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6.3 Periodic Trends >
Chapter 6
The Periodic Table
6.1 Organizing the Elements
6.2 Classifying the Elements
6.3 Periodic Trends
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6.3 Periodic Trends >
CHEMISTRY
& YOU
How are trends in the weather similar to
trends in the properties of elements?
Although the
weather changes
from day to day, the
weather you
experience is related
to your location on
the globe.
2
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6.3 Periodic Trends > Trends in Atomic Size
Trends in Atomic Size
What are the trends among the
elements for atomic size?
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6.3 Periodic Trends > Trends in Atomic Size
In general, atomic size increases from
top to bottom within a group, and
decreases from left to right across a
period.
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6.3 Periodic Trends > Trends in Atomic Size
One way to think about atomic size is to look at
molecules, the units that form when atoms of the
same element are joined together.
• Because the atoms are identical, the distance between
the nuclei (centers) of these atoms can be used to
estimate the size of the atoms.
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6.3 Periodic Trends > Trends in Atomic Size
• The atomic radius is one-half of the distance
between the nuclei of two atoms of the same
element when they are joined.
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6.3 Periodic Trends > Trends in Atomic Size
The distances between atoms in a molecule
are extremely small so the atomic radius is
measured in picometers (pm).
– There are one trillion, or 1012, picometers in a
meter.
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Trends in Atomic Size
The distance between the nuclei in an
iodine molecule is 280 pm.
Distance between the nuclei
280 pm
• Atomic radius is onehalf the distance
between the nuclei, a
value of 140 pm (280/2)
is assigned to the radius
of the iodine atom.
140 pm
Atomic radius
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6.3 Periodic Trends > Trends in Atomic Size
Group Trends in Atomic Size
As the atomic number increases within a
group 2 things occur.
1.) the charge on the nucleus increases
2.) number of occupied energy levels
increases.
• These variables affect atomic size in opposite
ways.
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6.3 Periodic Trends > Trends in Atomic Size
Group Trends in Atomic Size
1.) The increase in positive charge draws
electrons closer to the nucleus. (smaller)
2.) The increase in the number of occupied
orbitals shields electrons in the valence
level from the attraction of protons in the
nucleus. (larger)
• The shielding effect is greater than the effect
of the increase in nuclear charge.
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Interpret Graphs
Group Trends in Atomic Size
Atomic radius (pm)
The atomic radius within a group increases as the
atomic number increases.
• This increase
is an example
of a trend.
Atomic number
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Trends in Atomic Size
Periodic Trends in Atomic Size
Across a period, the electrons are added to the
same principal energy level.
Atomic radius (pm)
• The shielding effect
is constant for all
elements in a period.
Atomic number
• The increasing
nuclear charge pulls
the electrons in the
highest occupied
energy level closer to
the nucleus, and the
atomic size
decreases.
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6.3 Periodic Trends > Trends in Atomic Size
The figure below summarizes the group
and period trends in atomic size.
Atomic
Atomic
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What are the trends for atomic
size?
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What are the trends for atomic
size?
In general, atomic size increases
from top to bottom within a group
and decreases from left to right
across a period.
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6.3 Periodic Trends > Ions
An atom is electronically neutral because it
has equal numbers of protons and
electrons.
• An atom of barium (Ba) has 56 positively
charged protons and 56 negatively charged
electrons.
• The net charge on a barium atom is zero.
(+56) + (-56) = 0
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6.3 Periodic Trends > Ions
Ions
How do ions form?
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6.3 Periodic Trends > Ions
Positive and negative ions form when
electrons are transferred between atoms.
• Metals tend to form ions by losing one or
more electrons from their highest occupied
energy levels.
• Non-metals tend to form ions by gaining one
or more electrons in their highest occupied
energy levels.
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6.3 Periodic Trends > Ions
• An ion is an atom or group of atoms that has a
positive or negative charge.
An ion with a positive charge is called a
cation.
• The charge for a cation is written as a
superscript with a number followed by a plus
sign.
• If the charge is 1+, the number 1 is usually
omitted from the symbol for the ion.
– For example, Na1+ is written as Na+.
