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The elements show a periodic recurrence of chemical and physical properties when they are arranged in order of increasing atomic number. Elements in the same vertical column are known as groups and the horizontal rows of elements are called periods. All elements in a specific group have equivalent outermost, or valence electron, configurations which accounts for the similarity in the number and type of chemical bonds these elements form.
In moving from element to element across a period or down a group, chemical and physical properties change gradually.
Most of the elements are metals that chemically tend to lose electrons and form positive ions. The relatively few nonmetals appear in the upper right-hand section of the chart except for hydrogen. The nonmetals have a tendency to react chemically with metals and gain electrons to form negative ions. Nonmetals often bond to each other by forming covalent bonds.
Alkali Metals: Highly reactive elements that combine with many nonmetals to form ionic solids (salts). They also form compounds with oxygen that dissolve in water to create solutions that are strongly basic (alkaline).
Alkaline Earth Metals: Very reactive elements that form ionic compounds with nonmetals. Many of their oxygen compounds are found in deposits in the ground.
Transition Metals: Generally less reactive than the alkali and alkaline earth metals, these elements vary in physical and chemical properties. Many form important alloys with one another and other metals. Several of the transition elements can form more than one positive ion. For example, iron can form more than one compound with a given nonmetal since it exists as two different ions, Fe 2+ and Fe 3+.
Lanthanides: Series of transition elements between lanthanum (La) and hafnium (Hf). These elements are found together in nature and they are similar chemically.
Actinides: Series of transition elements between actinium (Ac) and rutherfordium (Rf). Many can be prepared in minute quantities only. They tend to form insoluble compounds.
Noble Gases: Elements exhibit limited chemical reactivity though the heavier noble gases show degrees of reactivity. These elements have generally complete electron shells.
Halogens: Reactive elements that form compounds known as halides. Several halogens including chlorine, fluorine and iodine, have important applications in everyday life.
Other Metals: Also referred to as post transition metals. Aluminum is the most abundant metal on earth. Lead, tin and mercury have industrial uses.
Other Nonmetals: Includes chemically reactive elements important for life on the planet, such as carbon, oxygen, nitrogen and phosphorous.