6 Electronegativity
Whenever two atoms form a bond, the nucleus of each atom attracts the other’s electrons.
Electronegativityis a measure of the strength of this attraction.
6.0.1 Periodic trends
Several traits of atoms are said to have ”periodic trends”, meaning that different atoms
in a period have identifiable relationships to one another based on their position. Is that
confusing? Think of the periodic table as a group picture, maybe of a very large basketball
team. Each period is a row of players in the picture, and the ”photographer” has decided
to arrange the ”players” by their characteristics. Of course, no conscious effort was made
to arrange the periodic table by any characteristic other than number of protons, but some
properties are consistent in its layout regardless.
Atomic size is one characteristic that shows a periodic trend. In case of atomic radius the
”photographer” (Mendeleev and others since) decided to arrange ”players” (atoms) by size
with the very shortest and smallest players at the top right. As you go left to right along
a row (a period) the atoms get sequentially smaller and smaller. Fluorine is smaller than
carbon, and carbon is smaller than magnesium. This is due to the number of protons in
the nucleus increasing, while the increasing number of electrons are unable to shield one
another from the attractive force of the positive charge from the nucleus.
REMEMBER:largest> Li > Be > B > C > N > O > F > Ne >smallest
Another characteristic with a periodic trend is ionization energy. This is the amount of
energy necessary to remove one electron from an atom. Since all the atoms favor an electron
configuration of a noble gas, the atoms at the extreme left of the table will give up their first
electron most readily. (In almost all cases, a metal will readily give up its first electron.)
The halogens, which need only one more electron to fill their outer shells, require a great
deal of energy to give up an electron because they would be much more stable if they gained
one electron instead. Ionization energy is the opposite of atomic radius, therefore, because
it increases from left to right across a period.
REMEMBER:least energy to ionize< Li < Be < B < C < N < O < F < Ne <most
energy to ionize
Electronegativity is perhaps the most important periodic trend, and it is not related to
ionization energy directly -- but its trend is the same, increasing from left to right. Also, the
elements in a group (like the halogen group) gain stability as they grow in atomic number,
so the smallest member of an electronegative group is often the most electronegative. In
general, it can be said that among periods (rows) or groups (columns) of the periodic table,
the closer an element is to fluorine, the more electronegative it will be. For Group VIIA (the
aforementioned halogens) of the periodic table, you memorize the following relationships: