元素及び化合物の分類

Atomic elements exist as single, stable atoms. Examples include helium, neon, and krypton, which can exist independently as monatomic elements. Molecular elements, by contrast, contain two or more atoms bonded together as their basic unit. Diatomic hydrogen and polyatomic sulfur are examples of molecular elements that form stable multi-atom structures.

Molecular compounds form when atoms of two or more nonmetallic elements combine by sharing valence electrons through covalent bonds. This electron sharing lowers the system's potential energy, creating stable molecules. The basic units of molecular compounds are discrete molecules composed of constituent atoms, such as ethanol molecules containing carbon, hydrogen, and oxygen atoms.

Ionic bonds form strong electrostatic attractions between oppositely charged ions. Metal atoms lose valence electrons to form positively charged cations, while nonmetal atoms gain electrons to form negatively charged anions. These ionic bonds lower the system's total energy and drive the assembly of stable, ordered three-dimensional ionic lattice structures characteristic of ionic compounds.

A formula unit is the smallest array of ions that forms an electrically neutral basic unit of an ionic compound. In table salt (NaCl), the formula unit contains one sodium ion and one chloride ion in a one-to-one ratio. Formula units represent the simplest whole-number ratio of ions needed to achieve electrical neutrality in the compound.

Simple ions like sodium or chloride consist of single atoms with a net charge. Polyatomic ions, however, are made of multiple covalently bonded atoms with a net negative or positive charge. Sodium acetate exemplifies this, containing positively charged sodium ions and polyatomic acetate ions carrying a net negative charge, allowing for more complex ionic compound structures.

Molecular compounds typically exist as gases, low-boiling liquids, or low-melting solids under normal conditions. Ionic compounds, conversely, are solids that melt and boil at high temperatures. In solid form, ionic compounds cannot conduct electricity because ions cannot move, but when molten, they conduct electricity as ions flow freely through the liquid.

In every ionic compound, the total positive charges from cations must equal the total negative charges from anions to achieve electrical neutrality. This balance is maintained regardless of whether the compound contains simple ions or polyatomic ions. For example, calcium phosphate Ca3(PO4)2 has three calcium cations (3×2+ = 6+) balanced by two phosphate anions (2×3− = 6−), ensuring overall neutrality.