Periodic Trends of the Elements

The properties of the elements exhibit certain trends, which can be explained in terms of the
position of the element in the periodic table, or in terms of the electron configuration of the
element. Elements in general seek to gain or lose valence electrons so as to achieve the stable octet
formation possessed by the inert or noble gases of Group VIII (last column of the periodic table). Two
other important general trends exist. First, as one goes from left to right across a period, electrons
are added one at a time; the electrons of the outermost shell experience an increasing amount of
nuclear attraction, becoming closer and more tightly bound to the nucleus. Second, as one goes
down a given column, the outermost electrons become less tightly bound to the nucleus. This is
because the number of filled principal energy levels (which shield the outermost electrons from
attraction by the nucleus) increases downward within each group. These trends help explain
elemental properties such as atomic radius, ionization potential, electron affinity, and
electronegativity.

ATOMIC RADII

The atomic radius is an indication of the size of an atom. In general, the atomic radius decreases
across a period from left to right and increases down a given group; the atoms with the largest
atomic radii will therefore be found at the bottom of groups, and in Group I.

As one moves from left to right across a period, electrons are added one at a time to the outer
energy shell. Electrons in the same shell cannot shield one another from the attractive pull of
protons very efficiently. Therefore, since the number of protons is also increasing, producing a
greater positive charge, the effective nuclear charge increases steadily across a period, meaning that
the valence electrons feel a stronger and stronger attraction toward the nucleus. This causes the
atomic radius to decrease.

As one moves down a group of the periodic table, the number of electrons and filled electron shells
will increase, but the number of valence electrons will remain the same. Thus, the outermost
electrons in a given group will feel the same amount of effective nuclear charge, but electrons will