How Atoms Differ
Isotopes and Mass Number
Atoms of a particular element have the same number of _______________________
and electrons, but the number of _________________________ may differ!
These atoms are called ______________________. Isotopes are atoms of the same
element that have an identical number of protons, but a different number of
_______________________.

For example, there are three types of potassium. All three types have _____
protons and 19 electrons, but one type contains 20 neutrons, another contains 21
neutrons, and the third type has 22 neutrons!
In nature most elements are found as a mixture of isotopes. No matter the source of the
element, its isotopes will always be found in the same ____________________.

For example, in a ______________________ which is a rich source of potassium
93.25% of the potassium atoms have 20 neutrons, 6.7302 % have 22 neutrons, and
only 0.0117% will have 21 neutrons. In another banana, or a totally different source
of potassium, the _______________________ composition of the potassium
isotopes will still be the same.
As you might expect, the isotopes do differ in ________________. Isotopes with more
__________________ have a greater mass. Although isotopes differ in mass and their
number of neutrons, they essentially have the same _____________________ behavior.

Why? Because chemical behavior is determined by the number of
____________________.
Isotopes are distinguished by their ____________ number, which is the sum of the
number of protons and ____________________ in the nucleus.

For example, the potassium isotope with 19 protons and 20 neutrons has a mass
number of 39, and thus is called potassium-39. Fill the table for the other isotopes.
Protons
Neutrons
Electrons
Potassium-39
19
20
19
Postassium-40
Potassium-41
Chemists also distinguish different isotopes by a shortened notation that involves the
mass number, atomic number, and chemical symbol.

For example, potassium-39 would be written:
39
19
K
For the following isotopes please calculate the number of protons, neutrons, and electrons.
Then write out the isotope name and isotope symbol.
Element
Neon
Calcium
Oxygen
Iron
Zinc
Mercury
Atomic
Number
10
20
8
26
30
80
Mass Number
22
46
17
57
64
204
Mass of Individual Atoms
What is my Final Grade?!
Grade Category
Tests
Labs
Homework
Category Average
85
95
70
Category Percent
50
40
10
Final Grade = Test Average x (.5) + Lab Average x (.4) + Homework Average x (.1)
Protons and neutrons make up the majority of an atom’s ________________. Chemists
have come up with a standardized way to represent the masses of subatomic particle.
Protons, neutrons, and electrons are measured in units called ______________
______________ units (amu).
Because protons and neutrons are each about equal to 1 amu, you would expect the atomic
mass of an element to be a whole number. This is not the case because the atomic mass of
an element is the ____________________ average mass of the isotopes of that
element. (Just like your Final Grade is the average of the weighted grade categories
above)!
You can calculate the atomic mass of any element if you know its number of naturally
occurring isotopes, their masses, and their percent abundances.
Practice Problems:
Isotope
Boron-10
Boron-11
Mass (amu)
10.013
11.009
Percent Abundance (%)
19.8
80.2
You can often tell which isotope for an element is the most __________________ by
comparing the atomic mass to the mass number of each isotope.

For example, the atomic mass of boron is 10.81.
o
Which of Boron’s isotopes is the average atomic mass closest too?
o
So which isotope is likely the most abundant?