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6.3 Periodic Trends > Ions
• An ion is an atom or group of atoms that has a
positive or negative charge.
An ion with a negative charge is called an
anion.
• The charge for an anion is written as a
superscript with a number followed by a
minus sign.
• If the charge is 1-, the number 1 is usually
omitted from the symbol for the ion.
– For example, Br1- is written as Br -.
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What type of element tends to
form anions? What type tends to
form cations?
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What type of element tends to
form anions? What type tends to
form cations?
Nonmetals tend to form anions.
Metals tend to form cations.
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6.3 Periodic Trends > Trends in Ionic Size
Trends in Ionic Size
What are the trends among the
elements for ionic size?
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6.3 Periodic Trends > Trends in Ionic Size
Ionic size tends to increase from top to
bottom within a group. Generally, the
size of cations and anions decreases
from left to right across a period.
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6.3 Periodic Trends > Trends in Ionic Size
During reactions, metal atoms tend to lose
electrons and nonmetal atoms tend to
gain electrons.
• This transfer of electrons has a predictable
effect on the size of the ions that form.
• Cations are always smaller than the atoms
from which they form.
• Anions are always larger than the atoms
from which they form.
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6.3 Periodic Trends > Trends in Ionic Size
Group Trends in Ionic Size
Metals tend to lose all their outermost
electrons when forming ions.
• The ion loses one occupied energy level.
Non-Metals tend to attract electrons when
forming ions.
• The ion fills its valence energy level.
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Trends in Ionic Size
Group Trends in Ionic Size
For each of these elements, the ion is much
smaller than the atom.
• The radius of a sodium ion (95
pm) is about half the radius of a
sodium atom (191 pm).
• When a sodium atom loses an
electron, the attraction between
the remaining electrons and the
nucleus is increased. As a
result, the electrons are drawn
closer to the nucleus.
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Trends in Ionic Size
Group Trends in Ionic Size
The trend is the opposite for nonmetals,
like the halogens in Group 17.
• For each of these elements, the
ion is much larger than the atom.
– For example, the radius of a fluoride
ion (133 pm) is more than twice the
radius of a fluorine atom (62 pm).
– As the number of electrons
increases, the attraction of the
nucleus for any one electron
decreases.
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Trends in Ionic Size
Period Trends in Ionic Size
From left to right across a period, two trends are
visible—a gradual
decrease in the
size of the positive
ions (cations),
followed by a
gradual decrease
in the size of the
negative ions
(anions).
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6.3 Periodic Trends > Trends in Ionic Size
The figure below summarizes the group
and period trends in ionic size.
Size of anions decreases
Ion size generally increases
Size of cations decreases
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What are the trends for ionic
size?
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What are the trends for ionic
size?
Ionic size tends to increase from
top to bottom within a group.
Generally, the size of cations and
anions decreases from left to right
across a period.
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6.3 Periodic Trends > Trends in Ionization Energy
Sometimes an electron overcomes the
attraction of the protons in the nucleus and
escape the atom.
• The energy required to remove an electron
from an atom is called ionization energy.
– This energy is measured when an element
is in its gaseous state.
• The energy required to remove the first electron
from an atom is called the first ionization
energy.
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6.3 Periodic Trends > Trends in Ionization Energy
Trends in Ionization Energy
What are the trends among the
elements for first ionization energy?
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6.3 Periodic Trends > Trends in Ionization Energy
First ionization energy tends to decrease
from top to bottom within a group and
increase from left to right across a period.
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Interpret Data
Ionization Energies of Some Common Elements
Symbol
First
Second
Third
H
1312
He (noble gas)
2372
5247
Li
520
7297
11,810
Be
899
1757
14,840
C
1086
2352
4619
O
1314
3391
5301
F
1681
3375
6045
Ne (noble gas)
2080
3963
6276
Na
496
4565
6912
Mg
738
1450
7732
S
999
2260
3380
1520
2665
3947
K
419
3096
4600
Ca
590
1146
4941
Ar (noble gas
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6.3 Periodic Trends > Trends in Ionization Energy
Ionization energies can help you predict what
charge will be on an ion when it forms.
