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Chapter 4 Lecture
Basic Chemistry
Fourth Edition
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.
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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
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11
Na
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.
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Atomic Models
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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
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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
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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.
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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
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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
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Composition of Elements
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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
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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
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Learning Check
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
(1) 14
(2) 16
(3) 34
B. Its mass number is
(1) 14
(2) 16
(3) 34
C. The element is
(1) Si
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(2) Ca
(3) Se
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
© 2014 Pearson Education, Inc.