Name_______________KEY_______________________
Date _______________________
Class _____________
What is an Isotope?
1. Look at your periodic table. The numbers above the element symbol are called atomic numbers.
~What is the atomic number of phosphorus?
15
~What is the atomic number of copper (Cu)?
29
~What is the atomic number of titanium (Ti)?
22
2. The atomic number tells us the number of protons in an atom of that element.
~What is the charge of a proton? +1
~Where are protons located in the atom? In the nucleus (center of the atom)
3. The number of protons identifies the element. CHANGE THE NUMBER OF PROTONS AND YOU CHANGE
THE ELEMENT!
~How many protons are in an atom of phosphorus?
15
~If another proton is added to an atom of phosphorus, what element do we have?
Sulfur (atomic number = 16)
4. Electrons are located in the electron cloud that surrounds the nucleus. They are so small, we ignore their mass in the
mass of an atom.
~What is the charge of an electron?
-1
5. To have a neutral atom, the positive charges and the negative charges must be in balance. THIS MEANS THAT THE
NUMBER OF PROTONS MUST = THE NUMBER OF ELECTRONS IN A NEUTRAL ATOM!
~How many electrons are in a neutral atom of phosphorus? 15
~How many electrons are in a neutral atom of beryllium?
4
6. Remember that electrons are arranged in energy levels around the nucleus. Each energy level can hold a certain
number of electrons.
~ 1st energy level holds ______2_____ electrons.
~ 3rd energy level holds _____8____ electrons.
~ 2nd energy level holds _____8_____electrons.
~ 4th energy level holds ____18______ electrons.
7. Neutrons are located in the nucleus with the protons.
~What is the charge of a neutron? 0 (neutral)
8. Although they do not have an electric charge, neutrons do have mass.
Mass of a proton = 1
Mass of a neutron = 1
Mass of an electron = 0 (tiny!)
~If an atom has 4 protons, 4 electrons, and 5 neutrons, what is the mass of the atom?
~What element would this be? Beryllium
~If an atom has 4 protons, 4 electrons, and 6 neutrons, what is the mass of the atom?
~What element would this be? Beryllium (still 4 protons  still beryllium)
~If an atom has 5 protons, 5 electrons, and 5 neutrons, what is the mass of the atom?
~What element would this be? Boron
~If an atom has 5 protons, 5 electrons, and 6 neutrons, what is the mass of the atom?
~What element would this be? Boron (still 5 protons  still boron)
4p + 5n = 9
4p + 6n = 10
5p + 5n = 10
5p + 6n = 11
9. You should have just seen that changing the number of neutrons does NOT change what element we are working
with. However, changing the number of neutrons DOES change the mass of the atom.
THE MASS NUMBER OF AN ATOM = NUMBER OF PROTONS + NUMBER OF NEUTRONS.
~How many protons does an atom of aluminum have? 13
~What is the mass of an atom of aluminum if it has 13 neutrons? 13p + 13n = 26
~If we change the number of neutrons to 14, does the element change? No-still 13 protons  still aluminum
~What is the mass of this atom now that is has 14 neutrons? 13p + 14n = 27
10. Now we know that elements are defined by how many protons are in an atom of that element. However, we can
change the number of neutrons without changing the element.
ATOMS OF THE SAME ELEMENT WITH DIFFERENT AMOUNTS OF NEUTRONS ARE CALLED ISOTOPES.
When you change the number of neutrons, you also change the mass of the atom. Remember:
PROTONS + NEUTRONS = MASS OF ATOM
~All atoms of nitrogen have _____7_____ protons.
~If one isotope of nitrogen has a mass of 14, it must have ____7_____ neutrons. (14 – 7p = 7n)
~If a second isotope of nitrogen has a mass of 15, it must have ___8____neutrons. (15 – 7p = 8n)
~These two atoms are isotopes of nitrogen because they have the same number of protons, but different numbers
of neutrons. They also have different masses.
11. One way to write isotopes is shown below.
Mass Number
Element Symbol
Atomic Number
16
8
~Write the symbol in this format for an isotope of oxygen with 8 neutrons.
O
17
8
O
18
8
O
~Write the symbols in this format for an isotope of oxygen with 9 neutrons.
~Write the symbol in this format for an isotope of oxygen with 10 neutrons.
40
20
~Write the symbol for an atom that has 20 protons and 20 neutrons.
Ca
~Write the symbol for an atom that has 20 protons and 21 neutrons. 41 Ca
20
12. Another way to write isotopes is shown below:
Element Symbol – Mass Number
~Write the symbol in this format for an atom with 2 protons and 2 neutrons. He-4
~Write the symbol in this format for an atom with 2 protons and 3 neutrons. He-5
~ In the atom C-12, there are ______6____ protons and _____6____ neutrons. This is one isotope of the
element carbon.
~Another isotope of this same element is C-14. In an atom of this isotope there are _____6___ protons and
________8_____ neutrons.
13. So, let’s recap:
~The atomic number tells us the number of ____________ in an atom.
~Where can atomic numbers be found?
_____________________________________________________________.
~If we change the number of ______________ in an atom, we change what element we’re
working with.
~The number of protons and _________________ must be equal in order for an atom to be
neutral (no overall charge).
~Every proton has a charge of _____________.
~Every electron has a charge of ____________.
~Every neutron has a charge of ____________.
~The mass of an atom = the number of ______________ + the number of _____________.
~An isotope is when we change the number of ______________, but do not change the number
of ____________,
~So, we still have an atom of the same ________________, it just has a different mass because it
has a different number of neutrons.
~2 ways to write isotopes:
_____ Number
Element Symbol
_____ Number
Mass number = ____________ + ____________
Atomic number = ___________
Now that you know all of this….PRACTICE, PRACTICE, PRACTICE!
Element symbol - ________ Number