Periodic Trends
Atomic Radius
Definition: Half of the distance
between nuclei in covalently bonded
diatomic molecule
Radius decreases across a period
 Increased Zeff due to decreased
shielding thereby pulling e- cloud
closer to the nucleus
Radius increases down a group
 Each row on the periodic table adds
a “shell” or energy level to the atom
increasing distance the nucleus must
pull
Table of
Atomic
Radii
Period Trend:
Atomic Radius
Ionization Energy
Definition: the energy required to remove an
electron from an atom
 Tends to increase across a period
 As radius decreases across a period,
the electron you are removing is closer
to the nucleus and harder to remove
 Tends to decrease down a group
 Outer electrons are farther from the
nucleus and easier to remove
Periodic Trend:
Ionization Energy
Electronegativity
Definition: A measure of the ability of an
atom in a chemical compound to attract
electrons
o Electronegativity tends to increase
across a period
o As radius decreases, electrons get
closer to the bonding atom’s nucleus
o Electronegativity tends to decrease
down a group or remain the same
o As radius increases, electrons are
farther from the bonding atom’s
nucleus
Periodic Table of Electronegativities
Periodic Trend:
Electronegativity
Ionic Radius
Definition: distance from the nucleus to the
outer edge of the electron cloud in a charged
ion. (same general trend as atomic radius)
Electron Affinity
Definition: the energy gained or lost with the
addition of an electron to a gaseous atom or
ion.
(becomes more negative)
Decreasing electron affinity (becomes more positive)
Summary of
Periodic Trends
Ionic Radii
Cations
(same trend as atomic radii)
 Positively charged ions formed when
an atom of a metal loses one or
more electrons
 Smaller than the corresponding
atom
 Negatively charged ions formed
when nonmetallic atoms gain one
Anions
or more electrons
 Larger than the corresponding
atom
Graphic courtesy Wikimedia Commons user Popnose
VALENCE ELECTRONS
• Valence electrons – the electrons available to be
lost, gained, or shared in the formation of
chemical compounds.
• They are the electrons located in the outermost
energy shell in an atom
• Atoms are most energetically stable when they
have 8 valence electrons