Periodic Trends Reading
Use this reading and/or Section 6.3 (p.170-178) in your textbook to fill in the organizer on page
20 of your packet. Also draw in the trends on the periodic table on p.19 in your packet.
Periodic Trends
The arrangement of the periodic table reveals trends in the properties of the
elements. A TREND is a predictable change in a particular direction. For example, there
is a trend in the reactivity of the alkali metals as you move down Group 1. Each of the
alkali metals react with water. However, the reactivity of the alkali metals varies. At the
top of Group 1, lithium is the least reactive, sodium is more reactive, and potassium is
still more reactive. In other words, there is a trend toward great reactivity as you move
down the alkali metals in Group 1.
Understanding a trend among the elements allows you to make predictions about
the chemical behavior of the elements. These trends in properties of the elements in a
group or period can be explained in terms of electron organization.
Ionization Energy
When atoms have equal amount of protons and electrons, they are electrically
neutral. But when enough energy is added, the attractive force between the protons and
electrons is removed from an atom. The neutral atom then becomes a positively charged
ion. The energy that is supplied to remove electrons is the ionization energy of the atom.
Ionization energy tends to decrease down a group. Each element has more occupied
energy levels than the one above it does. Therefore, the outermost electrons are furthest
from the nucleus near the bottom of a group. Similarly, as you move down a group, each
successive element contains more electrons in the energy levels between the nucleus and
the outermost electrons. These inner electrons shield the outermost electrons from the
full attractive force of the nucleus. This electron shielding causes the outermost electrons
to be held less tightly to the nucleus. Ionization energy, however, tends to increase as
you move from left to right across a period. From one element to the next in a period, the
number of protons and electrons increase by one each. The additional proton increase the
nuclear charge. The additional electron is added to the same outer energy level as the
previous element in the period. However, a higher nuclear charge attracts the outer
electrons more strongly. Therefore, they are held onto “tighter” and need more energy to
be removed.
Atomic Radius
The exact size of an atom is hard to determine. An atom’s size depends on the
volume occupied by the electrons around the nucleus, and the electrons do not move in
well-defined paths. Rather, the volume the electrons occupy is though of as an electron
cloud, with no clear-cut edges. In addition, the physical and chemical state of an atom
can change the size of an electron cloud. Instead, to measure an atom’s size, we measure
the atomic (bond) radius. This is the length that is half the distance between the nuclei of
two bonded atoms. The atomic radius increase as you move down a group. As you
proceed from one element down to the next in the group, another energy level is filled.
The addition of another level of electrons increases the size, or atomic radius, or the
atom. As you move from left to right in a period, each atom has one more proton and one
more electron than the atom before it has. All additional electrons go into the same outer
energy level-no electrons are being added to the inner levels. As a result, electron
shielding does not play a role as you move across a period. Therefore, as the nuclear
charge increases across a period, the effective nuclear charge pulls the outermost
electrons closer and closer to the nucleus and thus reduces the atomic radius.
Reactivity
Reactivity refers to how likely or vigorously an atom is to react with other
substances. This is usually determined by how easily electrons can be removed
(ionization energy) and how badly they want to take other atom’s electrons
(electronegativity) because it is the transfer/interaction of electrons that is the basis of
chemical reactions. Metals and nonmetals each have their own trends. When we look at
the metals, we see that the reactivity increases as you move down the group. However,
as we move across a period, the reactivity decreases as you move from left to right. The
farther to the left and down the periodic table that you go, the easier it is for electrons to
be given or taken away, resulting in higher reactivity. This is a direct result of the
nuclear charge and the attraction to the outermost electrons. The nonmetals, however,
show different trends. As you move from left to right in the period, we see that the
reactivity increases. On the other hand, as you move down a group in the nonmetals, the
reactivity decreases. The farther right and up you go on the periodic table, the higher the
electronegativity, resulting in a more vigorous exchange of electrons. Where metals will
easily give up an electron to gain stability, nonmetals want to gain electrons.
Atomic Weight
Almost every atom contains one or more neutrons in its nucleus. The number of
neutrons does not affect the charge of an atom, but it does affect its mass. The total
number of protons and neutrons in an atom is called its mass number. This number is
usually written with a hyphen after the element’s name. Recall that most elements have
more than one naturally occurring isotope. A sample of these elements, therefore, will
contain atoms with different atomic masses. The atomic weight is the average of the
atomic mass of the isotopes of an element. As you move left to right on the periodic
table, you are adding more neutrons to the atom. Therefore, the atom becomes heavier in
mass and the atomic weight (or the average of the element) increases. The same is true as
you move down a group, you are also adding more neutrons. Again, the atomic weight
increases as you move down a group in the periodic table.