Warm-Up
 What
is the difference between atomic
mass and atomic number?
 Can there be an atom with atomic mass
that has a decimal? Explain?
Counting Atomic Particles
5





Main Ways to Represent Atoms
Atomic Drawing
Using Periodic table
Using Specific Isotope
Nuclear Symbol
Hyphen Notation
Counting Subatomic Particles
Atom
Nucleus
Electron
Orbits
Periodic Table
74
Atomic Number
W
Atomic Symbol
Tungsten
Elemental Name
183.85
Atomic Mass
Using Specific Isotope
74
W
Tungsten
184
Mass Number
Nuclear Symbol
Mass #
Atomic #
W
184
74
W
Hyphen Notation
 Name

of Element – mass #
Ex. Tungsten -184 or W-184
Atomic Number
 Atomic
NumberAtomic #: Number given to
the elements according to properties
 Number
of protons determines the
identity of an element
 Number of electrons a neutral atom has
Ions

Ion: an atom with a charge due to the loss or gain
of electrons.
 If a neutral atom loses electrons, it becomes a
positive ion because there are more protons than
electrons.
 If a neutral atom gains electrons, it becomes a
negative ion because there are more electrons
than protons.

Ex: If S gains 2 electrons, it has __ (+)’s and
__ (-)’s for an overall charge of ___. This is
represented like this
Mass Number
 Mass
#: the sum of protons and neutrons
 Atomic Mass: average mass of all known
isotopes
 Isotopes: atoms that are chemically alike
but differ in mass; contain different number
of neutrons.

Ex: U-235 vs U-238
Isotopes

To find the atomic mass for any element, you
must first know 3 things about the element:




The number of stable isotopes of the element
Mass of each isotope
Natural abundance/% of each isotope.
Ex: If H has 3 isotopes, the first with a mass of
1.0078amu & 99.985% abundance, the second
with a mass of 2.0141amu & .15% abundance,
and the third with a mass of 1.0076amu & .302%
abundance, what is the atomic mass of H?
 **
Remember, the weight on the periodic
table for each element represents the
average weight for all of the naturally
occurring isotopes of that element
Few Examples
Atomic # vs. Atomic Mass

Mass #- protons and
neutrons of a specific
isotope
12
6
13
6
14
6
p
n
e
C
6
6
6
C
6
7
6
C
6
8
6
Atomic Mass

Atomic Mass- the weighted average of all
masses of all isotopes of a specific element
 Units- atomic mass unit (amu)
Isotope
Abundance
Mass
Boron-10
19.78%
1.978 amu
Boron-11
80.22%
8.8242 amu
Steps for Calculating Atomic Mass
1.
2.
3.
Convert the % abundance to a decimal.
Multiply the respective decimal by the mass of
the isotope.
Add using sig. fig.
Isotope
Abundance
Mass
Chlorine-35
75.53%
26.4355 amu
Chlorine-37
24.97%
9.2389 amu
Practice
Isotope
Silicon-28
Abundance
92.21%
Silicon-29
4.70%
Silicon-30
3.09%
Mass
More Practice
Isotope
Abundance
Lead - 122
1.37 %
Lead- 124
26.26%
Lead- 125
20.82%
Lead-126
51.55%
Mass