1. Unit 2: Atomic Theory & Structure
Isotopes

2. Introduction
Just as apples come in different varieties, a chemical element can come in different “varieties” called isotopes.

3. Atomic Number
Atomic number of an element is the number of protons in the nucleus of each atom of that element.
It equals the number of electrons.
Element
Atomic #
# of p+
# of e-
Carbon 6
Phosphorus 15
Gold 79

4. Mass Number Mass # = p+ + n0
Mass number is the number of protons AND neutrons in the nucleus of an atom.
Mass # = p+ + n0
Name p+ n0 e-
Mass #
Oxygen - 10 - 33 42
- 31 15 18 8 8 18
Arsenic 75 33 75
Phosphorus 16 15 31

5. Isotopes
Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.
Isotope Names Isotope Symbols
Hydrogen - 1
Mass # 1 H 1
Mass #
Atomic #

6. Isotopes
Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.
Isotope
Symbol
Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium) 1 H
Hydrogen-2
(deuterium) 2
Hydrogen-3
(tritium) 3

7. Quick (but challenging) Quiz!
1. The number of protons in the nucleus is…
the mass number.
the atomic number.
the same as the number of neutrons.
the total of protons and neutrons.

8. Quick (but challenging) Quiz.
2. The mass number is…
the number of protons.
the number of neutrons.
the total of protons and electrons.
the total of protons and neutrons.

9. Quick (but challenging) Quiz.
3. Isotopes of an element have…
the same mass number.
different atomic numbers.
the same number of protons but different mass numbers.
the same number of protons but different numbers of neutrons.

10. Quick (but challenging) Quiz.
4. How many neutrons are in an atom of potassium–40 ? (40K)
(Atomic Number = 19)
19 neutrons
21 neutrons
40 neutrons
59 neutrons