Four main-group families are given special names, which you should remember:
IA group (Group 1) alkali metals
IIA group (Group 2) alkaline earth metals
VIIA group (Group 17) halogens
VIIIA group (Group 18) noble gasesAnother way to label the groups is to consecutively number the groups from left to
right, 1–18. This method is newer than the other labeling method, and it has not gained
wide use. Most teachers and chemists still prefer and use the older method.Trends in Periodic Properties
Trends are useful on the multiple-choice portion of the AP Exam, but simply stating a trend
will notbe sufficient on the free-response portion of the exam. You must give the reason
behind the trend. For example, “higher on the periodic table” is a trend, but not a reason.
The overall attraction an electron experiences is due to the effective nuclear charge.
This attraction is related to the positive nuclear charge interacting with the negative elec-
trons. Electrons between the nucleus and the electron under consideration interfere with,
or shield, that electron from the full nuclear charge. This shielding lessens the nuclear
charge. Within a period, the shielding is nearly constant; however, the effective nuclear
charge will increase with an increasing number of protons (atomic number). Within the
same family or group, as the atomic number increases so does the shielding, resulting in
a relatively constant effective nuclear charge.
The size of an atom is generally determined by the number of energy levels occupied
by electrons. This means that as we move from top to bottom within a group, the size of
the atom increases due to the increased number of shells containing electrons. As we move
from left to right within a period (within the same valence shell), the atomic size decreases
somewhat owing to the increased effective nuclear charge for the electrons. This increased
attraction is related to the increasing number of protons within the nucleus. The size of
a cation is smaller than the neutral atom, because in many cases an entire energy shell has
been removed, while an anion is larger than the corresponding neutral atom since the
nuclear attraction is being distributed over additional electrons. As the number of electrons
changes so will the electron–electron repulsion. The greater the electron–electron repulsion,
the larger the species becomes, and vice versa.
The ionization energy (IE)is the energy needed to completely remove an electron
from an atom. It may be expressed in terms of 1 atom or a mole of atoms. Energy is
required in this process in order to overcome the attraction of the nucleus for the electrons.
There are two factors affecting the magnitude of the ionization energy. One is the size of
the atom. The closer the electrons are to the nucleus, the more energy is needed to over-
come the effective nuclear charge.
Therefore, ionization energy tends to decrease from top to bottom within
a group, since the valence electrons (the first ones to be lost) are farther away from the
nucleus.
The other factor is the magnitude of the effective nuclear charge. The greater the effec-
tive nuclear charge, the more energy is required to remove the electron. Since the effective
nuclear charge increases from left to right within a period, the ionization energies will also
increase from left to right. The increased effective nuclear charge results in the atom becom-
ing slightly smaller, which also leads to a greater nuclear attraction for the electrons.
The ionization energy for the removal of a second electron is greater in all cases than
the first, because the electron is being pulled away from a positively charged ion and the
attraction is greater than from a neutral atom.52 STEP 4. Review the Knowledge You Need to Score High
KEY IDEA