A chemical compound is a substance formed when atoms from two or more different elements are chemically bound together in a fixed, precise ratio. This chemical combination results in a new material that exhibits properties entirely distinct from those of the original component elements. For instance, water (H2O) is a compound with liquid properties at room temperature, unlike its component elements. Understanding how these compounds are organized is fundamental to chemistry and provides the foundation for comprehending biological and geological processes.
Compounds Classified by Chemical Bond
The way atoms connect dictates a compound’s fundamental structure and resulting physical characteristics. One primary classification method distinguishes compounds based on the nature of the chemical bond holding their constituent atoms together. This distinction separates compounds into two major structural groups: ionic and covalent.
Ionic compounds form through the complete transfer of one or more valence electrons from one atom to another, typically occurring between a metal and a nonmetal. The electron transfer creates oppositely charged particles called ions, a positive cation and a negative anion, which are then held together by strong electrostatic attraction. These forces result in an extended, three-dimensional arrangement known as a crystal lattice, rather than discrete molecules. Common examples like sodium chloride (NaCl), or table salt, exhibit typical ionic properties, such as high melting and boiling points, due to the substantial energy required to break the strong lattice structure.
Covalent compounds, often called molecular compounds, form when two atoms, usually nonmetals, share electrons between them. This sharing creates a strong, directional bond that results in distinct, neutral units called molecules. Unlike ionic compounds, covalent compounds melt at much lower temperatures because less energy is required to overcome the weaker forces between individual molecules. Water (H2O) and carbon dioxide (CO2) exemplify this group, demonstrating the wide variety of molecular shapes and sizes possible.
Compounds Based on Carbon Presence
A separate, highly relevant classification, particularly in biological and health sciences, categorizes compounds based on their elemental composition, specifically the presence of carbon. This system divides all chemical compounds into organic and inorganic groups.
Organic compounds are primarily defined by the presence of carbon atoms chemically bonded to hydrogen atoms. Carbon’s ability to form four stable bonds and link with itself in long chains or rings allows organic compounds to achieve structural complexity and diversity. This complexity makes them the foundational molecular building blocks of all known life, encompassing large molecules like proteins, carbohydrates, fats (lipids), and nucleic acids such as DNA.
In contrast, inorganic compounds generally lack the carbon-hydrogen framework, although there are a few exceptions like carbon dioxide (CO2) or simple carbonates. These compounds are often derived from non-living geological sources, such as minerals and metals. They tend to have simpler, smaller structures and often contain ionic bonds, exemplified by substances like sodium chloride (NaCl) or various metal oxides. Inorganic compounds play important roles in biological systems as well, such as in the structure of bones and the function of many cellular processes.
Functional Compounds
Beyond structure and composition, compounds can also be classified by how they function and behave when dissolved in water, particularly in terms of proton transfer. This functional classification focuses on acids, bases, and the resulting salts.
Acids are compounds defined by their ability to donate a proton when placed in an aqueous solution. This action increases the concentration of hydrogen ions, which is measured on the pH scale; acids register a pH value below 7. Common acids include hydrochloric acid (HCl), found in stomach acid, and acetic acid, which gives vinegar its characteristic sour taste.
Bases function as proton acceptors. Alternatively, many bases increase the concentration of hydroxide ions in water. Bases register a pH value above 7 and often feel slippery to the touch, with examples including sodium hydroxide (NaOH) and the sodium bicarbonate found in baking soda.
When an acid and a base react, they perform a neutralization reaction, resulting in the formation of water and an ionic compound known as a salt. This demonstrates the functional relationship between these compounds. Table salt (NaCl) is a simple example, formed from the reaction between hydrochloric acid and sodium hydroxide.