Ch. 3: “Atoms and the
Periodic Table”
Section 3.2: “A Guided Tour of the
Periodic Table”
Objectives
1.
2.
3.
4.
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 isotope has, given its
symbol, atomic number and mass number.
Describe how the abundance of isotopes
affects an element’s average atomic mass.
Organization of the Periodic Table
•
•
periodic law = properties of elements tend to change in a
regular pattern when elements are arranged in order of
increasing atomic number, or number of protons in their
atoms
How many protons does each of the following elements
have?
hydrogen 1
lithium 3
sodium 11
helium 2
beryllium 4
magnesium 12
boron 5
aluminum 13
carbon 6
silicon 14
nitrogen 7
phosphorus 15
oxygen 8
sulfur 16
fluorine 9
chlorine 17
neon10
argon 18
Organization of the Periodic Table
• period = a horizontal () row of elements in the periodic table
• In a period, the number of electrons increases as you move from
left to right.
• How many electrons does each of the following elements have?
hydrogen 1
helium 2
lithium 3
beryllium 4
boron 5
carbon 6
nitrogen 7
oxygen 8
fluorine 9
neon 10
sodium 11
magnesium 12
aluminum 13
silicon 14
phosphorus 15
sulfur 16
chlorine 17
argon 18
Organization of the Periodic Table
• group/family = a vertical column () of elements in the periodic
table
• Atoms of the same group/family have the same number of
valence electrons.
• How many valence electrons does each of the following
elements have?
hydrogen 1
lithium 1
sodium 1
helium 2
beryllium 2
magnesium 2
boron 3
aluminum 3
carbon 4
silicon 4
nitrogen 5
phosphorus 5
oxygen 6
sulfur 6
fluorine 7
chlorine 7
neon 8
argon 8
Some Atoms Form Ions
•
•
ionization = the process of adding electrons to or
removing electrons from an atom or group of atoms
ion = an atom or group of atoms that has lost or gained
one or more electrons and therefore has a net electric
charge
1. cation = an ion with a positive charge
What are the charges on the following cations?
H +1
Be +2
B +3
Li +1
Mg +2
Al +3
Na +1
2. anion = an ion with a negative charge
What are the charges on the following anions?
N -3
O -2
F -1
P -3
S -2
Cl -1
How Do Structures of Atoms Differ?
• atomic number (Z) = the number of
protons in the nucleus of an atom
• Atoms are neutral, therefore, the number
of protons = the number of electrons.
• mass number (A) = AKA atomic mass or
average atomic mass . It’s the total
number of protons and neutrons in the
nucleus of an atom.
How Do Structures of Atoms Differ?
• isotopes = any atoms having the same
number of protons but different numbers of
neutrons
• ex. hydrogen
1. protium (A = 1):
1 p+, 1 e-, & 0 nº
2. deuterium (A = 2): 1 p+, 1 e-, & 1 nº
3. tritium (A = 3):
1 p+, 1 e-, & 2 nº
Calculating the Number of
Neutrons in an Atom
• Uranium-235
mass # (A):
235
atomic # (Z): - 92
# of neutrons: 143
• Chlorine-37
A:
37
Z:
- 17
# of nº:
20