Periodic Trends!
Periodic Law
When elements are arranged
in order of increasing atomic
number, there is a periodic
pattern in their physical and
chemical properties.
There are many trends in the
periodic table, but you must
know the reasoning behind 3 of
them
Atomic Radius:
Gives us an idea of the size of an
atom
 The trend:
A.R. increases as you move down
a group
A.R. decreases as you move
across a period from left to right
Why?
 As atomic # increases, the number of protons and electrons
increases
 To accommodate the electrons, there must be more energy levels.
 This is why A.R. increases as you move down a group.
 As you go across a period, an atom is not gaining energy levels
 The atom is still gaining protons and electrons, though, so the atom
becomes more and more dense
 This causes the electrons to be drawn closer to the positive nucleus.
 This is why A.R. decreases as you move across a period.
Ionization Energy
 The energy required to remove an
electron from the valence shell of an
atom
 The trend:
 I.E. decreases as you go down a group
 I.E. increases as you go across a
period from left to right
Draw the arrows for the trend on the
periodic table sheet!
Why?
As you go down a group, the atomic radius increases
 This means that the valence electrons in a large atom are further
away from the nucleus, than the valence electrons in a small atom
 If the valence electrons are far away from the positive nucleus, it
will take less energy to remove them. Large atoms tend to be lower
down in a group.
 This is why I.E. decreases as you go down a group.
 As you go across a period, atomic radius decreases
 This means the electrons will be held closer and closer to the
positive nucleus as you go across the period
 This means that atoms of elements to the right of the periodic table
will have valence electrons that are harder to remove.
 This is why I.E. increases as you go across a period.
Electronegativity
 the tendency of an atom to attract
electrons to itself when it is chemically
combined with other elements
 The trend:
 E.N. decreases as you go down a
group
 E.N. increases as you go across a
period left to right
Draw the arrows for the trend on the
periodic table sheet!
Why?
 As you go down a group, the atomic radius increases
 This means that in elements lower down in the group,
the additional electrons will not feel a strong attraction
to the nucleus, which is already surrounded by
electrons.
 This is why E.N. decreases as you go down a group.
 As you go across a period, the atomic radius decreases
 This means that in elements further to the right,
additional electrons will feel a stronger attraction to the
nucleus than in elements further to the left.
 This is why E.N. increases as you go across a period.