Chemical element

Lore & Background

The concept of a chemical element has two related meanings: it can refer to a pure substance composed of a single kind of atom, or to that kind of atom itself as a component of compounds. For instance, water contains the elements hydrogen and oxygen, but not the elementary substances dihydrogen and dioxygen. Historically, an element was defined as a substance that could not be broken down into simpler substances by chemical reactions, a definition that remains practically useful. The modern understanding is that a chemical element is a species of atom defined by its number of protons, known as its atomic number. Atoms of the same element can have different numbers of neutrons, forming isotopes. Elements can be transformed into other elements through nuclear reactions that change the atomic number. The periodic table, first published by Dmitri Mendeleev in 1869, organizes elements by increasing atomic number into rows (periods) and columns (groups) that share recurring physical and chemical properties. This arrangement allows chemists to derive relationships between elements and predict properties of undiscovered ones. As of November 2016, 118 elements are recognized, with the first 94 occurring naturally on Earth and the remaining 24 being synthetic, produced in nuclear reactions. Almost all baryonic matter in the universe is composed of elements, with rare exceptions like neutron stars. The lightest elements, hydrogen and helium, were created in Big Bang nucleosynthesis, while most others were formed through various natural nucleosynthesis processes. On Earth, new atoms are also produced naturally through nucleogenic reactions, cosmic ray spallation, and radioactive decay.

Reader's Guide

The term 'chemical element' carries two closely related meanings: a kind of atom defined by its proton count, and a pure substance consisting of only that kind of atom. This duality is reflected in some languages, such as French (élément chimique vs. corps simple) and Russian (химический элемент vs. простое вещество). The periodic table remains the most convenient and traditional presentation, summarizing properties and allowing predictions about undiscovered elements. The discovery and synthesis of new elements continues as an ongoing area of scientific study.

Historically, the concept of a chemical element emerged from the search for substances that could not be broken down further by chemical reactions, a definition still practically valid. Early human societies used native minerals like carbon, sulfur, copper, and gold without understanding the modern concept. The modern framework was largely shaped by Dmitri Mendeleev, who published the first recognizable periodic table in 1869, organizing elements by increasing atomic number into periods and groups that share recurring physical and chemical properties. This table enables chemists to derive relationships and predict properties of yet-unobserved nuclides and their potential compounds. The International Union of Pure and Applied Chemistry (IUPAC) now recognizes 118 elements, with the first 94 occurring naturally on Earth and the remaining 24 being synthetic, produced in nuclear reactions. Nearly all elements, except for rapidly decaying radioactive ones, are industrially available. The lightest elements, hydrogen and helium, were created in Big Bang nucleosynthesis, while almost all others were formed through natural nucleosynthesis processes, including nucleogenic reactions, cosmic ray spallation, and radioactive decay. Ongoing research continues to discover and synthesize new elements.

Did You Know?

• Oxygen has an atomic number of 8, meaning each oxygen atom has 8 protons in its nucleus.
• Atoms of the same element can have different numbers of neutrons, known as isotopes.
• The first 94 elements occur naturally on Earth; the remaining 24 are synthetic.
• The periodic table organizes elements by increasing atomic number into periods and groups.