Periodicity Definition in Chemistry Chemistry Glossary Definition of Periodicity

Author
by Anne Marie Helmenstine, Ph.D.

PERIODICITY DEFINITION

In the context of chemistry and the periodic table, periodicity refers to trends or recurring variations in element properties with increasing atomic number. Periodicity is caused by regular and predictable variations in element atomic structure.

Mendeleev organized elements according to recurring properties to make a periodic table of elements. Elements within a group (column) display similar characteristics.

The rows in the periodic table (the periods) reflect the filling of electrons shells around the nucleus, so when a new row begins, the elements stack on top of each other with similar properties. For example, helium and neon are both fairly unreactive gases that glow when an electric current is passed through them. Lithium and sodium both have a +1 oxidation state and are reactive, shiny metals.

USES OF PERIODICITY

Periodicity was helpful to Mendeleev because it showed him gaps in his periodic table where elements should be. This helped scientists find new elements, because they could be expected to display certain characteristics based on the location they would take in the periodic table. Now that the elements have been discovered, scientists and students used periodicity to make predictions about how elements will behave in chemical reactions and their physical properties. Periodicity helps chemists predict how the new, superheavy elements might look and behave.

PROPERTIES THAT DISPLAY PERIODICITY

Periodicity can include many different properties, but the key recurring trends are:

• Ionization Energy - This is the energy needed to completely remove an electron from an atom or ion. Ionization energy increases moving left to right across the table and decreases moving down a group.

• Electronegativity - A measure of how readily an atom forms a chemical bond. Electronegativity increases moving left to right across a period and decreases moving down a group.
• Atomic Radius - This is half the distance between the middle of two atoms just touching each other. Atomic radius decreases moving left to right across a period and increases moving down a group. Ionic radius is the distance for ions of the atoms and follows the same trend. Although it might seem like increasing the number of protons and electrons in an atom would always increase its size, the atom size doesn't increase until a new electron shell is added. Atom and ion sizes shrink moving across a period because the increasing positive charge of the nucleus pulls in the electron shell.
• Electron Affinity - This is a measure of readily an atom accepts an electron. Electron affinity increases moving across a period and decreases moving down a group. Nonmetals usually have higher electron affinities than metals. The noble gases are an exception to the trend, since these elements have filled electron valence shells and electron affinity values approaching zero. However, the behavior of the noble gases is periodic. In other words, even though an element group might break a trend, the elements within the group display periodic properties.

If you're still confused or need additional information, a more detailed overview of periodicity is also available.

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