MATTER UNIT
STRUCTURE OF AN ATOM
WHAT IS AN ATOM?
Atom: the
smallest unit of
matter that retains the identity
of the substance
THE EVOLUTION OF ATOMIC STRUCTURE
ATOM
•THE NUCLEUS IS THE SMALLEST PART OF
THE ATOM, BUT CONTAINS ALMOST ALL OF
THE MASS OF AN ATOM.
•THE MAJORITY OF THE ATOM IS
EMPTY SPACE.
 Atoms
ATOMIC STRUCTURE
are composed of 2 regions:
Nucleus: the center of the atom that contains the
mass of the atom
Electron cloud: region that surrounds the nucleus
that contains most of the space in the atom
Electron
Cloud
Nucleus
ATOMIC STRUCTURE
The nucleus contains 2 of the 3
subatomic particles:
Protons: positively charged
subatomic particles
Neutrons: neutrally charged
subatomic particles
The 3rd subatomic particle resides
outside of the nucleus in the
electron cloud
Electron: the subatomic particle
with a negative charge and
relatively no mass
Atomic Particles
Particle
Charge
Electron
-1
Mass
#
0
Proton
+1
0
Neutron
Location
symbol
Electron
cloud
e-
1
Nucleus
p+
1
Nucleus
n0
HOW DO THESE
PARTICLES INTERACT?
Protons
and neutrons are in the tiny
positively charged nucleus accounting for
most of the mass of the atom
The negatively charged electrons are
small and have a relatively small mass but
occupy the larger volume of space outside
the nucleus
HOW DO THE SUBATOMIC PARTICLES
BALANCE EACH OTHER?
In
an atom:
 The
# of protons = The # of electrons
If
20 protons are present in an atom then 20
electrons are there to balance the overall charge
of the atom—atoms are neutral
The
neutrons have no charge
they
do not have to equal the number of
protons or electrons
THE NUMBER OF
PROTONS IDENTIFIES
THE ATOM.
• AN ATOM WITH 6 PROTONS = CARBON
• AN ATOM WITH 8 PROTONS = OXYGEN
• AN ATOM WITH 20 PROTONS = CALCIUM
ATOMIC STRUCTURE
He
2
Atomic number
the number of
protons in an atom
Atomic
mass
4
the number of protons and
neutrons in an atom
# of electrons = # of protons
# of neutrons = Atomic mass – # of protons
HOW EXACTLY ARE THE
PARTICLES ARRANGED?

Bohr Model of the atom:
All of the
protons and the
neutrons
The 3rd ring can
hold up to 8 eThe 4th ring and any
after can hold up to
18 e-
The 1st ring can
hold up to 2 eThe 2nd ring can
hold up to 8 e-
ATOMIC STRUCTURE
Electrons are arranged in Energy Levels or
Shells around the nucleus of an atom.
•
First Shell
a maximum of 2 electrons
•
Second Shell
a maximum of 8 electrons
•
Third Shell
a maximum of 8 electrons
•
Fourth Shell
a maximum of 18 electrons
DETERMINING THE NUMBER
OF PROTONS AND NEUTRONS

Li has a mass number of 7 and an
atomic number of 3
 Protons
= 3 (same as atomic #)
 Neutrons= 7-3 = 4 (mass # - atomic #)

Ne has a mass number of 20 and an
atomic number of 10
 Protons
= 10
 Neutrons = 20 - 10= 10
WHAT ABOUT THE ELECTRONS?

The electrons are equal to the
number of protons
 So

e- = p = atomic #
Ex: He has a mass # of 4 and an
atomic # of 2
 p+
 no
=2
=2
 e- = 2
DETERMINE THE NUMBER OF
SUBATOMIC PARTICLES IN
THE FOLLOWING:

Cl has a mass # of 35 and an atomic #
of 17
 p+

= 17,
no = 18,
e- = 17
K has a mass # of 39 and an atomic #
of 19
 P+
= 19,
no = 20
e- = 19
WHAT DOES CARBON LOOK
LIKE?
Mass # = 12
atomic # = 6
6 p and 6 n live
in the nucleus
p+ = 6
no = 6
e- = 6
PRACTICE BOHR DIAGRAMS