Classification of elements Across a period, the elements change from metals through metalloids to non-metals. For example, in period 3, sodium (the extreme left element) is a very reactive metal whereas, chlorine (before argon at extreme right) is a very reactive non-metal. Silicon located in between acts as a metalloid. Note that metals occupy a major portion of periodic table.
There are about 90 elements, up to Uranium, that are found naturally. Another 26 elements, beyond Uranium, called trans–uranium elements, have been produced artificially in the laboratories by nuclear reactions. The chemical behavior of all elements depends on their electronic configurations. It is the electrons that take part in chemical reactions and therefore their interactions are the main factors that determine the outcome of a chemical reaction.
Depending on the availability of "free" electrons, elements can be classified as metals, metalloids and non–metals. Free electrons are unbound electrons, beyond closed shells. In metals, these unbound electrons are given off or donated during compound formation. Examples of metals are Sodium (Na), Potassium (K), Gold (Au). In non–metals, there are no free–electrons, instead there is a deficiency of electrons. While forming compounds, non–metals borrow electrons. Examples of non–metals are Carbon (C), Oxygen (O), Nitrogen (N), Chlorine (Cl). Metals are shown on the left–hand side and center of the periodic table. Non–metals are placed on the right hand side of the periodic table. Chemically inert gases like Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr) and Xenon (Xe) are included in the category of non–metals.
The electrical and thermal conductivity is relatively high for metals, low for metalloids and negligible for non-metals.