Non-metals are elements that lack the physical characteristics typically associated with metals, such as being lustrous, malleable, or good conductors of heat and electricity. These elements, which include gases, liquids, and solids, form the basis of organic chemistry and are found in almost every biological molecule. The unique chemical identity of non-metals is defined by the behavior of their valence electrons, which governs how they interact with other atoms.
Periodic Table Placement and General Trends
Non-metals are primarily located on the upper right side of the periodic table, separated from metals by a zigzag line of metalloid elements. This placement reflects a consistent pattern in their atomic structure and resulting chemical nature. Moving from left to right across a period, the metallic character decreases while the non-metallic character increases.
This trend is related to the increasing effective nuclear charge atoms experience, resulting in a stronger pull on their outer electrons. Consequently, non-metals exhibit high ionization energies, meaning it takes significant energy to remove an electron. They also display high electron affinity, indicating a strong desire to acquire additional electrons. Elements higher up in a group and further to the right, such as Fluorine, display the most pronounced non-metallic properties.
High Electronegativity and Anion Formation
A defining chemical characteristic of non-metals is their high electronegativity, which measures an atom’s tendency to attract a shared pair of electrons in a chemical bond. Non-metals possess greater electronegativity values compared to metals, with Fluorine having the highest value of all elements. This strong electron-attracting power drives their reactivity, particularly when they encounter metallic elements.
When reacting with a metal, the large difference in electronegativity causes the non-metal to completely gain one or more electrons from the metal atom. This gain allows the non-metal to achieve a stable electron configuration, resembling that of the nearest noble gas. The process results in the formation of a negatively charged ion, known as an anion. For instance, a neutral Chlorine atom gains one electron to become the Chloride ion (\(\text{Cl}^-\)), forming the basis for ionic compounds like sodium chloride.
Predominance of Covalent Bonding
When non-metals interact with one another, their shared high electronegativity prevents the simple transfer of electrons seen in ionic bonding. Since both atoms strongly attract electrons, they instead achieve stability by sharing valence electrons between them. This sharing mechanism forms a covalent bond, which is the primary type of bonding found in compounds composed solely of non-metals.
This results in the creation of discrete molecules, where atoms are held together by strong, localized covalent bonds. Examples include water (\(\text{H}_2\text{O}\)) and carbon dioxide (\(\text{CO}_2\)). Because the forces between these separate molecules are relatively weak, covalently bonded non-metal compounds often have low melting and boiling points. Consequently, many exist as gases or liquids at room temperature.
Chemical Reactivity and Oxidation States
The inherent tendency of non-metals to gain electrons means they act as oxidizing agents in chemical reactions. An oxidizing agent accepts electrons from another substance, causing that substance to be oxidized while the non-metal is reduced. This strong oxidizing power is most pronounced in the Halogens (Group 17), with elemental Fluorine (\(\text{F}_2\)) being the strongest common oxidizing agent.
When non-metals form compounds with metals or less electronegative non-metals, they exhibit negative oxidation states, such as the \(-1\) state for chlorine in \(\text{NaCl}\). Non-metals can display positive oxidation states when they bond with an element that is more electronegative than themselves. For example, in perchloric acid (\(\text{HClO}_4\)), chlorine exhibits a positive oxidation state because it is bonded to the highly electronegative oxygen atoms. This flexibility allows non-metals to participate in a wide variety of chemical transformations.