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Section 16.2
Masses of Atoms
Atomic Masses
Most of an atom’s mass is found in the
nucleus

atomic mass is measured in atomic mass units
(amu)
 Mass
of 1 proton = 1 amu
 Mass of 1 neutron = 1 amu
Atomic Number
The number of protons in an atom is called
the atomic number

The atomic number identifies the element
8 protons is oxygen
 6 protons is carbon
 16 protons is sulfur

Atomic Number Practice
Use your periodic table to find the atomic
number of the following elements:
Aluminum (Al)
 Titanium (Ti)
 Tin (Sn)
 Osmium (Os)
 Erbium (Er)


Al = 13
Ti = 22
Sn = 50
Os = 76
Er = 68
How many protons does each atom have?
Mass Number (Atomic Mass)
The mass number of an atom is the sum of
the number of protons and neutrons in the
nucleus.
Element Protons Neutrons Mass Number
Boron
5
6
5+6
11
Carbon
6
6
6+6
12
Oxygen
8
8
8+8
16
Sodium
11
12
11+12
23
Copper
29
34
29+34
63
Using the Atomic Number &
Mass Number (Atomic Mass)
If you know the atomic number and mass
number, you can determine the number of
neutrons
Mass Number
- Atomic Number
Number of Neutrons
Mass Number Practice
Determine the number of protons and
neutrons in the following atoms:
Lithium –
Atomic Number = 3
Mass Number = 7
 Argon –
Atomic Number = 18
Mass Number = 39
 Strontium – Atomic Number = 38
Mass Number = 88

Answers
Determine the number of protons and
neutrons in the following atoms:
Lithium –
Protons = 3
Neutrons = 7 – 3 = 4
 Argon –
Protons = 18
Neutrons = 39 – 18 = 21
 Strontium – Protons = 38
Neutrons = 88 – 38 = 50

Isotopes
Atoms of one type of element must have the
same number of protons but can have
different numbers of neutrons
Atoms with the same number of protons and
different numbers of neutrons are called
isotopes
Isotopes
Examples

Carbon-12 has a mass of 12 amu
 All
carbon atoms must have 6 protons
 Neutrons = 12 – 6 = 6
 C-12 has 6 protons and 6 neutrons

Carbon-14 has a mass of 14 amu
 All
carbon atoms must have 6 protons
 Neutrons = 14 – 6 = 8
 C-14 has 6 protons and 8 neutrons
Isotopes
Examples

Uranium-235 has a mass of 235 amu
 All
uranium atoms must have 92 protons
 Neutrons = 235 – 92 = 143
 U-235 has 92 protons and 143 neutrons

Uranium-238 has a mass of 238 amu
 All
uranium atoms must have 92 protons
 Neutrons = 238 – 92 = 146
 U-238 has 92 protons and 146 neutrons
Average Atomic Mass
Because atoms can have different numbers
of neutrons, the mass numbers can be
different
To get an overall mass of one element a
weighted-average of all the isotopes is
used and determines the average atomic
mass for the element
Neutral Atoms
In the nucleus, which particles have charge?

Protons
What is their charge?

Positive
What particle is opposite that charge?

Electrons
Neutral Atoms
To balance the charges in the atom, the
number of protons must equal the number
of electrons
Li has 3 protons, so the nucleus has a charge
of +3
 To balance that charge, there must be 3
electrons that add up to -3
 Overall, the charge is (+3) + (-3) = 0, so the
atom is neutral

Practice
Determine how many electrons the following
atoms have:
Magnesium, Protons = 12
 Beryllium, Protons = 4
 Nitrogen, Atomic Number = 7
 Bromine, Atomic Number = 35

12 Electrons
4 Electrons
7 Electrons
35 Electrons
Practice
Element
Symbol
Iodine
Mercury
Neon
I
Hg
Atomic
Mass
Protons
Number Number
Neutrons
Electrons
12
41
12
48
51
75
111
53
80
10
Mg
Rb
127
201
20
12
37
Practice
Element
Symbol
Iodine
Mercury
Neon
I
Hg
Ne
53
80
10
127
201
20
Magnesium
Niobium
Mg
Rb
Nb
12
37
41
Rhenium
Re
75
Rubidium
Atomic
Mass
Protons
Number Number
Neutrons
Electrons
53
80
10
74
121
10
53
80
10
24
85
92
12
37
41
12
48
51
12
37
41
186
75
111
75
Bohr Models of the Atoms
Each atom has energy levels/shells where
the electrons orbit
1st Shell: maximum of 2 electrons
 2nd Shell: maximum of 8 electrons
 3rd Shell: maximum of 8 electrons
 4th Shell: maximum of 32 electrons

Electrons fill the smallest/closest level/shell first
Drawing Bohr Models
Hydrogen
AN = 1
AM = 1
e- = 1
1 p+, 0 no
5 p+, 6 no
Boron
AN = 5
AM = 11
e- = 5
Drawing Bohr Models
Chlorine
AN = 17
AM = 35
e- = 17
17 p+, 18 no