Transcript Trends in Atomic Radius, Ionization Energy and

Trends in Atomic
Radius, Ionization
Energy and
Electronegativity
Trends in Atomic Radius, Ionization
Energy and Electronegativity can be
explained by
• Strength of the Nucleus
(As the number of protons in the nucleus
increases, the nucleus is more attractive to
electrons.)
• Shielding
Shielding means “blocking”. If electrons are
shielded or “blocked” they won’t feel the
attractive pull of the nucleus.
Analogy
A street performer is like a nucleus. It could
be attractive to its audience, but one must
consider the following:
• How talented is the performer—how
strongly does he or she attract onlookers?
• How much are the onlookers blocked from
being able to see the performer?
By considering both the strength
of the nucleus and the amount
of shielding electrons
experience, we can describe
and explain trends in atomic
radius, ionization energy and
electronegativity.
Atomic Radius
• A way to measure the
volume of an atom
• Measured by
measuring the
distance between two
nuclei and halving
that distance
Trends in Atomic Radius
• As you go from
the top of a group
to the bottom,
atomic radius
increases.
• As you go from
the left of the
period to the right,
atomic radius
decreases.
Atomic Radius Trends Explained
• Group Trends
From the top of a group to the bottom of a group,
the number of protons increases, but so does
the number of energy levels. Nuclear charge
increases, but so does shielding.
Electrons in the outer energy levels are more
shielded, and they don’t experience the full
attractiveness of the nucleus and they tend to
“stray” further, making the radius larger.
Trends in Atomic Radius
Continued…
• Period Trends
As you go from left to right across the
period, the nuclear charge increases, but
the shielding does not.
The nucleus gets stronger, and all electrons
in the period feel the same “draw” toward
the nucleus.
We can explain trends in
ionization energy and
electronegativity in the
same way.
Ionization Energy
• The amount of energy required to remove
an electron
• A high ionization energy means a lot of
energy is required to remove an electron
• First ionization energy = the amount of
energy required to remove the first electron
• Second ionization energy = the amount of
energy required to remove the second
electron (after the first has already been
removed)
Trends in First Ionization Energy
• As you go from the top
of a group to the
bottom, 1st ionization
energy decreases (It
gets easier to remove 1
e-).
• As you go from the left
of the period to the
right, 1st ionization
energy increases (It
gets harder to remove 1
e-).
Trends in Ionization Energy
Explained
• Group Trends
From the top of a group to the bottom of a group,
the number of protons increases, but so does
the number of energy levels. Nuclear charge
increases, but so does shielding.
Electrons in the outer energy levels are more
shielded, and they don’t experience the full
attractiveness of the nucleus so it is easier to
pull them away from the nucleus.
Explanation of Trends in Ionization
Energy Continued…
• Period Trends
As you go from left to right across the
period, the nuclear charge increases,
but the shielding does not.
The nucleus gets stronger and electrons
are held more tightly, making them
harder to remove.
Comparing first, second and
third ionization energies
Not only is it useful to analyze
trends in first ionization energy,
but we can use differences in first,
second and third ionization
energies to predict the kind of ion
an ion will form
Some values for elements of the third period are given in
the following table:
Sodium
Magnesium
Aluminum
1st ionization
energy
(kJ/mole)
2nd ionization
energy
(kJ/mole)
3rd ionization
energy
(kJ/mole)
4th ionization
energy
(kJ/mole)
496
738
577
4,560
1,450
1,816
6,912
7,730
2881
10,542
11,600
Large jumps in the successive molar ionization energies occur when
passing noble gas configurations. For example, as can be seen in
the table above, the first two molar ionization energies of
magnesium (stripping the two 3s electrons from a magnesium atom)
are much smaller than the third, which requires stripping off a 2p
electron from the very stable neon configuration of Mg2+.
Practice
Identify which unknown element is Lithium
and which is Beryllium
1st ionization energy 2nd ionization
(kJ/mole)
energy (kJ/mole)
3rd ionization
energy
(kJ/mole)
Unknown A
520
7,297
11,810
Unknown B
899
1,757
14,840
Electronegativity
• The ability of an atom to attract electrons
to itself
• Only measured when the atom is
combined in a compound
• Linus Pauling was the first to define and
measure electronegativity. The units are
named after him and range from 0.7-4.0
Paulings
Trends in Electronegativity
• As you go from the
top of a group to the
bottom,
electronegativity
decreases.
• As you go from the left
of the period to the
right, electronegativity
increases.
Trends in Electronegativity
Explained
• Group Trends
From the top of a group to the bottom of a
group, the number of protons increases,
but so does the number of energy levels.
Nuclear charge increases, but so does
shielding.
Electrons in the outer energy levels are
more shielded, and they aren’t as attracted
to the nucleus.
Explanation of Trends in
Electronegativity Continued…
• Period Trends
As you go from left to right across the
period, the nuclear charge increases,
but the shielding does not.
The nucleus gets stronger and it is
better able to attract electrons
• Note: electronegativity of the Noble
gases can not be measured.
Based on these trends…
1) What do you expect to be the
most reactive metal?
2) What do you expect to be the
most reactive nonmetal?
(Hint: What do metals do to react? How do you measure
that? What is the trend in that property? Hint what do
nonmetals do to react? How do you measure that? What is
the trend in that property?)