Chapter 5
The Periodic Table
Organizing the Elements
Mendeleev’s original periodic table was
arranged by atomic mass.
A periodic table is an arrangement of
elements in columns, based on a set of
properties that repeat from row to row.
Mendeleev’s Periodic Table
Mendeleev’s Periodic Table
Mendeleev named missing elements after elements
in the same group. He gave the name eka-aluminum
to the missing element one space below aluminum
in the table. Mendeleev predicted that ekaaluminum would
• be a soft metal,
• have a low melting point, and
• have a density of 5.9 g/cm3.
Mendeleev’s Periodic Table
In 1875, a French chemist discovered a new
element. He named the element gallium (Ga) in
honor of France. (The Latin name for France is
Gallia.) Gallium
• is a soft metal,
• has a melting point of 29.7°C, and
• has a density of 5.91 g/cm3.
Assessment Questions
1.
In Mendeleev’s periodic table, elements with
similar properties were grouped
a. in the same row.
b. in the same column.
c. in diagonal lines that run from top left to the bottom
right.
d. in pairs of two.
Assessment Questions
1.
In Mendeleev’s periodic table, elements with
similar properties were grouped
a. in the same row.
b. in the same column.
c. in diagonal lines that run from top left to the bottom
right.
d. in pairs of two.
ANS: B
Assessment Questions
2.
For which element did Mendeleev correctly
predict the properties even before it had been
discovered?
a.
b.
c.
d.
gallium
hydrogen
bromine
aluminum
Assessment Questions
2.
For which element did Mendeleev correctly
predict the properties even before it had been
discovered?
a.
b.
c.
d.
gallium
hydrogen
bromine
aluminum
ANS:
A
The Periodic Law
The modern periodic table is arranged by
atomic number.
A period is a row in the periodic table.
A group is a column in the periodic
table.
The Periodic Law
Classes of Elements
Elements are classified as metals,
nonmetals, and metalloids.
Metals are elements that are good
conductors of electric current and heat.
Nonmetals are elements that are poor
conductors of heat and electric current.
Metalloids are elements with properties
that fall between those of metals and
nonmetals.
Atomic Mass
Atomic Mass Units
The mass of an atom in grams is extremely small.
In order to have a convenient way to compare
the masses of atoms, scientists chose one isotope
to serve as a standard.
• Scientists assigned 12 atomic mass units to the
carbon-12 atom, which has 6 protons and 6
neutrons.
• An atomic mass unit (amu) is defined as one
twelfth the mass of a carbon-12 atom.
Atomic Mass
There are four pieces of information for each
element.
Atomic number
Element symbol
Element name
Atomic mass
Atomic Mass
Isotopes of Chlorine
In nature, most elements exist as a mixture of
two or more isotopes. The element chlorine has
an atomic mass of 35.453 amu. Where does the
number 35.453 come from?
• There are two natural isotopes of chlorine, chlorine35 and chlorine-37.
• An atom of chlorine-35 has 17 protons and 18
neutrons.
• An atom of chlorine-37 has 17 protons and 20
neutrons.
Atomic Mass
Weighted Averages
This table shows the atomic masses for the two naturally
occurring chlorine isotopes. The value of the atomic mass
for chlorine is a weighted average. If you add the atomic
masses of the isotopes and divide by 2, you get 35.967,
not 35.453.
Classes of Elements
Metals
The majority of the elements on the periodic
table are classified as metals. Metals are elements
that are good conductors of electric current and
heat.
• Except for mercury, metals are solids at room
temperature.
• Most metals are malleable.
• Many metals are ductile; that is, they can be drawn
into thin wires.
Classes of Elements
The metals in groups 3 through 12 are called
transition metals. Transition metals are elements
that form a bridge between the elements on the left
and right sides of the table.
• Transition elements, such as copper and silver, were
among the first elements discovered.
• One property of many transition metals is their ability
to form compounds with distinctive colors.
Classes of Elements
Nonmetals
Nonmetals generally have properties opposite to
those of metals.
• Nonmetals are elements that are poor conductors
of heat and electric current.
• Nonmetals have low boiling points–many nonmetals
are gases at room temperature.
• Nonmetals that are solids at room temperature tend
to be brittle. If they are hit with a hammer, they
shatter or crumble.
Classes of Elements
Metalloids
Metalloid elements are located on the periodic table
between metals and nonmetals.
• Metalloids are elements with properties that fall
between those of metals and nonmetals.
• For example, a metalloid’s ability to conduct electric
current varies with temperature. Silicon (Si) and
germanium (Ge) are good insulators at low
temperatures and good conductors at high
temperatures.
Variations Across a Period
Across a period from left to right, the
elements become less metallic and more
nonmetallic in their properties.
Variations Across a Period
From left to right across Period 3, there are three
metals (Na, Mg, and Al), one metalloid (Si), and four
nonmetals (P, S, Cl, and Ar).
Assessment Questions
1.
What determines the atomic mass of an
element?
a. the natural distribution of isotopes and the atomic
numbers of those isotopes
b. the natural distribution of isotopes and the masses
of those isotopes
c. the mass of the isotope of the element that has
the most neutrons
d. the average number of protons in the element’s
nucleus
Assessment Questions
1.
What determines the atomic mass of an
element?
a. the natural distribution of isotopes and the atomic
numbers of those isotopes
b. the natural distribution of isotopes and the masses
of those isotopes
c. the mass of the isotope of the element that has
the most neutrons
d. the average number of protons in the element’s
nucleus
ANS: B
Assessment Questions
2.
Which of the following is not
characteristic of metals?
a.
b.
c.
d.
ductile
good electrical conductor
typically solid at room temperature
brittle
Assessment Questions
2.
Which of the following is not
characteristic of metals?
a.
b.
c.
d.
ductile
good electrical conductor
typically solid at room temperature
brittle
ANS:
D
Assessment Questions
3.
Within a period of the periodic table, how
do the properties of the elements vary?
a. Metallic characteristics increase from left to right.
b. Metallic characteristics decrease from left to
right.
c. Reactivity increases from left to right.
d. Reactivity decreases from left to right.
Assessment Questions
3.
Within a period of the periodic table, how
do the properties of the elements vary?
a. Metallic characteristics increase from left to right.
b. Metallic characteristics decrease from left to
right.
c. Reactivity increases from left to right.
d. Reactivity decreases from left to right.
ANS: B
Assessment Questions
1.
In the modern periodic table, elements
are arranged in order of increasing atomic
mass.
True
False
Assessment Questions
1.
In the modern periodic table, elements
are arranged in order of increasing atomic
mass.
True
False
ANS:
F, atomic number
Valence Electrons
Why do elements in a group have similar
properties?

