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
The Development of Atomic Theory
› Pg. 113-118

Matter is made up of particles called
atoms.

In the fourth century BCE, the Greek
philosopher Democritus suggested that
the universe was made of indivisible
units.
› He called these units atoms.
 Comes from atomos that means, “unable to
be cut or divided.”
Dalton’s
Atomic Theory
› According to Dalton, all atoms of a given
element were exactly alike, and atoms of
different elements could join to form
compounds.

Dalton used experimental evidence.

Law of definite proportions
› A chemical compound always contains the
same elements in exactly the same
proportions by weight or mass.

Dalton’s theory did not fit all
observations.
› It is considered the foundation for modern
atomic theory.

Like many scientific theories, the atomic
theory changed gradually over many
years as scientists continued to do
experiments and acquire more
information.

Thomson was experimenting with
electricity. He wasn’t planning on
learning about the atom.

He was studying cathode rays,
mysterious rays in vacuum tubes.

Thomson’s cathode ray tube experiment
suggested that cathode rays were made
of negatively charged particles that
came from inside atoms.

Thomson developed the plum-pudding
model.
› A cathode has a negative charge.
› An anode has a positive charge.

Thomson discovered electrons
› Negatively charged particles inside the
atom.
Look at Figure 3, Pg. 115
Thomson’s model, often called the plumpudding model, was named after a dessert
that was popular in his day.

Found that Thomson’s model needed to
be revised.

Rutherford proposed that most of the
mass of the atom was concentrated at
the atom’s center.

Rutherford conducted the gold-foil
experiment.

Rutherford discovered the nucleus.
› His experiment suggested that an atom’s
positive charge is concentrated in the
center of the atom.
 This positively charged, dense core of the
atom is called the nucleus.
› The nucleus contains particles called protons
and neutrons.
 Protons have a positive charge.
 Neutrons have NO charge (neutral)

The Structure of Atoms
› Pg. 119-127

The three main subatomic particles are
distinguished by mass, charge, and
location in the atom.

At the center of the atom is a small,
dense nucleus.
› It is made of protons and neutrons
 Protons have a positive charge
 Neutrons have NO charge.
 Electrons are negatively charged
Hydrogen has 1 proton
 Helium has 2 protons
 Lithium has 3 protons


Even though the protons and electrons in
atoms have electric charges, most
atoms do not have an overall charge.

If an atom gains or loses electrons, it
becomes charged.
› A charged atom is called an ion.

Positive and negative charges attract
each other with a force known as the
electric force.
› Protons and electrons are attracted to each
other.

Atoms of each element have the same
number of protons, but they can have
different numbers of neutrons.

The atomic number equals the number
of protons.
› IF the element is neutral, the atomic # also
tells the # of electrons in the atom.
› The atomic number for an element NEVER
changes!

The mass number equals the total
number of subatomic particles in the
nucleus.
› Fluorine has 9 protons & 10 neutrons, so A=19
for fluorine.
Atoms of an element can have different mass
numbers because the number of neutrons
can vary.

An isotope is an atom that has the same
number of protons but a different
number of neutrons relative to other
atoms of the same element.

Isotopes of an element vary in mass
because their number of neutrons differ.

Read on page 122, the second
paragraph.
.

Hydrogen has 3 isotopes:
› Protium, A=1 (most common)
› Deuterium, A=2 (2nd most common)
› Tritium, A=3 (Least common)
Unstable isotopes are known as radioisotopes.
 They emit radiation and decay into other
isotopes.

Subtract the atomic number from the
mass number.
› (atomic number is always the same, # of
protons. Mass number changes, # protons
and neutrons)

Because working with such tiny masses is
difficult, atomic masses are usually
expressed in unified atomic mass units.

A unified atomic mass unit (u) is equal to
one-twelfth of the mass of a carbon-12
atom.
› Also known as atomic mass unit (amu)

The average atomic mass for an
element is a weighted average.

A mole (mol) is a collection of a very
large number of particles.

A mole is written as 6.023 x 1023 and is
called Avogadro’s number.

The mass in grams of one mole of a
substance is called molar mass.

A mole of an element usually contains
several isotopes.

Look at page 126, Figure 8 and Math
Skills problem.

To find the molar mass of a compound,
you can add up the molar masses of all
of the atoms in a molecule of the
compound.

Example: To find the molar mass of
water, H20, the first thing you do is find
the mass of hydrogen and oxygen on
the periodic table.
› Look on page. 127.

Modern Atomic Theory

In the modern atomic model, electrons
can be found only in certain energy
levels, not between levels. Furthermore,
the location of electrons cannot be
predicted precisely.

Electrons can be only certain energy
levels.

They must gain energy to move to a
higher energy lever or must lose energy
to move to a lower energy level.

A new model, which no longer assumed
that electrons orbited the nucleus along
definite paths in the same way that
planets orbit the sun, was proposed.

One way to show visually the likelihood
of finding an electron in a given location
is by shading.
› The darker the shading, the better the
chance of finding an electron at that
location.
 The shaded region is the orbital.

The number of energy levels that are
filled in an atom depends on the number
of electrons.

The electrons in the outer energy level of
an atom are called valence electrons.
› This determines the chemical properties of
an atom.

There are four types of orbitals.
› s, p, d, f

S orbital is the simplest kind.
› Shaped like a sphere
› Lowest energy
› Can hold 2 electrons!! (ONLY 2!)

P orbital
› Shaped like a dumbbell
› Can be oriented in space in 3 ways.
› Each orbital can hold 2 electrons,
 The three p orbitals can hold a TOTAL of 6
electrons.

D orbital
› Five possible orbitals
 Can hold a TOTAL of 10 electrons (2 in each
orbital)

F orbital
-Seven possible orbitals
--Can hold a TOTAL of 14 electrons

s = 1 orbital

p = 3 orbitals

d = 5 orbitals

f = 7 orbitals

Energy level 1 = 1 orbital, 2 electrons

Energy level 2 = 4 orbitals, 8 electrons

Energy level 3 = 9 orbitals, 18 electrons

Energy level 4 = 16 orbitals, 32 electrons
Look at Figure 5 on Page 131.

Electrons jump between energy levels
when an atom gains or loses energy.

The lowest state of energy in an electron
is called the ground state.

If an electron gains energy, it is in the
excited state.
› Gains energy by absorbing a particle of
light, a photon.

The energy of a photon is related to the
wavelength of the light.
› High energy photons have short
wavelengths.
› Low energy photons have long wavelengths.
The wavelengths emitted depend on the
particular element.
--different for each