This document discusses atomic structure, isotopes, and how atomic masses are reported on the periodic table. It explains that each element has a unique number of protons, while isotopes of an element have the same number of protons but different numbers of neutrons. It provides examples of common isotopes of oxygen, carbon, calcium, and lithium and explains that the atomic masses reported on the periodic table represent weighted averages accounting for the relative abundances of each isotope.

2. Each element has a unique # of protons.
So…EVERY SINGLE ATOM of that element
MUST HAVE the SAME number of protons,
ALWAYS!
This is the reason why each element has a
unique Atomic Number that never changes!

3. Isotopes are atoms with same # of protons
(making them the SAME ELEMENT), but have a
different # of neutrons.
Having different # of neutrons for the same
element means that the isotopes will have
different mass numbers, too!

4. There are 3 isotopes oxygen:
How many protons does each isotope have?
How many neutrons does each isotope have?
8
O
Oxygen
16
8
O
Oxygen
17
8
O
Oxygen
18
ALL oxygen isotopes have an atomic # of 8,
so they all have 8 protons!
8
neutrons
9
neutrons
10
neutrons

5. There are 3 isotopes carbon:
How many protons does each isotope have?
How many neutrons does each isotope have?
6
C
Carbon
12
6
C
Carbon
13
6
C
Carbon
14
ALL carbon isotopes have an atomic # of 6,
so they all have 6 protons!
6
neutrons
7
neutrons
8
neutrons

6. Atomic Masses on the periodic table are
normally decimals. WHY??
Atomic Masses are decimals because it is an
AVERAGE mass of all of the element’s isotopes.
Some isotopes are more common than others,
so this is a weighted average.
So the Mass # from the periodic table is
always closest to the most common
isotope’s mass.
5
B
Boron
10.81

7. In the universe, there are MUCH more
carbon-12 atoms that exist than carbon-13
and carbon-14 atoms.
When you find the weighted average of the
masses of the carbon isotopes, the average
will be closest to 12!
6
C
Carbon
12.01
Carbon- 12

8. The Atomic Mass of calcium is 40.08.
Which one is the most common calcium isotope?
This is because 40.08 rounded to a whole # is
closest to calcium-40!
20
Ca
Calcium
40
20
Ca
Calcium
42
20
Ca
Calcium
43
20
Ca
Calcium
44

9. The Atomic Mass of lithium is 6.94.
Which one is the most common lithium isotope?
This is because 6.94 rounded to a whole # is closest
to lithium-7!
3
Li
Lithium
5
3
Li
Lithium
6
3
Li
Lithium
7
3
Li
Lithium
8

10. • Hyphen Notation:
• Element name – Mass #
• Ex: Carbon – 14
• Nuclear Notation:
X
Mass #
Atomic #
Element Symbol
c
14
6
Ex: