Oxygen’s classification is purely chemical, despite its fundamental importance to life and the planet. It is the third most abundant element in the universe and makes up nearly half of the Earth’s crust by mass. Understanding its chemical identity requires examining the specific physical and chemical properties used for elemental classification.
The Direct Answer: Oxygen’s Classification
Oxygen is classified as a nonmetal. It is represented by the chemical symbol O and has an atomic number of 8, meaning its nucleus contains eight protons. This element resides on the right side of the periodic table, in Group 16, which is designated for nonmetallic elements. Its physical and chemical behavior is opposite to the characteristic tendencies displayed by metals.
Defining Chemical Classes: Metals vs. Nonmetals
The division between metals and nonmetals is based on distinct physical and chemical attributes. Physically, metals are known for their high electrical and thermal conductivity, efficiently transferring heat and electricity due to the presence of delocalized electrons. They possess a shiny, reflective surface (metallic luster), and are malleable and ductile. Most metals exist as dense solids at standard room temperature, with mercury being an exception.
Nonmetals, by contrast, are poor conductors, acting as effective insulators for heat and electricity. They lack the characteristic metallic sheen, often appearing dull or colorless, and are brittle in their solid state, unable to be shaped without breaking. Chemically, the defining difference is how they interact with electrons during bonding. Metals possess low electronegativity and tend to lose their outer electrons to form positive ions, or cations. Nonmetals have high electronegativity and prefer to gain electrons to achieve a stable electron configuration, typically forming negative ions, or anions, or sharing electrons to create covalent bonds.
Oxygen’s Defining Nonmetallic Properties
Oxygen exhibits physical properties that disqualify it from being a metal. At standard temperature and pressure, oxygen exists as a colorless, odorless gas (O₂), a state different from the solid form of nearly all metals. Gaseous oxygen is a poor conductor of both heat and electricity, aligning with the insulating nature of nonmetals. When solidified at extremely low temperatures, oxygen is brittle and appears pale blue, lacking the reflective luster of a metal.
The element’s chemical behavior provides evidence for its nonmetallic status. Oxygen has a high electronegativity, a measure of an atom’s ability to attract a shared pair of electrons, second only to fluorine. This strong pull means oxygen readily seeks to gain two electrons to complete its outer shell, often forming the oxide ion (O²⁻) in ionic compounds. When bonding with other nonmetals, it forms covalent bonds by sharing electrons, a characteristic of nonmetallic chemical reactions.