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Chapter 3 Notes – Atomic Theory and Structure
Atoms

An ________ is the smallest unit of an _____________ that still has all the
_____________of that element.

Atoms are very, very ____________.

However, atoms are visible with the proper tools.

A ______________ _______________ ________________ can be used to visualize atoms.

The idea of atoms was first suggested by the Greek philosopher _________________ in the 4th
century B.C.

He stated that atoms ________________, _________________, fundamental units of matter.

Democritus did not do any _____________ to back up his ideas, so they never became a
_____________.

Dalton’s ideas were ignored for the next ____________ years for two reasons:
o
_________________, who was much more famous and influential, refuted the idea of
atoms.
o
Most scientists were preoccupied with the field of ________________.
Alchemy

The first chemists were ______________.

Alchemy began in ____________ and _____________ in 400 B.C.

The goal of alchemists was to

The alchemists did not accomplish this goal, but they did do many things that spurred the
development of science.
o
They performed the first
.
o
They discovered many
.
o
They designed the first
,
, and many types of
______________________.
o

They developed procedures like
and
.
Their experiments led to the development of two laws:
1. The law of
2. The law of
The Law of Conservation of Mass

Developed by
in 1792.

Lavoisier observed many chemical reactions occurring inside sealed containers.

He noticed that the mass of the
was always the same as the mass of the
.

This observation is now known as the
.

This law states that in any chemical reaction, mass is not ________ or ____________, only
________________.
The Law of Definite Proportions

Proposed by
in 1799.

Proust studied the composition of many

He found that compounds always contained the same
.
in the same
_______________________.

For example, water is always ________ oxygen and ________ hydrogen.

The led to the law of

This law states that compounds always contain the same elements in the same proportions.
.
Dalton’s Atomic Theory

In the 1800s, an English schoolteacher named

Unlike Democritus, Dalton performed

Based on his results, he proposed the first

Dalton’s atomic theory included the following key points:
1. All
also studied atoms.
in 1803.
are composed of
.
and
.
2. Atoms are
3. Atoms of the same element are
. Atoms of one element are
_______________ from any other element.
4. Atoms of different elements can
in small whole-number ratios to form
.
5. When chemical reactions occur, atoms are
,
, or
. But atoms of one element can never be changed to atoms of ________________
________________.

Most of Dalton’s theory is still accepted today.

But two important changes have been made.
1. One is that atoms of the same element are not always
2. The other is that atoms are not

Atoms can be divided into three subatomic particles:
1. ________________
2. _______________
3. ________________
Electrons

Can be abbreviated __________

__________________ charged.
.
.

Discovered by _______________________ in 1897 through a series of
___________________ _______ experiments.

Have a mass that is about ______________ of the smallest known atom.

The discovery of the electron allowed scientists to make two important assumptions:
1. Because the e- is so ____________, atoms must contain other particles to account for
the rest of their _______________
2. Because atoms are ______________, they must contain some sort of ______________
charge to balance out the _____________ electron.
Protons

Abbreviated ________.

______________ charged subatomic particle.

The discovery of the proton is not credited to any one person.

Have a mass that is about ___________ times greater than the electron.
Neutrons

Abbreviated ______

______________ subatomic particle.

Discovered by ____________ _______________ in 1932.

The mass of the neutron is about the same as the mass of the _____________.
Atomic Structure

Dalton imagined atoms as
.

Thomson imagined that atoms looked like a
: a solid sphere with
_______________ scattered throughout.

In 1911, Ernest Rutherford performed his famous
.

This experiment led to an entirely new view of the atom.

At this time, scientists knew that atoms contained both ___________ and ____________.
However, they did not know how these particles were _____________.

Rutherford’s work showed that atoms are mostly
with a very dense
at the center.

Rutherford realized that the
must be located in the

We now know that
are also located in the

He proposed that the
must float somewhere in the
.
.
around the nucleus.

Many scientists were reluctant to accept this model however, because it had one very obvious
flaw.

Since protons are
and electrons are
particles should

But according to Rutherford’s model, they do not.
, the oppositely charged

In 1913, a scientist named
proposed a new model of the
atom.

He suggested that electrons could only be found in certain places in the atom.

He called these places ___________, or ____________ ___________.

Bohr believed electrons orbit the _________ much like planets orbit the sun.

His model of the atom is also called the

In Bohr’s model, electrons filled the energy levels in an orderly manner.

_____ electrons fit into the first energy level.

_____ fit in the 2nd and 3rd.

The 4th and 5th could hold ________.
.
Atomic Number

Number of ____________ in an atom.

All atoms of an element have the same

Atomic numbers are used to
an element.

Can be found on the
, above the element’s symbol.

Ex: The atomic number of carbon is 6. There are 6 p+ in carbon.

Atoms are
.
. So they must contain the same number of _________________
as ______________. Ex: carbon also has 6 electrons.

Mass numbers can also be written:

You can determine the composition of an atom using its mass # and atomic #.
o
Atomic # = # protons
o
# of protons = # of electrons
o
Mass # = protons + neutrons
o
Neutrons = mass # - protons
Isotopes

Isotopes are atoms of the same element with different __________.

They have different masses because they have different numbers of

They also have different

Ex: Hydrogen has three isotopes:
o
Hydrogen 1 – protium
o
Hydrogen 2 – dueterium
o
Hydrogen 3 - tritium
.
.
Ions

Sometimes, atoms can gain or lose
.

When this happens, the atom is no longer
.

The atom becomes an

An ion has a
or

If the atom
electrons, it becomes a

If it

Ex: If a Lithium atom loses an electron, it becomes an ion with a charge of +1. This is written Li+1.

A positive ion is called a
.

A negative ion is called an
.

To determine the # of protons, neutrons, and electrons in an ion:
.
charge.
electrons, it becomes
o
p+ = atomic #
o
no = mass # - p+
o
e- = atomic # - charge of ion
ion.
.