All matter, including minerals, is composed of minute building blocks called atoms—the smallest particles that constitute specific elements and cannot be split by chemical means. Atoms, in turn, contain even smaller particles—protons and neutrons located in a central nucleus that is surrounded by electrons ▼.

Properties of Protons, Neutrons, and Electrons

Protons and neutrons are very dense particles with almost identical masses. By contrast, electrons have a negligible mass, about 1/2000 that of a proton. To visualize this difference, imagine a scale on which a proton or neutron has the mass of a baseball, whereas an electron has the mass of a single grain of rice.

Both protons and electrons share a fundamental property called electrical charge. Protons have an electrical charge of +1, and electrons have a charge of -1. Neutrons, as the name suggests, have no charge. The charges of protons and electrons are equal in magnitude but opposite in polarity, so when these two particles are paired, the charges cancel each other out. Because matter typically contains equal numbers of positively charged protons and negatively charged electrons, most substances are electrically neutral.

Illustrations sometimes show electrons orbiting the nucleus in a manner that resembles the planets of our solar system orbiting the Sun ▲. However, electrons do not actually behave this way. A more realistic depiction would show electrons as a cloud of negative charges surrounding the nucleus ▲. Studies of the arrangements of electrons show that they move about the nucleus in regions called principal shells, each with an associated energy level. In addition, each shell can hold a specific number of electrons, with the outermost shell generally containing valence electrons. These electrons can be transferred to or shared with other atoms to form chemical bonds.

Most of the atoms in the universe (except hydrogen and helium) were created inside massive stars by nuclear fusion and then released into interstellar space during hot, fiery supernova explosions. As this ejected material cooled, the newly formed nuclei attracted electrons to complete their atomic structure. At the temperatures found at Earth’s surface, free atoms (those not bonded to other atoms) generally have a full complement of electrons—one for each proton in the nucleus.

Elements: Defined by Their Number of Protons

The simplest atoms have only 1 proton in their nuclei, whereas others have more than 100. The number of protons in the nucleus of an atom, called the atomic number, determines the atom’s chemical nature. All atoms with the same number of protons have the same chemical and physical properties; collectively they constitute a chemical element. There are about 92 naturally occurring elements, and several more have been synthesized in the laboratory. You are probably familiar with the names of many elements, including carbon, nitrogen, and oxygen. All carbon atoms have 6 protons, whereas all nitrogen atoms have 7 protons, and all oxygen atoms have 8.

The periodic table, shown in the figure below ▼, is a tool scientists use to organize the known elements. In it, the elements with similar properties line up in columns, referred to as groups. Each element is assigned a one- or two-letter symbol. The atomic number and atomic mass for each element are also included in the periodic table.

Atoms of the naturally occurring elements are the basic building blocks of Earth’s minerals. Most elements join with other elements to form chemical compounds. Therefore, most minerals are chemical compounds composed of atoms of two or more elements. These include the minerals quartz (SiO2), halite (NaCl), and calcite (CaCO3). However, a few minerals, such as diamonds, sulfur, and native gold and copper, are made entirely of atoms of only one element ▼. (A metal is called “native” when it is found in its pure form in nature).

• Minerals are composed of atoms of one or more elements. All atoms consist of the same three basic components: protons, neutrons, and electrons.

• The atomic number represents the number of protons found in the nucleus of an atom of a particular element. For example, an oxygen atom has eight protons, so its atomic number is eight. Protons and neutrons have approximately the same size and mass, but protons are positively charged, whereas neutrons have no charge.

• Electrons weigh only about 1/2000 as much as protons or neutrons. They occupy the space around the nucleus, where they form what can be thought of as a cloud that is structured into several distinct energy levels called principal shells. The electrons in the outermost principal shell, called valence electrons, are responsible for the bonds that hold atoms together to form chemical compounds.

• Elements that have the same number of valence electrons tend to behave similarly. The periodic table is organized so that elements with the same number of valence electrons form a column, called a group.

• atomic number: The number of protons in the nucleus of an atom; determines the atom’s chemical nature.

• atoms: The smallest particles that constitute specific elements.

• chemical compounds: Substances formed by the chemical combination of two or more elements and usually having properties different from those of its constituent elements.

• chemical element: A substance with unique chemical and physical properties that cannot be broken down into another substance.

• electrons: Negatively charged subatomic particles that have a negligible mass and are found outside an atom’s nucleus.

• neutrons: Subatomic particles found in the nucleus of an atom. A neutron is electrically neutral and has a mass approximately that of a proton.

• periodic table: The organized arrangement of the elements into rows and columns in order of increasing atomic number.

• proton: A positively charged subatomic particle in the nucleus of an atom.

• valence electrons: The electrons involved in the bonding process; the electrons occupying the outermost shell of an atom.