Ionization energy
In the last Chapter, on slides 59-61, we saw
the ionization energy required to remove an
electron from the ground state of hydrogen
and ions with one electron.
We can now talk about the ionization energy
of atoms with more than one electron. Each
electron that is removed has it’s own
ionization energy which changes because of
changes in electron configuration.
Chapter 9
The Periodic Table and Some Atomic Properties
Dr. Peter Warburton
[email protected]
http://www.chem.mun.ca/zcourses/1050.php
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Ionization energy
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Ionization energy
Ionization energies
decrease as atomic
radii increase.
The ionization energy is the amount of
energy a gaseous atom (or ion) must
absorb to eject one of the outermost
electrons (which are the least
electrostatically bound to the nucleus).
Mg(g) → Mg+(g) + e-
I1 = 738 kJ
Mg+(g) → Mg2+(g) + e-
I2 = 1451 kJ
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Notice you can see changes from s
block to p/d block and d block to p/f
block! Also, within the d or f block
ionization energy is fairly constant.
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Ionization energy
Ionization energy
In actuality, the ease of removing the electron is a
function of the effective nuclear charge. As this gets
smaller, the ionization energy gets smaller. Since Zeff
changes dramatically when we change shells, we see
dramatic changes in ionization energies when we
change the shell the electron comes from.
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Since Zeff changes significantly when we change
subshells (p to s block, for example), we see
significant changes in ionization energies when we
change the subshell the electron comes from. This is
also true when we pair the first electron of a subshell,
since pairing increases the energy of an orbital
(electron repulsion).
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Problem
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Problem answers
Using just a periodic table:
a) organize Cl, K, Mg and S in terms of
increasing first ionization energy.
b) Which element is in the middle in terms
of first ionization energy in the set: Rb,
As, Sb, Br, Sr
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a) K< Mg < S < Cl in terms of I1
b) Sb
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Electron affinity
Electron affinity
We’ve seen that nonmetals tend to gain
electrons to form ions. The molar enthalpy
change for a gaseous atom to gain an
electron is called the electron affinity.
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Second electron affinity
∆Hea in kJ mol-1
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Magnetic properties
We saw in the Stern-Gerlach experiment (slide
139 of the last Chapter) that silver atoms can be
deflected by a magnetic field due to the spin of
one unpaired electron.
If we try and put a second electron into the
atom, this will be the second electron
affinity. This is usually energetically
unfavorable since we are adding a
negative charge to an ion that is already
negative!
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Generally electron affinities are large and negative (exothermic) when the
electron is put “close” to the nucleus. However, no clear pattern is seen.
Atoms or ions that have unpaired
electrons can be deflected by a
magnetic field and are called
paramagnetic. More unpaired
electrons mean stronger deflection.
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Magnetic properties
Manganese and its ions are paramagnetic
Atoms or ions that have no
unpaired electrons and
can not be deflected by a
magnetic field (actually
slightly repelled by it) are
called diamagnetic.
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Problems
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Problem answers
a) Which of the following are
paramagnetic, and which are
diamagnetic: Zn, Cl, K+, O2- and Al?
b) Which has the greater number of
unpaired electrons (and explain): Cr2+ or
Cr3+?
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a) Paramagnetic: Cl, Al
Diamagnetic: Zn, K+, O2b) Cr2+
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Summary of some properties
Variations down a group
Many properties of elements tend to change
uniformly as we gown down the period (column
of the periodic table). With this in mind, we can
estimate the properties of Br2 or At2 by knowing
the properties of Cl2 and I2.
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Problem
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Problem answer
Estimate the melting and boiling points of Br2
and At2 using the data below.
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melting point of Br2: 280 K (actual: 266 K)
boiling point of Br2: 349 K (actual: 332 K)
melting point of At2: 494 K (actual: 575 K)
boiling point of At2: 567 K (actual: 610 K)
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Variations down a group
Variations across a period
Many properties of the
molecular compounds of
the elements of a period
ALSO tend to change
uniformly as we gown
down the period (column
of the periodic table).
However, certain major
exceptions occur. We
can explain this later with
intermolecular forces!
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Because of the wide variety of effects of atomic
size and other features, it’s hard to predict most
properties across a row of the periodic table!
Only ability to conduct heat and electricity
decrease somewhat regularly across a row.
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Text pages 383-385
Read and understand the abilities of
elements to reduce and oxidize other
chemicals, as well as acid-base properties
of elemental oxides yourself.
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