An element is a pure substance defined by atoms having the same number of protons, whereas a compound is a pure substance containing atoms of at least two different elements chemically combined in a defined structural or stoichiometric relationship. The atomic number is therefore the fundamental criterion for elemental identity, while the presence of chemically bonded different elements defines a compound. Hydrogen, oxygen, iron and carbon are elements.
Water, H2O, carbon dioxide, CO2, sodium chloride, NaCl, and ammonia, NH3, are compounds. An element cannot be broken down into a simpler substance by ordinary chemical reactions because chemical reactions rearrange electrons and bonds without normally changing the nuclei. A compound can be chemically decomposed into simpler substances or transformed into other compounds because its constituent elements are chemically connected. Nuclear reactions can transform one element into another by changing the nucleus, but that is fundamentally different from ordinary chemical decomposition.
Elements and compounds differ in the nature of their chemical identity and composition. An element may consist of individual atoms, diatomic or polyatomic molecules, metallic lattices or extended covalent structures depending on the element and its allotrope. A compound may likewise be molecular, ionic, metallic or network covalent. The important distinction is not whether the material contains molecules but whether it contains atoms of different elements chemically combined.
Oxygen gas, O2, is an elemental substance because both atoms are oxygen atoms. Water is a compound because each molecule contains hydrogen and oxygen. Sodium chloride is a compound even though its solid structure is an ionic lattice rather than a collection of discrete NaCl molecules. This demonstrates why the distinction should be based on atomic identity and chemical bonding rather than on superficial physical form.
The proportions of elements in a compound are governed by its chemical structure and stoichiometry, whereas a pure element contains only one atomic number even though isotopic abundance can vary. Water has a molecular composition corresponding to two hydrogen atoms for every oxygen atom. Carbon dioxide contains one carbon atom for every two oxygen atoms. Sodium chloride has a one to one stoichiometric ratio of sodium and chlorine in its ideal formula unit.
By contrast, a mixture of hydrogen and oxygen can contain those substances in many different proportions while remaining a mixture rather than automatically becoming water. If they react under suitable conditions, the resulting water is a new compound with its own chemical identity. This difference between physical combination and chemical combination is one of the foundational distinctions in introductory chemistry.
The properties of an element and the compounds formed from it can be dramatically different. Sodium is a soft, highly reactive metal, while chlorine is a reactive greenish yellow gas under ordinary conditions, yet sodium chloride is a stable crystalline ionic solid commonly known as table salt. Hydrogen and oxygen are gases under ordinary conditions, whereas water is a liquid under many ordinary terrestrial conditions.
Carbon forms several elemental allotropes with very different properties, and its compounds range from simple hydrocarbons to complex biomolecules and polymers. The new properties of a compound arise from its bonding, structure, electron distribution and intermolecular or lattice interactions. Consequently, knowing which elements are present does not by itself determine the physical and chemical properties of the resulting compound. Structure and chemical arrangement must also be considered.
Elements and compounds can be distinguished experimentally through chemical and physical methods. A compound can often be decomposed by chemical processes such as electrolysis, thermal decomposition or chemical reaction. Water can be separated into hydrogen and oxygen through electrolysis, although the process requires electrical energy and suitable electrodes and conditions. A mixture such as salt dissolved in water can instead be separated by physical methods such as evaporation and crystallisation because the salt and water retain their chemical identities.
Distillation can separate liquid components based on volatility without necessarily chemically transforming them. Analytical methods such as spectroscopy, mass spectrometry and chromatography can determine whether a sample contains elemental species, molecular compounds or mixtures. The appropriate method depends on the structure, volatility, solubility, charge and other properties of the substances involved.
Elements and compounds also differ in how they are represented in the periodic table and chemical nomenclature. Elements have symbols such as H, O, Fe and C and occupy individual positions in the periodic table according to atomic number. Compounds are represented using chemical formulas that indicate their elemental composition and, where appropriate, structural relationships. Chemical names identify compounds according to systematic naming conventions that can convey composition, oxidation state, molecular structure or functional groups.
A compound may contain an element in different oxidation states in different substances, producing compounds with distinct names and properties. Iron, for example, can form compounds involving iron in different oxidation states. The periodic table therefore provides the fundamental inventory of elements, while chemical formulas and nomenclature describe the enormous range of compounds that can be constructed from those elements.
There are important qualifications to the simple introductory distinction. Some materials have variable composition because their crystal structures contain vacancies, substitutions or nonstoichiometric defects, so real solid compounds may not always have perfectly fixed ratios in every sample. Intermetallic compounds and alloys can occupy complex regions of composition and may not fit elementary categories neatly. Coordination compounds can involve complicated bonding descriptions that do not reduce simply to electron transfer or straightforward covalent sharing.
Molecular compounds can also exist as mixtures of conformers, isotopologues or stereoisomers. Nevertheless, the central distinction remains robust: an element is defined by atomic number, while a compound contains chemically combined atoms of different elements. The concepts of element and compound are therefore complementary, with elements representing fundamental chemical identities and compounds representing structured combinations of those identities.
Understanding the difference between elements and compounds is essential for interpreting chemical reactions and conservation laws. During ordinary chemical reactions, the atoms of the elements present are rearranged into different combinations, but the nuclei remain essentially unchanged. A reaction such as the formation of water from hydrogen and oxygen does not create hydrogen or oxygen atoms from nothing; it changes their bonding arrangement.
The law of conservation of mass in ordinary chemical reactions reflects this atomic rearrangement, while balanced chemical equations ensure that the number of atoms of each element is conserved. Nuclear reactions are an important exception to the simple chemical picture because changes in nuclear composition can transform one element into another and release much larger amounts of energy. The distinction between elements and compounds therefore provides the conceptual foundation for understanding chemical composition, reaction equations, stoichiometry, decomposition, synthesis and the structure of matter.
