1. Chapter 3 Atoms and Elements
3.4
Atomic Number and Mass Number
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

2. 11 Na Atomic Number Atomic Number Symbol The atomic number
is specific for each element
is the same for all atoms of an element
is equal to the number of protons in an atom
appears above the symbol of an element in the Periodic Table
Atomic Number
Symbol 11 Na
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

3. Atomic Number = Protons in Atom
For example, atomic number = number of protons:
Atomic number H is 1; every H atom has one proton
Atomic number of C is 6; every C atom has six protons
Atomic number of Cu is 29; every Cu atom has 29 protons
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

4. Atomic Number = Protons in Atom
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

5. Atoms Are Neutral For neutral atoms, the net charge is zero.
number of protons = number of electrons
Aluminum has 13 protons and 13 electrons. The net
(overall) charge is zero.
13 protons (13+) electrons (13–) = 0
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

6. Learning Check
Use the periodic table to fill in the atomic number, number of protons, and number of electrons for each of the following elements:
Element
Atomic Number
Protons
Electrons N Zn S
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

7. Solution
Use the periodic table to fill in the atomic number, number of protons, and number of electrons for each of the following elements:
Element
Atomic Number
Protons
Electrons N 7 Zn 30 S 16
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

8. Mass Number The mass number
represents the number of particles in the nucleus
is equal to the number of protons + the number of neutrons
is always a whole number
does not appear in the periodic table
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

9. Composition of Some Atoms of Different Elements
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

10. Study Tips: Protons and Neutrons
Number of protons = atomic number
Number of protons + neutrons = mass number
Number of neutrons = mass number – atomic number
Note: Mass number is given for specific isotopes only.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

11. Learning Check An atom of lead (Pb) has a mass number of 207.
A. How many protons are in the nucleus?
B. How many neutrons are in the nucleus?
C. How many electrons are in the atom?
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

12. Solution An atom of lead (Pb) has a mass number of 207.
A. How many protons are in the nucleus?
atomic number = 82; number protons = 82
B. How many neutrons are in the nucleus?
mass number – number protons = number neutrons
207 – 82 = 125 neutrons
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

13. Solution An atom of lead (Pb) has a mass number of 207.
C. How many electrons are in the atom?
An atom is neutral, which means that the
number of electrons is equal to the number of
protons. An atom of Pb has 82 protons and
82 electrons.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.