The classification of chemical compounds into organic and inorganic is a fundamental division in chemistry. This distinction originated in the early 19th century when scientists believed that compounds produced by living organisms possessed a mysterious “vital force” and were labeled organic. This historical view was disproven in 1828 with the laboratory synthesis of the organic compound urea from inorganic starting materials. The terms persist, now based purely on the molecular composition and structure of the substance. The modern chemical definition relies on the presence of carbon to separate these two vast categories of compounds.
The Defining Role of Carbon
The modern definition states that organic compounds contain carbon atoms, typically bonded to hydrogen atoms, forming C-H bonds. Carbon’s unique structure, with four valence electrons, allows it to form four strong covalent bonds, making it exceptionally versatile. This capability enables carbon atoms to link together almost endlessly, a process known as catenation, forming long chains, branched structures, and rings. The ability to create these varied carbon skeletons is why there are millions of known organic compounds, far outnumbering inorganic ones. These structures form the basis for all biological molecules, including DNA, proteins, and carbohydrates.
However, simply containing carbon is not enough to qualify a substance as organic. A few specific carbon-containing compounds are exceptions and fall under the inorganic category, typically lacking the characteristic C-H bonds. For example, carbon dioxide (\(CO_2\)) is classified as inorganic because it lacks hydrogen atoms bonded to the carbon. Similarly, carbon monoxide (\(CO\)) and compounds containing the carbonate ion (\(CO_3^{2-}\)), such as calcium carbonate, are considered inorganic. Cyanides and carbides are also traditionally categorized as inorganic.
Contrasting Chemical and Physical Behaviors
The structural differences between the two compound classes result in distinct chemical and physical properties. Organic molecules are primarily held together by strong covalent bonds, involving the sharing of electrons, typically resulting in non-polar or weakly polar molecules. Inorganic compounds, in contrast, frequently feature ionic bonds, where electrons are transferred between atoms to form charged ions, or highly polar covalent bonds.
Organic compounds generally exhibit lower melting and boiling points due to weaker intermolecular forces between individual molecules. Less energy is required to transition the substance into a liquid or gaseous state, making many organic compounds volatile. Inorganic compounds, particularly those with ionic lattices like table salt, are held together by powerful electrostatic attractions. This necessitates much higher temperatures to break, meaning inorganic substances are less volatile and exist as solids with elevated melting points.
Solubility patterns also differ significantly based on the compounds’ polarity. Organic compounds are often soluble in non-polar organic solvents, such as gasoline or ether, but are poorly soluble in water. Inorganic compounds, especially those with ionic bonds, readily dissolve in water, which is a highly polar solvent. Furthermore, the reactions of organic compounds are often slow, require specific catalysts, and can yield complex mixtures of products. Inorganic reactions, especially those involving ions in water, generally occur much faster and produce simpler, more predictable outcomes.
Common Examples in Everyday Life
Many compounds associated with biological processes and energy storage are organic. Sugars, fats, proteins, and nucleic acids (DNA and RNA) are fundamental organic molecules that build and manage living cells. Common examples also include fuels like methane and gasoline, which are complex mixtures of hydrocarbons, and synthetic materials such as plastics.
Inorganic compounds form the bulk of the non-living world, including the earth’s crust and the atmosphere. Water (\(H_2O\)) is a classic inorganic example, serving as a solvent for many inorganic substances. Minerals and salts, such as sodium chloride, iron oxide, and quartz, are all categorized as inorganic. The gases that make up most of the air we breathe, like nitrogen (\(N_2\)) and oxygen (\(O_2\)), along with metals used in construction, are also inorganic.