Untitled - Kelly Walsh High School

124 CHEMISTRY FOR THE UTTERLY CONFUSED
Trends for the elements may be either horizontal or vertical. The combination
of these leads to diagonal relationships that increase either from the lower left
to the upper right on the periodic table or from the upper right to the lower left.
There are few trends that increase along another diagonal or that are only horizontal
or vertical. You should remember that the noble gases are not included
in some trends. In addition, hydrogen is often an exception to many trends.
There are two general classes of questions concerning periodic properties. One
type simply asks for ranking elements in either increasing or decreasing order.
You should be careful not to accidentally put the values in reverse order. Simply
locating the elements on the periodic table often allows answering this category
of question. Exceptions are usually not the focus. The other type asks for an
explanation of values. In this case, you will need to utilize atomic radius and the
effective nuclear charge. In many cases, the electron configuration of the elements
are involved. In this kind of question, position on the periodic table is
often not very important. To answer this type of question often requires you to
deal with one or more exceptions.
Another type of question may ask for a comparison of apparently unrelated
information. For example, rank the following in order of increasing radius: Ne,
F , and Na . In this case, we are comparing ionic and atomic radii of elements
from the second and third periods on the periodic table. If we consider the electron
configuration of each of these species, we find they are all 1s 2 2s 2 2p 6 . Since
the electron configurations are identical, the screenings of the core electrons
are identical. If there were no additional factors, all three of the species would
be identical in size. The cation, Na , has a higher attraction for the electrons
due to its positive charge, which leads to an increase in the effective nuclear
charge. For this reason, the cation will be the smallest of the three. The anion,
F , has additional repulsion due to the extra electron indicated by the charge.
This additional electron leads to an increase in repulsion of the electrons leading
to an increase in size. The anion is the largest of the three species. Thus, the
answer to the question is Na < Ne < F .
There are other exceptions. For example, the EA of fluorine is lower than
expected. This is because of the repulsion of the electrons in the small fluoride
ion. If the ion were larger, the repulsion would be lower. The larger chlorine
atom has the highest EA of all the elements.
The elements on the periodic table exhibit trends in ionization energies and
electron affinities. These trends are related to the effective nuclear charge of
the atom. The ionization energy (IE) is the energy required to remove an electron
from a gaseous atom in its ground state. The ionization energies tend to
decrease going down a family and increase from left to right across a period.
The energy change accompanying the addition of an electron to a gaseous atom