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periodicity
With increasing atomic number, the electron
configuration of the atoms display a periodic variation. Because of
this the elements show periodic variations of both physical and
chemical behavior. The periodic law is a law stating that when the
elements are arranged by atomic number, their physical and chemical
properties vary periodically. We are going to be looking at three
physical properties of an atom: atomic radius, ionization energy,
and electron affinity.
atomic radius
The size of the electron cloud increases as the
principal quantum number increases. Therefore, as you look down the
periodic table, the size of atoms in each group is going to
increase. When you look across the periodic table, you see that all
the atoms in each group have the same principal quantum number.
However, for each element, the positive charge on the nucleus
increases by one proton. This means that the outer electron cloud
is pulled in a little tighter. One periodic property of atoms is
that they tend to decrease in size from left to right across a
period of the table. So finally we have a good definition for how
the atomic radii increases: the atomic radii increases top to
bottom and right to left in the periodic table
ionization energy
The energy needed to remove the most loosely held
electron from an atom is known as ionization energy. Ionization
energies are periodic. The ionization energy tends to increase as
atomic number increases in any horizontal row or period. In any
column or group, there is a gradual decrease in ionization energy
as the atomic number increases. Metals typically have a low
ionization energy. Nonmetals typically have a high ionization
energy.
electron affinity
The attraction of an atom for an electron is
called electron affinity. Metals have low electron affinities while
nonmetals have high electron affinities. The general trend as you
go down a column is a decreasing tendency to gain electrons. As you
go across a row there is also a trend for a greater attraction for
electrons. |