ATOMS
&
Atomic
Theory
DEFINING THE ATOM
An
atom is the smallest particle
of an element that retains its
identity in a reaction.
The
basic building blocks of
matter that make-up everyday
objects.
ATOMIC THEORY

Democritus was an early Greek Scholar.

He was the first to suggest the existence of atoms

Democritus believed that atoms were indivisible and
indestructible.


He never developed a theory because he did not have
experimental support nor did he explain chemical
behavior.
It took 2000 years after Democritus for the real
nature of atoms and events at the atomic level to be
established
DALTON’S ATOMIC THEORY
Using experimental methods, Dalton transformed
Democritus’s ideas on atoms into a scientific
theory
 1. All matter is composed of tiny 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.
 3. Atoms from different elements can physically mix
together or can chemically combine in simple wholenumber ratios to form compounds.
 4. Chemical reactions occur when atoms are
separated, joined, or rearranged. Atoms of one
element, however, are never changed into atoms of
another element as a result of a chemical reaction.
DALTON’S THEORY REVISED
Most
of Dalton’s theory is still
accepted today EXCEPT that
atoms are known to be divisible.
Atoms
can be broken down into
3 subatomic particles: electrons,
protons and neutrons.
J.J. THOMSON
 Used
a cathode ray tube to prove the
smallest particles present must have
a negative charge.
 He discovered the ELECTRON!
J.J. THOMSON
Atoms were
electrically neutral, so
there must be a +
particle to cancel out
the – charge from the
electron.
 Developed the Plum
Pudding Model
(positive ball
containing scattered
electrons)

RUTHERFORD’S GOLD-FOIL
EXPERIMENT



Former student of
Thomson, disproved the
Plum Pudding Model of
the atom.
The Gold Foil Experiment:
Sent a beam of + charges
(alpha particles) through a
piece of very thin gold foil.
Angles of deflection were
measured.
RUTHERFORD
 Results:
Most of the alpha particles
passed straight through, most of the
foil must be regions of “empty” space
– not a + sphere like Thomson
believed.
 + charges and the atoms mass must
be found in the center 
discovered the nucleus
RUTHERFORD’S ATOMIC MODEL


In Rutherford’s atomic
model, the protons and
neutrons are located in
the nucleus.
The electrons are
distributed around the
nucleus and occupy
almost all the volume
of the atoms.
ATOMIC STRUCTURE
Particle
Charge
Mass
proton
+ ve charge
1
neutron
No charge
1
electron
-ve charge
nil
ATOMIC STRUCTURE
He
2
4
Atomic number
the number of protons in an atom
Atomic mass
the number of protons and
neutrons in an atom
number of electrons = number of protons
HELIUM ATOM
Shell
proton
+
-
N
N
+
-
electron
Are atoms electrically neutral? Why?
neutron
COMPLETE THE FOLLOWING TABLE IN
YOUR NOTES
Atomic Mass # # of
# of
# of
#
Protons Neutron Electro
s
ns
9
10
14
47
55
15
22
25
ATOMIC NUMBER
Elements are different because they contain different
numbers of protons
Atomic Number: The number of protons in the
nucleus of an atom of that element. This is used to
identify an element.
Example: Carbon’s atomic number is 6 because there
are 6 neutrons in each Carbon atom’s nucleus
For each element the number of protons equals the
number of electrons. Atoms are electronically neutral,
so the negative charge must equal the positive charge.
MASS NUMBER
The total number of protons and neutrons in an atom
If you know the atomic number and mass number of an atom
of any element, you can determine the atom’s composition.
The number of neutrons in an atom is the difference between
the mass number and atomic number.
Number of neutrons= mass # - atomic #
The composition of any atom can be represented in shorthand
notation using atomic number and mass number
ISOTOPES
Atoms that have the same number of protons but
different numbers of neutrons
ISOTOPES


Because isotopes have a different number of neutrons, they also
have different mass numbers.
Chemically alike because they have identical numbers of protons
and electrons, which are responsible for chemical behavior.
Ex: Three known isotopes for H
H: Hydrogen (no neutrons, mass # of 1)
 H-2: Deuterium (one neutron, mass # of 2)
 H-3: Tritium (two neutrons, mass # of 3)

CALCULATING AVERAGE ATOMIC
MASS OF ISOTOPES



In nature, isotopes occur in various percentages.
The higher the percent the more abundant.
In order to figure out the average mass of each
element the percent abundance and mass of each
isotope need to be considered
We can calculate average atomic mass in much
the same way as we calculate your grade in this
class…
CALCULATING AVERAGE ATOMIC
MASS
1.
Divide the percent abundances by 100. (natural
occurrence)
2.
Multiply each isotope mass by its natural
occurrence. (maintain sigfigs)*
3.
Add up all the masses (maintain place values)
4.
Include a unit (amu)
WHAT ARE THE DIFFERENT
CATEGORIES THAT YOU ARE GRADED ON
IN THIS CLASS?
 Classwork:
79pts
 Practice: 12pts
 Final: 14pts
 What would your semester grade be if you
received an 81% for classwork, 52% for
practice, and 73% on your final?

0.81 x 79 = 64
0.52 x 12 = 6.2
0.73 x 14 = 10
Add all answers together to get % semester grade

63.2 + 3.5 + 10.1 = 80.2 80 % (a B)



NOW LETS TRY WITH AN ELEMENT!

Copper has two isotopes: copper-63 and copper65. The relative abundances of these isotopes are
69.2% and 30.8% respectively. Calculate the
average atomic mass of copper.
0.692 x 63 = 43.6
0.308 x 65 = 20.0
43.6 + 20.0 = 63.6
ONE MORE EXAMPLE…

Uranium has three naturally occurring isotopes
with the following percent abundances: U-234
(0.0058%), U-235 (0.71%), and U-238 (99.23%).
What do you expect the average atomic mass to be
and why?
 What is the average atomic mass?


237.9
SUMMARY
1. The Atomic Number of an atom = number of
protons in the nucleus.
2. The Atomic Mass of an atom = number of
Protons + Neutrons in the nucleus.
3.
The number of Protons = Number of Electrons.
4.
Electrons orbit the nucleus in shells.
5.
Each shell can only carry a set number of electrons.
THE PERIODIC TABLE



A periodic table allows you to easily compare the
properties of one element (or a group of elements)
to another element (or group of elements)
Period- Each horizontal row of the periodic
table. The properties of the elements vary as you
move across it from element to element.
Group (or family)- Each vertical column of the
periodic table. Elements within a group have
similar chemical and physical properties.
THE PERIODIC TABLE