THE ATOM
Four important pieces of information…
Atomic Number
• Number of protons in an element
• Defines the element
Chemical Symbol
• 1 or 2 letter symbol
• 1st letter is always capitalized
• If there is a second letter is it
lowercase
Element Name
Atomic Mass - More on this later
Atomic Number
The atomic number of an element = the number of protons
• The number of protons is what defines an element!
IF the element is neutrally charged (not an ion) then…
the number of protons = the number of electrons
Mass Number
Mass number = number of protons + number of neutrons
This is NOT the number shown on the periodic table, the
periodic table shows the atomic mass – more on this later.
Determining an Atom’s Composition
If you know the atomic number and the mass number of an atom
then you know how many protons, neutrons, & electrons it has.
Atomic Number = Number of protons & electrons
Mass Number = Protons + Neutrons
SO… Neutrons = Mass Number – Atomic Number
Practice – Complete the chart below
Element
Helium
Titanium
Sodium
Krypton
Protons
Electrons
Neutrons
Practice – Complete the chart below
Element
Protons
Electrons
Neutrons
2
2
2
Titanium
2
2
26
Sodium
11
11
12
Krypton
36
36
42
Helium
Isotopes
• For many elements there are multiple types of that element,
•
•
•
•
called isotopes.
Isotopes have the same number of protons, but different
numbers of neutrons.
Because they have different numbers of neutrons their
mass numbers are different.
Isotopes have the same chemical properties because they
have the same number of protons and electrons
Isotopes can (but don’t always) have different physical
properties because of their difference in neutrons.
Isotopes are identified by the element name,
followed by a dash with the mass number of that isotope.
For example: Carbon-12 and Carbon-13 are two isotopes of
Carbon.
• Because they are still Carbon, they both have 6 protons (where
did I find this?)
• Carbon - 12 has a mass number of 12
• Carbon - 13 has a mass number of 13
Since the Number of neutrons = Mass Number – Atomic Number
Carbon – 12 has 6 Neutrons (12 – 6)
Carbon – 13 has 7 Neutrons (13 - 6 )
HW due Wednesday
Side 1 (chart) of worksheet
Atomic Mass
• The mass that is found on your periodic table is the
atomic mass.
• Atomic mass is the average of the masses of an
element’s different isotopes adjusted for how frequently
each one is found in nature.
So what does this mean? Let’s look at an example…
Atomic Mass Example 1
Carbon-12 is found
in 99% of all
Carbon atoms while
Carbon -13 is found
in only 1%
So the atomic mass
of carbon is 99% of
the Carbon-12
mass + 1% of the
Carbon-13 mass
Atomic Mass Example 2
What is average atomic mass of Lithium if 7.42% exists
as Lithium–6 and 92.58% exists as Lithium–7?
Average Atomic Mass = (6 * .0742) + (7 * .9258) = 6.9258
round to the thousands
= 6.926
For these problems the mass will not be the exact same as
the mass listed on the periodic table, other isotopes have
been left out or percentages may be slightly changed…
otherwise you could just look at your periodic table for the
answers 
Atomic Mass Example 3
Neon has two major isotopes, Neon-20 and Neon-22.
Out of every 250 neon atoms, 225 will be Neon-20 and 25
will be Neon-22.
What is the average atomic mass of Neon?
= (20 * (225/250)) + (22 * (25/250)) OR (20*.9) + (22 *.1)
Atomic Mass Example 4
Without doing any math, are there more Bromine-79 atoms
or more Bromine-80 atoms on earth? (Hint: look at the
periodic table)
Answer:
Bromine on the periodic table has a atomic mass of 79.8,
because the number is closer to 80 then it is to 79 we know
that there is a greater percentage of Bromine-80 on Earth.
Atomic Mass Example 5
Magnesium has three naturally occurring isotopes.
78.70% of Magnesium atoms exist as Magnesium-24
10.03% exist as Magnesium-25
11.17% exist as Magnesium-26
What is the average atomic mass of Magnesium?
= (24*.787)+(25*.1003)+(26*.1117) = 24.2997 or 24.300