1. Chapter 4 Atoms and Elements
4.4 Atomic Number and Mass Number
Learning Goal Given the atomic number and the mass number of an atom, state the number of protons, neutrons, and electrons.
© 2014 Pearson Education, Inc.

2. 11 Na Atomic Number The atomic number
is specific for each element and the same for all atoms of that element
is equal to the number of protons in an atom
appears above the symbol of an element
Atomic Number
Chemical Symbol 11 Na
© 2014 Pearson Education, Inc.

3. Atomic Numbers and Protons
Hydrogen has atomic number 1; every H atom has 1 proton.
Carbon has atomic number 6; every C atom has 6 protons.
Copper has atomic number 29; every Cu atom has 29 protons.
Gold has atomic number 79; every Au atom has 79 protons.

4. Atomic Models

5. Learning Check
State the number of protons in each atom.
A. A nitrogen atom
(1) 5 protons (2) 7 protons (3) 14 protons
B. A sulfur atom
(1) 32 protons (2) 16 protons (3) 6 protons
C. A barium atom
(1) 137 protons (2) 81 protons (3) 56 protons

6. Solution
State the number of protons in each atom.
A. A nitrogen atom (2) 7 protons
B. A sulfur atom (2) 16 protons
C. A barium atom (3) 56 protons

7. Atoms Are Neutral
An atom of any element
is electrically neutral; it has a net charge of zero
has an equal number of protons and electrons
A neutral atom of calcium, atomic number 20, contains 20 protons and 20 electrons. It has zero net charge.

8. Mass Numbers
The mass number
represents the number of particles in the nucleus
is equal to the
number of protons + number of neutrons
does not appear on the periodic table because it applies to a single atom only

9. Mass Numbers - Neutrons
We calculate the number of neutrons in an atom from its mass number and atomic number:
mass number – atomic number
= number neutrons
Potassium has a mass number of 39 and an atomic number of 19. To find the number of neutrons, subtract the atomic number from its mass number.
39 (mass number) – 19( atomic number) = 20 neutrons

10. Composition of Elements

11. Learning Check
An atom of zinc has a mass number of 65.
A. How many protons are in the nucleus?
(1) 30 (2) 35 (3) 65
B. How many neutrons are in the nucleus?
(1) 30 (2) 35 (3) 65
C. What is the mass number of a zinc atom that has 37 neutrons?
(1) 37 (2) 65 (3) 67

12. Solution
An atom of zinc has a mass number of 65.
A. How many protons are in the nucleus?
(1) 30
B. How many neutrons are in the nucleus?
(2) 35
C. What is the mass number of a zinc atom that has 37 neutrons?
(3) 67

13. Learning Check
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
(1) (2) 16 (3) 34
B. Its mass number is
C. The element is
(1) Si (2) Ca (3) Se

14. Solution
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
(1) 14
B. Its mass number is
(3) 34
C. The element is
(1) Si