Distinguishing Among Atoms
4.3
Standards/Objectives
Students Will Be Able To:
 1a: Relate the position of an element in the
periodic table to its atomic number and
atomic mass
 Calculate the # of electrons, protons,
neutrons, and atomic mass of atoms
Atoms
 Elements have different # of protons or electrons
 Elements are NEUTRAL
Everyone think, what do I mean by that?
I will call on someone to answer
# protons (+) = # electrons (-)
Positive charge of protons = Negative charge of
electron
Atomic Number
 Top # on periodic table
 Atomic # = # of protons
 Since # protons = # electrons
 Therefore atomic # also equals # electrons
Atomic Number
 Everyone think of the following questions
(don’t answer yet):
 What is lithium’s atomic number? 3
 How many protons does lithium have? 3
 How many electrons does lithium have? 3
Mass Number
 Bottom # on periodic table rounded to WHOLE #
 Average atomic mass rounded to whole #
Mass # = 6.941 ~ 7
 Where is most of the MASS in an atom?
Nucleus
Neutrons (no charge) and protons (+ charge)
 Mass # = # neutrons + # protons
# Neutrons
 # neutrons = mass #
(neutrons + protons)
-
atomic #
(protons)
OR
(electrons)
Example
 What is the atomic # of lithium?
 What is the mass # of lithium?
 How many protons, electrons, and neutrons does
lithium have?
 Protons = 3 (atomic #)
 Electrons = 3 (atomic #)
 Neutrons
# neutron = mass # - atomic #
Mass # = Bottom # or (average atomic mass)
rounded to whole #
6.941 ~ 7
# neutron = 7 - 3 = 4
Now You Try
Find the atomic #, mass #, # protons, electrons, and
neutrons
1. Beryllium
Beryllium = Be
2. Neon
1. Atomic # = 4
3. Sodium
2. Mass # = 9.0122 ~ 9
3. # protons = 4
4. # electrons = 4
5. # neutrons = 9 – 4 = 5
Now You Try
Find the atomic #, mass #, # protons, electrons, and
neutrons
1. Beryllium
Neon = Ne
2. Neon
1. Atomic # = 10
3. Sodium
2. Mass # = 20.179 ~ 20
3. # protons = 10
4. # electrons = 10
5. # neutrons = 20 -10 = 10
Now You Try
Find the atomic #, mass #, # protons, electrons, and
neutrons
1. Beryllium
Sodium = Na
2. Neon
1. Atomic # = 11
3. Sodium
2. Mass # = 22.990 ~ 23
3. # protons = 11
4. # electrons = 11
5. # neutrons = 23 -11 = 12
Shorthand Ways of Writing
mass #
atomic #
108Ag
47
 # protons = 47
 # electrons = 47
 # neutrons = 108 - 47 = 61
Shorthand Ways of Writing
Carbon - 12
element name
mass #
(bottom #) rounded to whole #
OR (average atomic mass) rounded
to whole #)
Isotopes
 Isotopes = Atoms with DIFFERENT # of neutrons
 Different mass # = # proton + # neutron
 Same # electrons and protons
Mass # = 10p + 10n
= 20
Neon-20
20
10
Ne
Mass # = 10p + 11n
= 20
Neon-21
21
10
Ne
Mass # = 10p + 12n
= 20
Neon-22
22
10
Ne
Now You Try
 Find the following:
a) Atomic #
b) Mass #
c) # protons
d) # electrons
e) # neutrons
1)
32S
16
2) Helium-4
3) Manganese
4) 16O
8
5) Copper
6) Chlorine-37
Atomic Mass Unit
 Atomic mass unit (amu)
 Used as a standard for masses of atoms
 Carbon-12 is exactly 12 atomic mass units (amu)
 1 amu = one-twelfth or (1/12) mass of carbon-12
 Helium-4 is 1/3 of the mass of carbon-12
 Nickel-60 is about 5 times larger than the mass of
carbon-12
Average Atomic Mass
 Most atoms have
many isotopes
 Average atomic mass
is the weighted mass
of the different
isotopes
 Reason why average
atomic mass is a
decimal #
Calculating Atomic Mass
 ExampleMultiply mass of each isotope by
its natural abundance (decimal) AND add
the products
 : Calculate the atomic mass of carbon.
Isotope
Carbon-12
Carbon-13
Natural
Abundance
98.89%
1.11%
Atomic mass
12.000 amu
13.003 amu
Isotope
Carbon-12
Carbon-13
Natural Abundance
98.89%
1.11%
Atomic
mass
12.000 amu
13.003 amu
 Atomic mass carbon = (atomic mass carbon-12) + (atomic mass carbon-13)
 Atomic mass carbon = 11.8668 + 0.1443333 = 12.0111333
~ 12.011 amu
Now You Try
Calculate the atomic mass for the following.
1.
Chlorine
1.
Chlorine-35
 75.77%
 34.969 amu
2.
Chlorine-37
 24.23%
 36.966 amu
2.
Oxygen
1.
Oxygen-16
 99.759%
 15.995 amu
2.
Oxygen-17
 0.037%
 16.995 amu
3.
Oxygen-18
 0.204%
 17.999 amu
1. Chlorine = 35.453 amu
2. Oxygen = 15.999 amu