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The Periodic Table
Chapter 4, Section 2
• Elements cannot be broken down chemically in to
different kinds of matter.
• Each element has its own unique symbol
– Sometimes the symbol is a single capitalized letter
– If the symbol consists of more than one letter, the first letter
is capitalized and the other letters are lower case.
– Usually the symbol is the same as the first letter of the
• If it is not, it may be the first letter of the Latin word for the same
element. Example – Iron’s symbol is Fe…short for Ferrium
• If the single letter has already been used, then the second letter is
added. Example: Helium – H was used for Hydrogen so Helium’s
symbol is He
• You must memorize the symbol for
elements 1-20 and the following
Atomic Number
• Atomic Number: the number of
protons in an element
• The atomic number is the smaller
whole number found next to the
• The atomic number never changes –
atoms are differentiated by how many
protons they have
• Each element has a different atomic
Atomic Mass
• The atomic mass is the sum of the
number of protons and neutrons in
an element
• The atomic mass is the larger
number that is NOT a whole
• Question: Why aren’t the
electrons included in the mass
Calculating the # of Neutrons
1. Take the atomic mass and round it to the
nearest whole number
2. Subtract the atomic number from this
• Example: Carbon
12.01 rounds to 12
12 – 6 = 6 neutrons
• Isotopes contain a different number of
neutrons than the normal atom.
• This will change the atomic mass but the
number of protons and electrons remains
the same. Element
New Mass
Example: C-14
14 is the new mass…14 – 6 = 8 neutrons
Atomic Mass Explained
• Isotopes explain why the atomic
mass is NOT a whole number…
% of Chlorine
• The atomic mass is often
referred to as the “Average
Atomic Mass.”
Cl 37
• It is an average of all of the
isotopes that exist.
• Chlorine’s Avg. Atomic Mass is
35.453 – There must be more
Cl-35 than Cl-37
Cl 35
Bohr Models
The Bohr Model is the
easiest way to draw an atom
(the solar system model).
1. Draw a nucleus
2. Determine the number of
protons and neutrons found
in the nucleus and write this
number in the nucleus
3. Determine how many
electrons there are and
place each one in its
appropriate energy level
Let’s Practice Making Bohr Models
See the worksheet! 
Valence Electrons
• Valence electrons are the electrons found
in the outermost energy level.
• Example: Carbon has 4 valence electrons
The Periodic Table
• The periodic table groups similar elements
together. This makes it easier to predict
properties of an element based on where it is in
the periodic table.
• This is called the periodic law.
Periodic Table Organization
• Period: horizontal rows
– Tells you the number of energy levels found in an
• Group/Family: vertical columns
– Elements within a group have similar properties and
contain the same number of valence electrons
– There are 18 groups
– Groups 3-12: transition metals
– Group 18: inert or noble gases
– Group 17: Halogens
– Group 1: Hydrogen and the Alkali Metals