The Periodic Table and its Trends
Effective Nuclear Charge (Zeff)
• e- are pulled to the nucleus because they are
negative, and the nucleus is positive.
• In larger atoms the outer electrons are
shielded from the positive nucleus because
the core electrons block the “positive” charge
of the nucleus.
• Zeff is essentially the positive charge that a
valence electron "sees".
Effective Nuclear Charge Cont.
• Z = nuclear charge
• S = screening constant
• S is usually close to the number of core electrons in the
atom
• Zeff = Z - S
Trends in Zeff
• As you go across a row of the PT, the Zeff
increases.
– WHY?
• Because # of core electrons is the same, but the nuclear
charge is increasing because of more protons
(increasing atomic number)
• As you go down a column of the PT, the Zeff
only slightly increases.
• We would calculate them to all be the same, however a
larger nucleus is harder to shield
Atomic Radii
• There are two ways that scientists can look at the
size of an atom: 1. The radius of an atom when it
is not bonded (say in a gas phase) or 2. when it is
bonded to something
• Which do you think is a larger radius, nonbonding
or bonding?
• (Nonbonding)
– WHY?
• Nonbonding- the atoms don’t want to be together, so they
will just bounce off each other. When they are bonded, they
are attracted in some way, and therefore the radius will be
smaller.
Bonding Radii
• What is most often used when talking about
atom size
• Found by looking at bond lengths between
atoms.
• Figure 7.7 page 261 shows the different
atomic radii
Trends in Atomic Radii
• As you go down the rows, the atoms get ________
– bigger
– WHY??
• Electrons move out to outer shells that are further away
from the nucleus
• As you go right across the rows, atomic radii get
_________.
– smaller
– WHY?
• the effective nuclear charge increases, pulling the e- in
further. Even though e- are added too….they are still in the
same shell
Radii of IONS
• Cations are ________ than their parent atom
• Smaller
– WHY?
• Cations have given up electrons, and gotten rid of an
electron shell
• Anions are ________ than their parent atom
• Larger
– WHY?
• More electons means more e-/e- repulsions, which
results in the electrons spreading out more.
Ionization Energy
• The energy required to remove an electron
from an atom. (to create a POSITVE ion)
• A Lithium atom wants its electron to be
removed, so that it has a full shell (Li+)
• Flourine DOES NOT want an electron
removed, it wants to gain another electron.
1st, 2nd, 3rd Ionization Energies
• The 1st Ionization energy is the energy required to
remove the first electron from an atom, the 2nd
ionization energy is the energy required to
remove the second electron from an atom……etc.
• Which is higher, the 1st or 2nd ionization energy
for Lithium?
– 2nd
• The 1st electron wants to be removed, so it doesn’t take a lot
of energy, but the 2nd is in a full outer shell (like a noble gas)
so it doesn’t want to be removed.
1st Ionization Energy Trend
• As you move ACROSS the rows, it become
harder to pull off electrons (therefore the
energy required to remove an electron is
higher, ionization energy increases)
• As you move DOWN the columns, it becomes
easier to pull off electrons (they are further
from the nucleus and are not held in as
tightly)
The orange color shows where a large change in ionization energy occurs. This
large change occurs once the full octet (Noble Gas Configuration) has been
achieved, because electrons really don’t want to leave that stable octet.
Electron Affinity
• The change in potential energy when an
electron is added.
• The ease at which an atom gains an electron
• Electron affinities are thought of as negative
(release of energy- exothermic process)
• If there is a large energy change, then a stable
ion is formed. If there is a small energy
change, then the ion is not stable.
Noble gases are not
included in the trend.
Halogens have the highest
affinity. Noble gases don’t
want any more electrons
(their outer shells are full)
Trends in Electron Affinity
• Electron affinity increases across a row
because the neutral atoms are more likely to
gain an electron to have a full shell.
• Electron affinity decrease only slightly down a
column because the additional electron will
be entering an orbital farther away from the
nucleus, and thus would experience a lesser
effective nuclear charge.
Interesting trends in Electron Affinities
• The electron affinity values are not what they
are expected to be for atoms in group 15.
• It seems as though this group doesn’t want
that electron as much as one would think…
– Why?
• Because the p orbital is currently half filled….adding
one more electron disrupts this partial stability.
» Both half filled and fully filled orbitals create stability in
the electronic structure of an atom
Electronegativity
• (very similar to electron affinity)
• The tendency for an atom to attract an electron
• Fluorine has the highest electronegativity
• Trend is the same as electron affinity (and noble
gases are excluded once again from the trend)