Trends on the Periodic Table
Trends exist among elements both vertically and horizontally
on the periodic table. Knowing the trends helps you predict
chemical behavior.
Atomic Radius
Atomic radii are obtained by
measuring ½ the distance
between two atoms in a covalent
compound.
Atomic radii are typically listed in
picometers.
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What affects the size of the radius?
 The attraction of the protons
 The repulsion of inner shell
electrons
Lithium
Valence shell = 2nd energy level
2 electrons repelling the outer
shell electrons shielding the
effect of the protons pulling
Sodium
Valence shell = 3rd energy level
10 electrons repelling the outer
shell electrons (3nd energy level)
shielding the effect of the protons
pulling
The valence shell is farther out,
and the shielding effect is the
greater, so, the radius is larger as
you go down a family
2
Lithium
3 protons pulling
2 electrons repelling the outer
shell electrons shielding the
effect of the protons pulling
Neon
10 protons pulling
2 electrons repelling the outer
shell electrons shielding the
effect of the protons pulling
The shielding effect is the
same, and the effective
nuclear charge is greater, so,
the radius is smaller as you go
across a period.
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Electronegativity
Valence electrons are responsible for holding atoms together
in a chemical compound. Electronegativity is a measure of
how strongly one atom attracts another atom’s valence
electrons within a compound.
Pauling Scale
Electronegativity is measured using the Pauling scale
4.0 = largest EN value
arbitrary
no units
every other atom is relative to this value
Lithium oxide = Li2O
Each Li = 1.0
Oxygen = 3.5
The difference in electronegativity is so great that each lithium loses its
valence electron to the oxygen (which has a much higher EN).
*He, Ne and Ar don’t have EN values – why? They don’t
form compounds. (Notice, some noble gases do have EN
values, that means these noble gases CAN form compounds)
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Which atoms have higher electronegativity?
Fluorine can pull more strongly on the
shared electrons (green), so it has a higher
electronegativity. Fluorine is able to pull
more strongly because the shared electrons
are closer to the fluorine nucleus. (So, the
smaller the nucleus, the higher the EN)
Fluorine
Chlorine
Ionization Energy
Ionization energy is the amount of energy required to
remove a valence electron from a gaseous atom or ion.
Al(g) --> Al+1(g) + eAl+1(g) --> Al+2(g) + eAl+2(g) --> Al+3(g) + eAl+3(g) --> Al+4(g) + e-
I1
I2
I3
I4
=
=
=
=
580 kJ/mol
1815 kJ/mol
2740 kJ/mol
11,600 kJ/mol
kJ/mol =
measure of
energy
To remove a valence e-, you have to work against the
attraction of the protons in the nucleus, so…
…it shouldn’t take a lot
of energy to remove a
valence electron from
Francium because it is
so far away from the
nucleus. (The larger the
radius, the lower the
ionization energy.)
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Review questions (remind students to draw Bohr models)
1. Which atom has…
 a larger atomic radius, chlorine or silicon? Explain.
Si – the shielding effect is the same for both atoms
(same number of inner shell electrons interfering with
the protons pull on the valence shell), and silicon has
less protons pulling on the valence shell, so there is a
lower effective nuclear charge.
 a smaller atomic radius, phosphorus or arsenic? Explain.
(P – less energy levels, lower shielding effect)
2. Which atom has…
 higher ionization energy, bromine or iodine?
Explain. Br has a smaller radius, so it would be
harder to remove a valence electron.
 lower ionization energy, tin or iodine? Sn has a
larger radius, so it would be easier to remove a
valence electron.
3. Which atom has…
 higher EN, sulfur or oxygen? Sulfur, a smaller atomic
radius is able to pull more strongly on a shared electron
within a compound.
 lower EN, phosphorus or chlorine? Phosphorus has a
larger atomic radius, so it isn’t able to pull as strongly
on shared electrons within a compound.
6
What metals do for a living…
Metals form cations, to do this they lose valence electrons,
so, the metals that lose electrons the easiest are the best
metals (or, in other words, are the most metallic). Which
metals on the periodic table lose electrons the most readily?
(those with the lowest ionization energy, so, Fr is the most
metallic element on the periodic table)
What do nonmetals do for a living?
Nonmetals form anions, to do this, they gain electrons. This
means that the nonmetals that gain electrons the easiest are
the most nonmetallic, or in other words, least metallic
elements on the periodic table. The elements that gain
electrons the easiest are those with the highest EN, so F is
the least metallic element on the periodic table.
Periodic Trends of Atomic Radius,
Ionization Energy and Metallic
Character
Electronegativity
Electronegativity
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