Elements in the same group have similar
properties because they have the same
number of valence electrons.
A valence electron is an electron that is
in the highest occupied energy level of an
atom.
Valence Electrons

The number of valence electrons is equal to
the group number for groups 1A – 8A in the
periodic table.

Elements in the same group have similar
properties because they have the same
number of valence electrons.

The properties are not identical because the
valence electrons are in different energy
levels.
The Groups
The elements in Group 1A are called
alkali metals.
 All have 1 valence electron
 Extremely reactive
 Found only in compounds
 Stored in oil or argon gas to keep
from reacting with water in air.
The reactivity for Group 1A elements
increases from top to bottom.
The Groups
The elements of Group 2A are called
alkaline earth metals
 All have 2 valence electrons
 Harder than Group 1A metals
 Less reactive than Group 1A metals
 Higher melting points than Group
1A metals
The reactivity for Group 2A elements
also increases from top to bottom.
The Groups
Group 3A elements are called
the boron family.
 All elements have 3 valence
electrons
 Boron is a metalloids,
aluminum, gallium, indium, and
thallium are metals.
The Groups
Group 4A elements are called
the carbon family.
 All elements have 4 valence
electrons
 Carbon is a nonmetal, silicon
and germanium are metalloids,
and tin and lead are metals.
 Carbon is found in most
compounds in your body,
except water
The Groups
Group 5A elements are called the
nitrogen family.
 All elements have 5 valence
electrons
 Nitrogen and phosphorus are
nonmetals, arsenic and antimony
are metalloids, and bismuth is a
metal
The Groups
Group 6A elements are called the
oxygen family.
 All elements have 6 valence
electrons
 Oxygen, sulfur, and selenium are
nonmetals, tellurium and
polonium are metalloids
The Groups
Group 7A elements are called the
halogens.
 Each element has 7 valence electrons
 Fluorine and Chlorine are gases, Bromine
is a liquid, and Iodine is a solid
 They are all nonmetals, except astatine is
a metalloid
 React most easily with metals (Group 1A
especially)
The reactivity for Group 7A decreases
from top to bottom. Fluorine is the most
reactive halogen.
The Groups
Group 8A elements are called the
noble gases.
 Helium has 2 valence electrons
 Each of the other noble gases has
8 valence electrons
 When electric current passes
through noble gases, they emit
different colors.
The noble gases are colorless,
odorless, and extremely unreactive.
Assessment Questions
1.
Why do elements in a group have similar
chemical properties?
a) They have the same number of valence
electrons
b) They have the same ratio of protons to
neutrons.
c) Their atomic masses are multiples of the
least massive element of the group.
d) They have the same number of total
electrons.
Answer: A
Assessment Questions
2.
True or False: In general, the metallic
characteristics of A group elements
within a period decreases from left to
right.
Answer: True