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PERIODIC TRENDS
Atomic size
Ionization energy
Atomic Size
• For the main group elements (s & p blocks),
atomic radii increase going down a group
and decrease from left to right across a
period.
3
Atomic Radius
2.5
K
Rb
Na
2
Li
1.5
1
Ar
Kr
Ne
0.5
He
0
0
20
40
Atomic Number
60
Putting Trend to Use
• Using the periodic table, arrange the
following in order of increasing
atomic radius:
Br, Te, Se
Ionization energy, IE, is the amount of energy
required to remove an electron from an isolated,
gas phase atom.
Li(g)
Li1+(g) + e1-
• Energy is always required for this process.
• IE generally decreases going down a group
and generally increases from left to right
across a period.
Electron Affinity
• Energy released when a lone, gaseous atom
gains an electron.
F + e-  F-
Electron Affinity, EA, is the amount of
energy released when a free, gas phase
atom gains an electron.
A(g) + e1-
A1-(g)
EA increases (i.e., more energy is released)
as we move to the top and right of the
periodic table.
Ionic Size
•All positive ions are smaller than
the atoms from which they were
formed
•All negative ions are larger than
the atoms from which they were
formed.