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Atomic Number
The primary way elements are classified in the Periodic Table is by their atomic number. An atomic number is the number of protons located in the nucleus of an element. The elements change as the atomic number increases. The number of protons an element has is equivalent to the number of electrons the element has. The electrons are found on shells or orbitals surrounding the nucleus.
Metals, Nonmetals and Metalloids
Elements are also classified in the Periodic Table as metals, nonmetals and metalloids. Metals are solid substances that conduct electricity and heat and are ductile and malleable. For example, silver and gold are metals. All metals form positive ions called cations because they readily lose their outermost electrons. Nonmetals have the opposite properties of metals. Nonmetals are brittle and don't conduct heat and electricity. Nonmetals also gain electrons in their outermost shell to form a negative ion called an anion. Carbon and iodine are examples of nonmetals. Metalloids, also called semimetals, have properties of both metals and nonmetals. Silicon is an example of a metalloid.
Groups
Elements in the Periodic Table are classified into groups and periods. A group is a column of elements that share similar chemical and physical properties. For example, the Group 1B (also called Group 11 or coinage metals) elements copper and silver have similar properties. Additionally, Groups 1 and 2 (except hydrogen), as well as Groups 13 through 18 are called the main group elements. Groups 3 through 12 are labeled transition elements. Transition elements are the elements whose valence electrons are in more than one shell.
Periods
A period is a row of elements whose chemical properties change when moving from left to right on the Periodic Table. For example, sodium and magnesium have different chemical properties. Additionally, all the elements in a period will have something in common -- the same number of electron orbitals. Elements in the top row will have one orbital, those in the second row will have two orbitals, elements in the third row will have three orbitals and so on down the Periodic Table.
Rare Earth Elements
The elements in the Periodic Table also classify the rare earth elements. The lanthanide and actinide series are in this classification. They are named as such because of the first element in each series -- lanthanum and actinium. Every rare earth metal is found in group 3 and in the 6th and 7th periods. Many of the elements in the actinide series are called transuranium, or those elements with an atomic number greater than that of uranium.