the Atom
Atom
1.All matter is composed of
particles called atoms.
2.The atom is the smallest
particle of an element that
retains its identity in a
chemical reaction.
A. Democritus' Atomic Philosophy
1. The Greek philosopher
Democritus (460 B.C- 370 B.C)
was among the first to suggest the
existence of atoms.
Democritus believed
that atoms were
indivisible and
indestructible.
4. Chemical reactions take place when
atoms link up, separate, or are
rearranged with one another. Atoms
are not created or destroyed by
chemical changes.
B. Dalton’s Atomic Theory
1.John Dalton (1766-1844) was an
English chemist and schoolteacher.
His theory:
1.All elements are composed of tiny
particles called atoms.
2. Atoms of a given element
are identical.
3. Atoms of different
elements are different.
II. Sizing Up the Atom
The radii of most atoms fall within
the range of 5x10-11 meters to
2 x 10-10 meters.
Despite their small size, individual
atoms are observable with
instruments such as
scanning tunneling
microscopes.
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Structure of the Nuclear Atom
Electrons
A.Subatomic particles
1.In 1897, the English
physicist, J.J.Thomson
discovered the electron.
1.Atoms are now known to be divisible.
They can be broken down into even
smaller, more fundamental particles,
called subatomic particles.
2.Three kinds of subatomic particles
are electrons, protons, and neutrons.
3. Thomson worked with sealed glass
tubes with electrodes at both ends
called a cathode ray tube.
He concluded that the cathode ray is
a stream of negatively charged
particles.
2. Electrons are
negatively
charged
subatomic
particles.
4. The U.S. physicist, Robert A.
Millikan carried out experiments
to calculate the mass of the
electron. His calculations from
1916 are very similar to those
accepted today.
5. An electron carries exactly one
unit of negative charge.
Protons and Neutrons
1.Protons are positively
charged subatomic
particles. Each proton
has a mass about 1840
times that of an
electron.
2. In 1932, the English physicist
James Chadwick confirmed the
existence of the neutron.
Neutrons are subatomic particles with
no charge but with a mass nearly
equal to that of a proton.
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The Atomic Nucleus
Even though
protons and neutrons
are very small,
scientists believe
they are composed of
yet smaller particles
called quarks.
3.
1. In 1911, Ernest Rutherford
decided to test what was then the
current theory of atomic structure.
2. His classic experiment is
called the gold-foil
experiment. Based on this
experiment, he came up
with the theory that the
atom is mostly empty
space.
Rutheford aimed a beam of alpha particles at a
sheet of gold foil surrounded by a fluorescent
screen.
Alpha particles
gold foil
3. He concluded that all the positive
charge and most of the mass of the
atom are concentrated in a small
region that is positive.
He called this region the nucleus.
Alpha particles
He found that while most of the particles passed
through the foil, only a small number were
4. The nucleus is the tiny central
core of an atom and is composed of
protons and neutrons.
deflected and, to his surprise, some particles
bounced straight back.
5. The Rutherford model is known as
the nuclear atom. In the nuclear
atom, the protons and neutrons are
located in the nucleus. The
electrons are distributed around the
nucleus and occupy almost all the
volume of the atom.
In fact, if the atom was the size of
the football field, the nucleus would
be a marble on the 50-yard line!
The stands of the football field would
be the area where the electrons are!
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MODELS OF THE ATOM
J.J. Thomson developed the idea that
an atom was made up of positive
charges from protons. Spread
throughout these positive charges
Niels Bohr (1885-1962) did important
work that led to a new atomic model.
Bohr stated that electrons in atoms could
exist only at certain energy levels.
He believed electrons circled around
protons similar to the way planets travel
around the sun.
were negative charges from electrons.
Like plums in a pudding!
Today’s Model:
Bohr’s Atom
electrons in orbits
Today’s model of the atom describes the
locations of electrons in terms of
probabilities.
The probabilities are described by
charge-cloud models.
nucleus
In this model, an electron is
most likely to be found where
the dots are close together.
Distinguishing Among Atoms
A.Atomic Number
1.Atoms are composed of protons,
neutrons, and electrons. Protons
and neutrons make up the nucleus.
Electrons surround the nucleus.
2.Elements are different because
they contain different numbers of
protons.
3. The atomic number of an element
is the number of protons in the
nucleus of an atom.
Ex: all hydrogen atoms have one
proton, so the atomic number of
hydrogen is 1.
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The atomic number
identifies the element.
4.
5. Remember that atoms are
electrically neutral, so the
number of electrons equals
the number of protons.
B. Mass Number
1.Most of the mass of an atom is
concentrated in the nucleus and
depends on the number of protons
and neutrons.
2. The total number of protons and
neutrons is called the mass number.
Ex: a helium atom has 2 protons and
2 neutrons so its mass number is 4.
(2 + 2 = 4)
3. Number of neutrons=mass number
minus atomic number.
4. The composition of any atom can
be represented in shorthand notation
using atomic number and mass
number. See page 111 for the
notation for gold.
ATOMIC STRUCTURE
Au
197
79
Mass number (A)
Number of protons +
number of neutrons
We can also refer to atoms by using
the mass number and the name of
the element.
Ex: gold-197
Atomic number (Z)
Number of protons
in nucleus
The atomic number is the subscript. (under)
The mass number is the superscript. (above)
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C. Isotopes
1.Isotopes are atoms of the same
element that have the same
number of protons but different
numbers of neutrons.
3. Despite these differences,
isotopes are chemically alike because
they have the same numbers of
protons and electrons.
2. Because isotopes of an element
have different numbers of
neutrons, they also have
different mass numbers.
4. There are (3) known isotopes of
hydrogen. Each has 1 proton in its
nucleus. The most common isotope has
0 neutrons. It has a mass number of
1 and is called hydrogen-1.
The second isotope has one neutron
and a mass number of 2.
The third isotope has 2 neutrons and
a mass number of (3).
D. Atomic Mass
It is called hydrogen-3 or tritium.
It is hydrogen-2 or deuterium.
1.It is useful to compare the
relative masses of atoms using a
reference isotope as a standard.
The isotope chosen is carbon-12.
2.The atomic weight of an element is
the weighted average mass of the
atoms in a naturally occurring
sample of the element.
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The atomic mass unit (amu) is
defined as 1/12 of the mass
of a carbon-12 atom.
Isotope
Isotope
Notation
Atomic
Number
Protons electrons
Neutrons
Oxygen-16
Bromine-80
Uranium235
Copper-64
Atomic
Number
Atomic
Mass/
Mass
Number
Number
of
Protons
Number of
Neutrons
Number of
Electrons
Chemical
Symbol
10
12
15
15
Al
64
Chapter 7 Pre-Assessment
Please answer the following questions to the
best of your ability-This helps me prepare
for the upcoming topic!
1.What is a chemical compound?
2.What part of the atom takes part in
forming a chemical compound?
3.Define the term ion.
4.Name two types of chemical bonds.
5.Where are the metals on the periodic
table?
6.What do we call the elements in group 18?
(last group on the table)
7. Why do atoms combine to form
compounds?
8. How would you describe a chemical
formula?
9. Can you name the following compound?
NaCl
10. What is a molecule?
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