A Map of the Elements
The periodic table of elements is one of the most important tools in all of science. It's a chart that organizes all the known chemical elements in a systematic way. An element is a pure substance consisting of only one type of atom.
The table was created by the Russian chemist Dmitri Mendeleev in 1869. He arranged the elements in order of increasing atomic number (the number of protons). He discovered that when he did this, the properties of the elements repeated in a regular, or periodic, pattern.
How the Table is Organized
Periods: These are the horizontal rows on the periodic table. There are 7 periods. Elements in the same period do not have very similar properties, but they do have the same number of electron shells (energy levels).
Groups (or Families): These are the vertical columns on the periodic table. There are 18 groups. Elements in the same group have similar chemical properties because they have the same number of valence electrons (the electrons in the outermost shell).
Metals, Nonmetals, and Metalloids
The periodic table has a 'stair-step' line that divides the elements into three main categories.
Metals: Located to the left of the stair-step line. The vast majority of elements are metals.
Properties: They are typically shiny (have luster), are good conductors of heat and electricity, are malleable (can be hammered into sheets), and are ductile (can be drawn into wires). Most are solid at room temperature.
Nonmetals: Located to the right of the stair-step line (plus Hydrogen).
Properties: They are typically dull, are poor conductors of heat and electricity (insulators), and are brittle when solid. Many are gases at room temperature.
Metalloids (or Semimetals): Located directly on either side of the stair-step line.
Properties: They have properties that are intermediate between metals and nonmetals. For example, they are 'semiconductors', meaning they can carry an electrical charge under special conditions. This property makes them essential for computers and calculators. Silicon (Si) is the most famous metalloid.
Important Families
Some groups on the periodic table have special names.
Group 1: Alkali Metals: Very reactive metals.
Group 2: Alkaline Earth Metals: Reactive metals, but less so than Group 1.
Groups 3-12: Transition Metals: The large block in the middle. Less reactive, hard metals.
Group 17: Halogens: Very reactive nonmetals.
Group 18: Noble Gases: Very unreactive (inert) gases. They are stable because they have a full outer shell of valence electrons.