Download Atomic Number

4.4 Atomic Number and Mass Number
All atoms of an element
have the same number
of protons and the
same atomic number.
Learning Goal Given the atomic number and the mass
number of an atom, state the number of protons, neutrons,
and electrons.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Atomic Number
The atomic number
• is a whole number 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
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
11
Na
© 2016 Pearson Education, Inc.
Atomic Number = Protons in an Atom
Atomic number = number of protons—for example,
• the atomic number of H is 1; every H atom has one proton.
• the atomic number of C is 6; every C atom has six protons.
• the atomic number of Cu is 29; every Cu atom has 29 protons.
Core Chemistry Skill Counting Protons and Neutrons
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Atomic Number = Protons in an Atom
All atoms of lithium (left) contain three protons, and all atoms of carbon (right) contain six protons.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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+) + 13 electrons (13–) = 0
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study 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
N
Protons
Electrons
Zn
S
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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
N
7
Protons
Electrons
7
7
Zn
30
30
30
S
16
16
16
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Composition of Some Atoms of Different
Elements
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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 numbers are given for specific isotopes only.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study 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?
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.