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Periodicity
AP Chemistry
By
Diane Paskowski
Valence electrons
• The valence electrons (outer e-) in atoms are those
shared/transferred to form bonds between atoms.
• The number of valence electrons will affect the type(s) of
bonds an atom can form with a specific element.
• The number of valence electrons in an atom can be
determined from the electron configuration or just by
looking at its location on the periodic table.
Valence electrons cont’d
Valence electrons can be predicted for the
main or representative elements (s and p
blocks) very easily.
Effective Nuclear Charge
The determining factor
in most periodic
properties
The "pull" that the
specific electron
"feels" from the
nucleus
Ionization energy
• The energy required (added to an atom) to
remove 1 valence electron. This value is
always positive.
– First ionization energy is the energy required
to remove the first electron
– Second ionization energy is the energy
required to remove the second electron. It is
“always” more energy than the first ionization.
– Third ionization energy is . . .
Ionization Energy cont’d
Students should be able to
predict which metal ion
would most likely be
formed from the ionization
energies of an element
Trend on the periodic table
• Increases across because
the effective nuclear
charge increases.
• Decreases down a column
because the effective
nuclear charge decreases
Ionization Energy cont’d
Atomic Radii
The size of an atom. Measured by bonding the atom
with another, measuring the distance between them,
and then subtracting to find the unknown radius.
Atomic Radii cont’d
 Trends
 Decreases across a period (greater ENC)
 Increases down a column (more energy levels)
Atomic Radii in 3-D
Main Group Elements Only
Electron Affinity
• The energy released (or needed) when an
atom takes in an extra electron.
Electron Affinity
• Elements that have high electron affinities
release a lot of energy when an electron is
taken in
• Some elements that have low electron
affinities require energy to be added to the
atom in order to take in an electron
• General Trends
• increase across a row
• decrease down a column
Electron Affinity
There are many exceptions to this general
trend due to the lower energy of half-filled
sublevels and orbitals. This is a “bumpy”
trend.
Electron Affinity Graph
Electron Affinity “Bumps”
• The difference between Na and Mg is due
to an additional of an electron in Na
completes the 3s sublevel which is a more
favorable state than a half-filled 3s
sublevel.
• Carbon will achieve a lower energy state
by taking an electron and half-filling the 2p
sublevel than nitrogen which on additional
electron will disturb a half-filled sublevel.
Ionic Radii
Cations are smaller than the corresponding atom.
Ionic Radii cont’d
Anions are larger than the corresponding atoms.
Applications of Periodic Trends
• Predict chemical properties of elements
based on their location on the periodic
table
– Metals: lower electron affinity and lower
ionization energy. Tend to contribute
electrons to bonds
– Nonmetals: higher electron affinity and higher
ionization energy. Tend to share or take in
electrons in bonds.
Student Constructed Periodic
Tables
Ionization Energy
Density
Different Design