8th Grade
Physical Science
Packet #17.2
Name ___________________________________
Period _______________________________
Date
________
Today’s objective(s):
AS.02.b
Use the periodic table to obtain atomic number and mass number for an element, and
identify the number of protons, neutrons, and electrons
Do Now (3 minutes)
You will need your periodic table today
To the best of your ability, give the name of each part of an element’s entry in the Periodic
Table, and explain what it tells you about what an atom of cobalt (Co) is made up of.
A.
B.
C. (This is mostly new!)
Quick Notes
This is the atomic number. It tells you how many protons
one atom of cobalt has.
Why is the atomic number never going to be a decimal or a
fraction?
Minute Madness: Use your periodic table to find the missing information
Element Name
Oxygen (O)
Atomic Number
Tungsten (W)
21
55
This is the chemical symbol.
You should be pretty comfortable with chemical symbols
already. But there are a few important rules you may not have
noticed.
1. Chemical symbols can have ______ or ______ letters.
2. The first letter is always ________________.
3. The second letter (if there is one!) is always ______________ ____________.
4. When chemical symbols don’t make sense (ex. Au for gold, and Pb for lead), it’s usually
because they are based on the Latin name for an element (gold is aurum, lead is plumbum)
Minute Madness: Use your periodic table to find the missing information
1. Find two elements (other than gold and lead) whose chemical symbols don’t match their
English names.
2. Find two elements that you eat in your diet.
New Information
The last number you’ll find when you look up an element
in the periodic table is its atomic mass.
Yesterday, we learned how to calculate the mass number of an atom.
Review from yesterday:
This is NOT the same as the atomic mass, but the difference
can be tricky to understand at first.
 The atomic mass is the average mass an atom of an element.
 The mass number is the actual mass of one specific atom of an
element.
Mass number =
number of _________
+ number of ________
in an atom.
If you are confused, here are two ways of thinking about the difference between atomic mass
and mass number.
#1 The Penny Comparison
Imagine you find a bag of pennies. Some pennies are new and shiny.
Others are old and oxidized (chemically bonded to oxygen in the air
– kind of like rusting). Some are so old they are worn down.
Do you think every penny has exactly the same mass or weight?
Explain why or why not.
In this situation, mass number is like the mass of one individual penny. For example, a certain
2005 penny might have a mass of 2.5 grams, and a certain 1983 penny might have a mass of 2.1
grams.
In this situation, atomic mass is like the average mass of a penny in the bag. For example, if
you have 200 pennies, and the total mass is 440 grams, you would do the calculation.
440 𝑔𝑟𝑎𝑚𝑠
= 𝐴𝑛 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑜𝑓 ______𝑔𝑟𝑎𝑚𝑠/𝑝𝑒𝑛𝑛𝑦
200 𝑝𝑒𝑛𝑛𝑖𝑒𝑠
Challenge: Do you think there are more 2005 pennies or 1983 pennies in the bag?
Atomic mass vs. mass number – Part 2
We can also think about this difference by thinking about actual atoms.
Usually, a carbon atom has a mass number of 12. Scientists call
this type of carbon carbon-12 or C-12.
Quick check: Why is it called “carbon-12”? Where did the 12
come from?
But some carbon atoms have a mass number of 14. This is because they have extra protons /
neutrons (circle one, think carefully!) compared to carbon-12. This form of carbon is called
carbon-14.
Any carbon that you will find in real life is a mix of carbon-12 and carbon-14 (and some other
types of carbon as well!), just like any bag of pennies will have some heavier pennies and some
lighter pennies.
The atomic mass for an element is a real-life calculation of the average of all of the mass
numbers for that element. To find the atomic mass of carbon, you would average out the mass
numbers you find in a real life chunk of carbon.
What is the atomic mass for carbon? (Use your periodic table)
Do you think there is more carbon-12 or carbon-14 found in real life? How do you know?
If we want to find the most common mass number for an element, we can just round the
atomic mass.
For example, the most common mass number for an atom of cobalt is
58.93  59. For carbon, the most common mass number is 12.
Find the most common mass number for the elements below, using your
rounding skills and the periodic table.
Element
Most common
mass #
Oxygen (O)
Aluminum (Al)
Neon (Ne)
Zinc (Zn)
Check yourself: The difference between mass number and atomic mass is…
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Now we know enough about the periodic table to put it all together. You will soon be able to
draw a model for any element on the periodic table, using just the three pieces of information
we saw in the “Do Now.” Let’s try the first one together.
Question: Draw a model of the most common form of fluorine (F).
Step 1: What is the atomic number of fluorine? (Look this up in the periodic table)
Step 2: How many protons does fluorine have?
Step 3: What is the atomic mass for fluorine?
Step 4: What is the most common mass number for fluorine (Remember: round to the nearest
whole number)
Step 5: Plug your values from above into the formula for mass number
Mass number
# protons
# neutrons
Step 6: Use math to solve for the missing value.
Step 7: An atom always has balanced charges. This means that the number of protons is equal
to the number of electrons. Use this information to complete the chart
below and to sketch what a fluorine atom looks like.
# protons
Fluorine
# neutrons
# electrons
Practice
Use the seven steps from the previous page to figure out the number of protons, neutrons, and
electrons in a beryllium (Be) atom.
Step 1:
Step 2:
Step 3:
Step 4:
Step 5:
Mass number
# protons
# neutrons
Step 6: Use math to solve for the missing value.
Step 7: An atom always has balanced charges. This means that the number of protons is equal
to the number of electrons. Use this information to complete the chart below and to sketch
what a fluorine atom looks like..
# protons
# neutrons
# electrons
Practice
Use the same process as the previous page to figure out the number of protons, neutrons, and
electrons in a sodium (Na) atom.
Use the seven steps from the previous page to figure out the number of protons, neutrons, and
electrons in a beryllium (Be) atom.
Step 1:
Step 2:
Step 3:
Step 4:
Step 5:
Mass number
# protons
# neutrons
Step 6: Use math to solve for the missing value.
Step 7: An atom always has balanced charges. This means that the number of protons is equal
to the number of electrons. Use this information to complete the chart below and to sketch
what a fluorine atom looks like..
# protons
# neutrons
# electrons
Use the same process as the previous page to figure out the number of protons, neutrons, and
electrons in a nitrogen (N) atom.
Use the seven steps from the previous page to figure out the number of protons, neutrons, and
electrons in a beryllium (Be) atom.
Step 1:
Step 2:
Step 3:
Step 4:
Step 5:
Mass number
# protons
# neutrons
Step 6: Use math to solve for the missing value.
Step 7: An atom always has balanced charges. This means that the number of protons is equal
to the number of electrons. Use this information to complete the chart below and to sketch
what a fluorine atom looks like..
# protons
# neutrons
# electrons