Download Period Trend

Objectives:
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Define atomic and ionic radii, ionization energy, electron affinity, and electronegativity
Compare the periodic trends of atomic radii, ionization energy, and electronegativity, and state the reasons for
these variations
Define valence electrons, and state how many are present in atoms of each main-group element
Compare the atomic radii, ionization energies, and electronegativity of the d-block elements with those of the
main-group elements
PERIODIC TRENDS
Atomic Radii
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Atomic radius: one-half the distance between the nuclei of identical
atoms that are bonded together
Period Trends
 Atomic radii decrease from left to right across a period
 Trend is caused by the increasing positive charge of the nucleus
Group Trends
 In general, the atomic radii of the main-group elements increase
down a group
 As electrons occupy the sublevels in successively higher main
energy levels located farther from the nucleus, the sizes of the
atoms increase

Atomic radii decreases from left to right across a period and increases down a group
Atomic Radius vs. Atomic Number
Plot of atomic radius versus atomic number shows period and group trends
Sample Problem
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Of the elements magnesium, Mg, chlorine, Cl, sodium, Na, and
phosphorus, P, which has the largest atomic radius? Explain your
answer in terms of trends in the periodic table.
 1. Find the elements listed in the periodic table.
 2. Put the elements in order and determine the trend.
 Sodium, Na.
 Because atomic radii decreases across a period.
Of the elements calcium, Ca, beryllium, Be, barium, Ba, and
strontium, Sr, which has the largest atomic radius? Explain your
answer in terms of trends in the periodic table.
 1. Find the elements listed in the periodic table.
 2. Put the elements in order and determine the trend.
 Barium, Ba.
 Because atomic radii increases down a group.
You Try!
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Of the elements Li, O, C, and F, identify the
one with the largest atomic radius and the
one with the smallest atomic radius.
 Answer:
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largest: Li ; smallest: F
Of the elements Br, At, F, I, and Cl, identify
the one with the smallest atomic radius
and the one with the largest atomic
radius.
 Answer:
smallest: F ; largest: At
Ionization Energy
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An electron can be removed from an atom if enough energy is
supplied
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Process can be expressed as follows for an atom of any element:
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Ion: an atom or group of bonded atoms that has a positive or
negative charge
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A + energy  A+ + e-
Positive ion = removal of electron
Negative ion = addition of electron
Ionization: any process that results in the formation of an ion
Ionization energy (IE) : the energy required to remove one
electron from a neutral atom of an element

Indication of how strongly an atom’s nucleus holds onto its
electrons
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Period Trends
 Ionization energy generally increases as you move across
each period
 Trend caused by the increase nuclear charge
  a higher charge more strongly attracts electrons in
the same energy level
Group Trends
 Ionization energies generally decreases down the groups
 Trend caused by an increase in atomic size as you move
down a group
  with outer electrons farther from the nucleus, it
requires less energy to remove them
Electron Affinity
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Electron affinity: the energy change that occurs when
an electron is acquired by a neutral atom
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Most atoms release energy when they acquire an
electron
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A + e-  A- + energy
Quantity of energy released noted with a negative number
Some atoms must be “forced” to gain an electron by
the addition of energy
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A + e- + energy  A+
Quantity of energy absorbed noted with a positive number
Ion produced this way will be unstable and will lose the
added electron spontaneously

Period Trends
In general, electron affinities become more negative
across each period within the p block
 Halogens gain electrons most readily
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Group Trends
Trends within group not as regular; electron affinities
change little moving down a group
 General rule: electrons add with greater difficulty down
a group
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Pattern due to increase in nuclear charge down a group
(increases electron affinity) and increase in atomic radius
down a group (decreases electron affinity)
Ionic Radii
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Cation: positive ion
Formation by the loss of one or more electrons
 Leads to a decrease in atomic radius because the
removal of the highest-energy-level electrons
 Remaining electrons are drawn closer to the
nucleus by its unbalances positive charge
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Anion: negative ion
 Formation
by the addition of one or more electrons
 Leads to an increase in atomic radius
 Electron cloud spreads out due to greater repulsion
between the increased number of electrons

Period Trend
Metals to the left tend to form cations and the
nonmetals at the upper right tend to form anions
 In general, as you move left to right across a period, the
size of the positive ions gradually decreases
 Cation radius decreases across a period due to the
electron cloud shrinking
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Caused by increase nuclear charge acting on the electrons
in the main energy level
Group Trends
Gradual increase in ionic size as you move down a
group
 Trend caused by the outer electrons in both cations and
anions being in higher energy levels

Valence Electrons
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Valence electrons: the electrons available to be
lost, gained, or shared in the formation of
chemical compounds
Chemical compounds form because electrons
are lost, gained, or shared between an atom
Electronegativity
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Electronegativity: measure of the ability of an
atom in a chemical compound to attract
electrons
 Valence
electrons hold atoms together in chemical
compounds

Period Trends
 Electronegativities
tend to increase across each
period, although there are exceptions
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Group Trends
 Electronegativities
tend to either decrease down
a group or remain about the same
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Noble gases are unusual in that some of
them do not form compounds and cannot
be assigned electronegativities
The lowest electronegativities are found at
the lower left side of the periodic table,
while the highest electronegativities are
found at the upper right
Periodic Properties of the d- and
f-Block Elements

Properties of the d-block element vary less
and with less regularity than those of the
main-group elements
 Difference
is due to the presence of electrons in
incompletely filled d sublevels in the atoms of the
d-block elements