离子、分子和化合物

A cation is an atom that has lost electrons and becomes positively charged, while an anion is an atom that has gained electrons and becomes negatively charged. Both form when atoms participate in chemical reactions to achieve a full valence shell. For example, potassium loses one electron to become a cation, and fluorine gains one electron to become an anion.

Ions are vital to physiological processes including nerve impulse transmission and muscle contraction. Ion imbalances can cause serious health problems: sodium ion imbalance may lead to cardiovascular disease, while low blood calcium levels can cause tetany, which involves uncontrollable involuntary muscle contractions. Maintaining proper ion balance is essential for normal body function.

Molecules are combinations of two or more atoms chemically bonded together and may consist of atoms of the same element, like oxygen gas, or different elements, like carbon dioxide. Compounds are pure substances formed by bonding atoms of different elements in a fixed proportion. All compounds are molecules, but not all molecules are compounds.

Ionic compounds form through electrostatic interactions between metallic and nonmetallic ions, such as sodium and chloride in table salt. Molecular compounds form when nonmetal atoms bond via covalent bonds, where electrons are shared. In ionic compounds, electrons are transferred; in molecular compounds like methane, electrons are shared between atoms in specific proportions.

An atom becomes an ion when it gains or loses electrons during a chemical reaction, typically to achieve a full valence shell. An atom with a half-full or less-than-half-full valence shell tends to donate electrons, while an atom with a more-than-half-full valence shell tends to accept electrons. This electron transfer results in a net electrical charge on the atom.

Glucose is a compound composed of three elements—carbon, hydrogen, and oxygen—in a fixed proportion. It always contains six carbon atoms and six oxygen atoms for every twelve hydrogen atoms. This consistent composition makes glucose an important body fuel and demonstrates how compounds maintain specific elemental ratios regardless of their source.

Potassium has only one electron in its valence shell, making it highly likely to donate that electron to achieve a full shell. Fluorine has seven electrons in its valence shell, making it highly likely to accept one electron to complete its shell. Both elements' electron configurations drive them to participate in chemical reactions to reach stable, full valence shells.