All matter in the universe is composed of fundamental building blocks known as elements. An element is defined as a pure substance that cannot be broken down into simpler substances through ordinary chemical means, such as heating or chemical reactions. Elements represent the simplest forms of matter that retain their distinct chemical identities, serving as the core materials from which all other substances are constructed.
The Defining Feature of an Element
The scientific definition of an element rests entirely on the structure of its atoms, which are composed of protons, neutrons, and electrons. The defining feature of an element is the number of protons located in its nucleus. This count is known as the atomic number, symbolized by the letter Z.
Every atom of a specific element possesses an identical and unique atomic number. For instance, every atom with six protons is, by definition, a carbon atom.
The number of neutrons can vary, creating different versions of the same element called isotopes (e.g., carbon-12 and carbon-14). Similarly, the number of electrons can change to form ions, altering the atom’s charge. However, neither the neutron nor the electron count changes the element’s identity, which remains fixed solely by its atomic number.
Elements vs. Compounds and Mixtures
To understand elements, it is helpful to contrast them with compounds and mixtures. An element is a pure substance made up of only one type of atom, such as pure gold or oxygen gas.
A compound is a pure substance formed when two or more different elements are chemically bonded together in a fixed ratio. Water (H2O) is a familiar example, consisting of two hydrogen atoms bonded to one oxygen atom. The resulting compound has entirely different properties from its constituent elements; for example, hydrogen and oxygen are gases, while water is a liquid. Compounds can only be separated through a chemical reaction.
A mixture involves the physical combination of two or more substances (elements or compounds) that are not chemically bonded. For instance, salt water is a mixture of salt and water. In a mixture, components retain their individual properties and can be combined in variable ratios. Because there is no chemical bond, mixtures can usually be separated using simple physical methods, such as filtration or evaporation.
Organizing the Chemical World
Scientists have identified a total of 118 elements, both naturally occurring and synthetically produced. These elements are systematically arranged on the Periodic Table of Elements.
The table organizes elements primarily by their increasing atomic number, starting with hydrogen (atomic number 1). The arrangement places elements with similar chemical behaviors into vertical columns, known as groups or families. This structure allows scientists to predict an element’s properties based on its position.
The horizontal rows on the table are called periods, which generally correspond to the number of electron shells an atom possesses. The Periodic Table systematizes the fundamental building blocks of all matter, illustrating patterns and relationships between all known elements.