Download Chemistry Ch 5-3 Notes: Periodic Trends

Chemistry Ch 5-3 Notes: Periodic Trends
The chemical and physical properties of
elements vary predictably based on their location
in the periodic table. Examples:
1.) Atomic Radius: The distance from the
center of the nucleus to the outermost
electron in the cloud. Radius increases
going down a family (more electrons but
the same valence number, so a larger
outer valence). Radius decreases as you
move from left to right on a period (row).
Why??? By increasing the atomic number
without increasing the sublevel, we have a
stronger attraction to the nucleus, so the
size of the electron cloud shrinks. This is
why the alkali metals are very reactive
(outer electron is far from the nucleus),
and noble gases are unreactive (outer
electrons near the nucleus, and held
tightly).
2.) Ionic Radius: Gaining or losing electrons
influences the size of the radius. Cations
(+) become smaller than the neutral atom,
and are therefore more stable. Anions (-)
become larger due to repulsive forces
between electrons, but are more stable
because their valence is full.
3.) Ionization energy: Is the amount of energy
needed to remove the outermost electron
from an atom. Ionization energy decreases
as you move down a family because the
electron cloud is larger, and the outermost
electron is farther from the nucleus. Also,
the inner electrons shield the nucleus from
the outer electrons. Ionization energy
increases as we move from left to right,
because the larger number of protons
holds onto electrons more strongly. The
most difficult elements to remove electrons
from are the noble gases (full valence)
Also: The more electrons we try to
remove, the harder it is to remove them, so
first Ionization energy is lower than
second, third etc.
4.) Electron Affinity: The energy change when
an atom gains an electron. This is
extremely low (often negative) for the
Halogens, since they need only one
electron to fill their valence. Affinity
increases as we move from right to left.
The vertical trend is less pronounced, but
generally affinity decreases as we move
down a column.
5.) Electronegativity: The tendence of an
atom to attract extra electrons when it is
involved in chemical bonds.
Electronegativity increases from left to
right, and decreases from top to bottom;
similar to Ionization energy.