DETERMINE THE NUMBER
OF PROTONS, NEUTRONS
AND ELECTRONS IN AN
ATOM
Counting the Pieces
Atomic Number = number of
protons
n # of protons determines kind of
atom.
n the same as the number of
electrons in the neutral atom.
n
Counting the Pieces
n
Mass Number = the number of
protons + neutrons.
n
All the things with mass.
n
NOT on the periodic table
Isotopes
Atoms of the same element can
have different numbers of
neutrons.
n different mass numbers.
n called isotopes.
n
Writing Isotopes
n
n
n
Two ways:
Hyphen notation
–Name of element – mass number
Ex:
–carbon- 12
–carbon -14
–uranium-235
Nuclear Symbol Notation
n
Contain the symbol of the element, the
mass number and the atomic number.
Mass
number
Atomic
number
X
Symbols
n
Find the
– number of protons
– number of neutrons
– number of electrons
– Atomic number
– Mass Number
– Name
24
11
Na
Symbols
n
Find the
– number of protons = 11
– number of neutrons = 13
– number of electrons = 11
– Atomic number = 11
– Mass Number = 24
– Name = sodium – 24
24
11
Na
Symbols
n
Find the
–number of protons
–number of neutrons
–number of electrons
–Atomic number
–Mass Number
– Name
80
35
Br
Symbols
n
Find the
–number of protons = 35
–number of neutrons = 45
80
–number of electrons = 35
35
–Atomic number = 35
–Mass Number = 80
– Name = bromine - 80
Br
Symbols
n
if an element has an atomic
number of 34 and a mass number
of 78 what is the
–number of protons
–number of neutrons
–number of electrons
–symbol
– Name
ANSWERS
n
if an element has an atomic
number of 34 and a mass
number of 78 what is the
–number of protons = 34
–number of neutrons = 44
–number of electrons = 34
– Name = selenium – 78
78
34
Se
Symbols
n
if an element has 91 protons and
140 neutrons what is the
–Atomic number
–Mass number
–number of electrons
– Name
– symbol
ANSWERS
n
if an element has 91
protons and 140 neutrons
what is the
–Atomic number = 91
–Mass number = 140
–number of electrons = 91
– Name = Protactinium - 140
140
91
Pa
Average Atomic Mass
n
n
n
A weighted average of all naturally occuring
isotopes of an element.
There is a formula used to calculate average
atomic mass:
= (mass of isotope 1) (% abundance/100)
(mass of isotope 2) (%abundance/100)
+...________________________________
Unit for Atomic Mass
n
n
n
The atomic mass unit (amu)
amu definition - one twelfth the
mass of a carbon-12 atom.
Each isotope has its own atomic
mass relative to the carbon-12
atom
Atomic Mass
n
You try…
n
Calculate the atomic mass of copper if
copper has two isotopes. 69.1% has a mass
of 62.93 amu and 30.9% has a mass of
64.93 amu.
Answer
(62.93 amu) (69.1% / 100) = 43.48 amu
+(64.93 amu) (30.9% / 100) = 20.06 amu
63.54 amu
Atomic Mass
n
Magnesium has three isotopes. 78.99%
magnesium-24 with a mass of 23.9850
amu, 10.00% magnesium-25 with a mass of
24.9858 amu, and 11.01% magnesium-25
with a mass of 25.9826 amu. What is the
atomic mass of magnesium?
Answer
(23.9850 amu) (78.99% / 100) = 18.9457
(24.9858 amu) (10.00% / 100) = 2.49858
(25.9826 amu) (11.01% / 100) = 2.86068
24.304 amu
Atomic Mass
n
n
Is not a whole number because it is an
average.
are the decimal numbers on the periodic
table.