Periodic Table Notes and Review

Periodic Table Notes and Review

The periodic table is an important tool in chemistry. These notes review the periodic table, how it is organized, and periodic table trends.

Invention and Organization of the Periodic Table

In 1869, Dmitri Mendeleev organized the chemical elements into a periodic table much like the one we use today, except his elements were ordered according to increasing atomic weight, while the modern table is organized by increasing atomic number. The way the elements are organized makes it possible to see trends in element properties and to predict the behavior of elements in chemical reactions.

Rows (moving left to right) are called periods. Elements in a period share the same highest energy level for an unexcited electron. There are more sublevels per energy level as atom size increases, so there are more elements in periods further down the table.

Columns (moving top to bottom) form the basis for element groups. Elements in groups share the same number of valence electrons or outer electron shell arrangement, which gives elements in a group several common properties. Examples of element groups are alkali metals and noble gases.

Periodic Table Trends or Periodicity

The organization of the periodic table makes it possible to see trends in properties of elements at a glance. The important trends relate to atomic radius, ionization energy, electronegativity, and electron affinity.

• Atomic Radius
  Atomic radius reflects the size of an atom. Atomic radius decreases moving from left to right across a period and increases moving from top to bottom down an element group. Although you might think atoms would simply become larger as they gain more electrons, electrons remain in a shell, while the increasing number of protons pulls the shells in closer to the nucleus. Moving down a group, electrons are found further from the nucleus in new energy shells, so the overall size of the atom increases.
• Ionization Energy
  Ionization energy is the amount of energy needed to remove an electron from an ion or atom in the gas state. Ionization energy increases moving from left to right across a period and decreases moving top to bottom down a group.
• Electronegativity
  Electronegativity is a measure of how easily an atom forms a chemical bond. The higher the electronegativity, the higher the attraction for bonding an electron. Electronegativitydecreases moving down an element group. Elements on the lefthand side of the periodic table tend to be electropositive or more likely to donate an electron than accept one.
• Electron Affinity
  Electron affinity reflects how readily an atom will accept an electron. Electron affinity varies according to element group. The noble gases have electron affinities near zero because they have filled electron shells. The halogens have high electron affinities because the addition of an electron gives an atom a completely filled electron shell.