Download Section 4.3 Notes

Sec. 4.3—Trends in the Periodic Table and their Cause
Objectives:
1. Describe periodic trends in the ionization energy, atomic radius, and electronegativity.
2. Relate the periodic trends to the atomic structures of the elements.
3. Describe periodic trends in ionic size, electron affinity, and melting and boiling points, and relate them
to periodic trends in the atomic structure of the elements.
Vocabulary:
 ionization energy
 electron shielding
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bond radius
electronegativity
Periodic Trends
A trend is a predictable change in a particular direction. Understanding a trend among the elements enables you
to make predictions about the chemical and physical behavior of elements. Trends are explained by the electron
configurations of the elements. The three trends discussed below play a critical role in our understanding of
chemical bonding and compound formation.
The cause of the trends can be explained by the affect of electron shielding. The positively charged nucleus
attracts the negatively charged outermost electrons. Electron shielding occurs when this attractive force is
diminished by the negative charges of the inner electrons.
The ionization energy of an atom is the energy needed to completely remove the outermost electron from an
atom. When this happens, the atom loses a negative charge and becomes an ion.
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Decreases down a group due to electron shielding and the addition of a full shell of electrons with each
step down. The additional shell of electrons reduces the attractive force between the positively charged
nucleus and the outermost electrons.
Increases across a period from left to right due to the addition of one more proton and one more electron
as one steps from element to element. The added protons increase the attractive force on the outermost
electron.
Atomic radius is determined by measuring the bond radius, or half the distance from center to center, of two like
atoms bonded together.
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Increases down a group because each step down adds another filled shell that increases the size or radius
of the atom. The electron shielding of the inner electrons also increases the size due to the lessening of
the force of attraction between the outer electrons and the nucleus.
Decreases across a period from left to right due to the addition of one more proton and one more
electron as one steps from element to element. The added protons increase the attractive force between
the positive nucleus and the outermost electrons, which pulls them in tighter and decreases the radius.
Electronegativity is a measure of the ability of an atom in a chemical compound to attract electrons. In other
words, electronegativity determines what atom wins the tug-of-war for electrons between atoms that are bonded
together in a compound.
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Decrease down a group because each step down adds another filled shell. The addition of a new shell
insulates the outer electron from the attraction of the nucleus due to electron shielding.
Increases across a period from left to right due to the addition of one more proton and one more electron
as one steps from element to element. The added protons increase the attractive force between the
positive nucleus and the outermost electrons, which pulls them in tighter increasing electronegativity.
Other periodic trends discussed in Section 3 are the trends in ionic size and electron affinity. There is also an
interesting discussion of why the melting points of the elements change in a predictable manner across a period.
Summary
Atomic Trait
Ionization Energy
Atomic Radius
Electronegativity
Ionic Radius
Electron Affinity
Melting/Boiling Point
Trend Across Period (left to right)
Increases
Decreases
Increases
Decreases
Increases
Increases then Decreases
Trend Down Group (top to bottom)
Decreases
Increases
Decreases
Increases
Decreases