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How are oxides classified as ionic or molecular compounds?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — chemical compound Methane, in which four hydrogen atoms are bound to a single carbon atom, is an example of a basic chemical compound. The structures of chemical compounds are influenced by complex factors, such as bond angles and bond length.

Oxides are classified as ionic or molecular based on the nature of the chemical bonding between oxygen and the other element, which generally follows a trend across the periodic table.1

Ionic Oxides

Ionic oxides are typically formed by metals, particularly alkali metals and alkaline earth metals, as well as transition metals in lower oxidation states. These compounds consist of metal cations and oxide anions ( $O^{2 -}$ ) held together by electrostatic forces. They are characterized as crystalline solids that often react with water to form basic hydroxide solutions.12

Molecular (Covalent) Oxides

Nonmetals typically form covalent oxides, which are usually molecular in character. In these compounds, atoms are held together by the sharing of electrons (covalent bonding). As the periodic table is traversed from metals on the left to nonmetals on the right, the bonding in oxides shifts from ionic to covalent. Transition metals in high oxidation states (+4 to +7) also form oxides with a more covalent nature. Most nonmetal oxides react with water to form acidic solutions.132