Download Organization of the Periodic Table

Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Objectives
• Relate the organization of the periodic table to the
arrangement of electrons within an atom.
• Explain why some atoms gain or lose electrons to
form ions.
• Determine how many protons, neutrons, and
electrons an atom has, given its symbol, atomic
number, and mass number.
• Describe how the abundance of isotopes affects an
element’s average atomic mass.
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Organization of the Periodic Table
• The periodic table groups similar elements together.
• This organization makes it easier to predict the
properties of an element based on where it is in
the periodic table.
• Elements are listed in order of number of protons,
because the periodic law states that when
elements are arranged this way, similarities in their
properties will occur in a regular pattern.
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Organization of the Periodic Table, continued
• The periodic table helps determine electron
arrangement.
• Horizontal rows in the periodic table are
called periods.
• Just as the number of protons an atom has
increases as you move from left to right across a
period, so does its number of electrons.
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Organization of the Periodic Table, continued
• Elements in the same group have similar properties.
• A group is a vertical column of elements in the
periodic table.
• Atoms of elements in the same group have the
same number of valence electrons, so these
elements have similar properties.
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Some Atoms Form Ions
• An ion is an atom or group of atoms that has lost
or gained one electrons and has a negative or
positive charge.
• A lithium atom loses one electron to form a 1+ charged ion:
• A fluorine atom gains one electron to form a 1 charged ion:
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
How Do the Structures of Atoms Differ?
• The atomic number, Z, of an atom equals the
number of protons in the nucleus.
• The mass number, A, of an atom equals the
number of protons plus the number of neutrons in
the nucleus.
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Nucleus
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Atomic Number
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Mass Number
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
How Do the Structures of Atoms Differ?
continued
• An isotope is an atom that has the same number of
protons as other atoms of the same element do but
that has a different number of neutrons.
• Example: Hydrogen has three isotopes, shown
below.
• Some isotopes are more common than others.
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
How Do the Structures of Atoms Differ?
continued
• If you know the atomic number and mass number
of an atom, you can calculate the number of
neutrons it has.
• Example: uranium-235 has a mass number of
235. Like all uranium atoms, it has an atomic
number of 92. The number of neutrons it has is
therefore:
Mass number (A):
Atomic number (Z):
Number of neutrons:
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235
–92
143
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Isotopes
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
How Do the Structures of Atoms Differ? continued
• Because the mass of a single atom is so tiny, atomic
masses are usually expressed in atomic mass units.
• An atomic mass unit (amu) is equal to one twelfth of
the mass of a carbon-12 atom.
• The average atomic mass for an element is a
weighted average of the masses of all naturallyoccurring isotopes of an element.
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