• It is easy to remove one electron from Group 1
atoms, but difficult to remove a second
electron. (1+ charge)
• It is easy to remove two electrons from a
Group 2 atoms, but difficult to remove a third
electron. (2+ charge)
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6.3 Periodic Trends >
Interpret Graphs
Group Trends in Ionization Energy
First ionization energy (kJ/mol)
Look at the data for noble gases and alkali metals.
Atomic number
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6.3 Periodic Trends > Trends in Ionization Energy
Group Trends in Ionization Energy
• Due to the shielding effect, less energy is
required to remove an electron so the first
ionization energy is lower.
• Due to the effect of nuclear charge increase,
more energy is required to remove an electron
so the first ionization energy is higher.
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Ionization Energy generally decreases
(shielding, electrons are farther away)
6.3 Periodic Trends > Trends in Ionization Energy
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The figure below summarizes the group
and period trends for the first ionization
energy.
Ionization Energy generally increases
(nucleus is stronger)
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6.3 Periodic Trends > Trends in Electronegativity
A property called electronegativity is used
to predict the type of bond that will form
during a reaction.
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6.3 Periodic Trends > Trends in Electronegativity
A property called electronegativity is used
to predict the type of bond that will form
during a reaction.
• Electronegativity is the ability of an atom of
an element to attract electrons when the atom
is in a compound.
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6.3 Periodic Trends > Trends in Electronegativity
Trends in Electronegativity
What are the trends among the
elements for electronegativity?
43
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6.3 Periodic Trends > Trends in Electronegativity
In general, electronegativity values
decrease from top to bottom within a
group. For representative elements, the
values tend to increase from left to right
across a period.
• Metals at the far left of the periodic table
have low values.
• By contrast, nonmetals at the far right
(excluding noble gases) have high values.
• Values among transition metals are not as
regular.
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6.3 Periodic Trends >
Interpret Data
This table lists electronegativity values for
representative elements in Groups 1 through 17.
Electronegativity Values for Selected Elements
H
2.1
45
Li
1.0
Be
1.5
B
2.0
C
2.5
N
3.0
O
3.5
F
4.0
Na
0.9
Mg
1.2
Al
1.5
Si
1.8
P
2.1
S
2.5
Cl
3.0
K
0.8
Ca
1.0
Ga
1.6
Ge
1.8
As
2.0
Se
2.4
Br
2.8
Rb
0.8
Sr
1.0
In
1.7
Sn
1.8
Sb
1.9
Te
2.1
I
2.5
Cs
0.7
Ba
0.9
Tl
1.8
Pb
1.9
Bi
1.9
• The data
in this
table is
expressed
in Pauling
units.
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6.3 Periodic Trends > Trends in Electronegativity
The least electronegative elements are
cesium and francium, with an
electronegativity of 0.7.
• They have the least tendency to attract
electrons.
• When they reacts, they tends to lose
electrons and form cations.
46
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6.3 Periodic Trends > Trends in Electronegativity
The most electronegative element is
fluorine, with a value of 4.0.
• Because fluorine has such a strong
tendency to attract electrons, when it is
bonded to any other element it either
attracts the shared electrons or forms an
anion.
47
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6.3 Periodic Trends > Trends in Electronegativity
This figure summarizes several trends that
exist among the elements.
48
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What are the trends for
electronegativity values?
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What are the trends for
electronegativity values?
In general, electronegativity values
decrease from top to bottom within
a group. For representative
elements, the values tend to
increase from left to right across a
period.
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6.3 Periodic Trends > Key Concepts
In general, atomic size increases from top
to bottom within a group and decreases
from left to right across a period.
Positive and negative ions form when
electrons are transferred between atoms.
First ionization energy tends to decrease
from top to bottom within a group and
decrease from left to right across a
period.
51
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6.3 Periodic Trends > Key Concepts
Ionic size tends to increase from top to
bottom within a group. Generally, the
size of cations and anions decreases
from left to right across a period.
In general, electronegativity values
decrease from top to bottom within a
group. For representative elements, the
values tend to increase from left to
right across a period.
52
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6.3 Periodic Trends >
END OF 6.3
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