1. Essential Question: How do atoms of the same element differ?
Isotopes
Essential Question: How do atoms of the same element differ?

2. ISOTOPES What are they?
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. All elements consist of naturally occurring isotopes.

3. How are isotopes of an element similar? How are they different?
The isotopes of an element have
Different Physical Properties (different mass and different number of neutrons)
Identical Chemical Properties (this is because they have the same numbers of protons and electrons and subatomic particles are responsible for chemical behavior)

4. NUCLEAR SYMBOLS
Nuclear symbols are used to represent specific isotopes. To write a nuclear symbol, the symbol of the element is written first, the mass number is written as a superscript to the left of the symbol and the atomic number is written as a subscript to the left. Study the illustration below.
Li-7 isotope

5. WRITING ISOTOPES
You can also use the mass number and the name of the element to designate the atom or isotope. For example, two isotopes of carbon are carbon-12 and carbon-13. The symbols for these two isotopes would be: C 12 6 C 13 6

6. The mass number refers to the total number of protons and neutrons in a specific nucleus of an atom.
The atomic number always refers to the total number of protons in an atom.
To determine the number of neutrons in a specific isotope, subtract the atomic number from the mass number. Study the formula below.
Mass Number (neutrons + protons)
- Atomic Number (number of protons)
(number of neutrons)

7. Boron consists of two isotopes, B-10 and B-11
Boron consists of two isotopes, B-10 and B-11. Determine the number of neutrons in each of the isotopes. From the periodic table, write the symbol and atomic number for Boron.
Determine the number of neutrons by subtracting the atomic number from the mass number.
B-10: – 5 = 5 neutrons
B-11: – 5 = 6 neutrons

8. Mass Number and Atomic Mass
To identify isotopes, the mass number is added after the element’s name. Ex: potassium-39 potassium-40 Atomic mass of an element is the weighted average mass of the isotopes of that element It’s calculated by summing the products of each isotope’s percent abundance times its atomic mass. Refer to pg 103 in the text for the example (6.015 amu)(0.0750) + (7.016amu)(0.925) = 6.941amu

9. Nuclear Symbol
Complete based on the following information:
29 protons, mass 65
Barium - 138
17 protons, mass 36
Tin - 120
16 protons, mass 30