Here you will find the key terms and their definitions related to the periodic table of the elements, explained clearly and simply. Use the letters menu and the search box to find concepts.
A way of writing an electron configuration by replacing an inner part with the symbol—in square brackets—of the noble gas with that same electron distribution.
A chemical element belonging to the actinide series, which comprises the elements from actinium (Ac, atomic number 89) to lawrencium (Lr, atomic number 103). These elements belong to the f-block and exhibit chemical properties related to the filling of the 5f orbitals, although there are exceptions in their electron configurations. All actinides are radioactive.
A metallic element in Group 1 of the periodic table. It has one valence electron and typically forms ions with a +1 charge; hydrogen does not belong to this group.
An element in Group 2 of the periodic table. Its atoms have two valence electrons and typically form ions with a +2 charge.
Each of the structural forms of the same chemical element that exhibits different properties. Diamond and graphite, for example, are allotropes of carbon.
An electrically neutral particle of a chemical element, consisting of a nucleus containing protons and, with exceptions such as hydrogen-1, neutrons, surrounded by electrons. The number of protons determines which element the atom belongs to, while the number of neutrons can vary, giving rise to different isotopes. Electrons occupy atomic orbitals, which describe regions of space where they are most likely to be found.
The mass of an atom, usually expressed in atomic mass units (u), also called daltons (Da). Atoms of the same element can have different masses because isotopes contain different numbers of neutrons. The atomic mass of an individual atom corresponds to its own mass, whereas the relative atomic mass of an element is related to the masses of its isotopes and their relative abundances.
A unit of mass, symbol u, defined as one-twelfth of the mass of a free carbon-12 atom at rest and in its ground state. It is also called a dalton, symbol Da.
The number of protons in an atom’s nucleus. It is denoted by Z and determines which element the atom belongs to.
A mathematical function that describes a possible state of an electron in an atom and allows for the calculation of its probability distribution. It is not a trajectory; each orbital can hold at most two electrons with opposite spins.
A measure of an atom’s size obtained using a defined criterion. Since the electron cloud has no precise boundary, its value depends on the type of radius considered.
A set of models and principles that explain matter based on atoms, their structure, and their interactions. Its current formulation incorporates quantum mechanics.
A criterion for constructing electronic configurations by first filling the lowest-energy available orbitals. It explains many fundamental configurations, although there are exceptions to this simplified filling order.
The formation of ions from a substance without the addition of an external ionizing agent. In water, two molecules can exchange a proton and produce the ions H₃O⁺ and OH⁻.
The exact number of entities contained in one mole: 6.02214076 × 1023. Avogadro’s constant has the same numerical value and is expressed in mol−1.
Regions designated as s, p, d, and f based on the type of sublevel being occupied as one moves through the elements. They relate the position of the elements to their electronic structure.
An ion with a positive electric charge. An atom becomes a cation when it loses one or more electrons.
An interaction that holds atoms or ions together in a stable structure, such as a molecule or a crystal.
A class of atoms that have the same number of protons in the nucleus. That number, called the atomic number, determines the identity of the element.
A set of rules that allows for the systematic assignment of names to elements, compounds, and other chemical species.
A process in which one or more substances are transformed into other substances through the rearrangement of their atoms and chemical bonds. During a chemical reaction, atoms are conserved, but the way they are bonded changes. Reactions may release or absorb energy and, in some cases, involve electron transfer, as occurs in oxidation-reduction reactions.
An abbreviation that identifies an element. Current permanent symbols consist of one or two letters, with the first letter capitalized and the second, if present, lowercase.
A bond between atoms in which electron density is shared, typically represented by one or more electron pairs.
A measure of an atom’s size when it forms covalent bonds. It is typically estimated as half the distance between the nuclei of two identical bonded atoms.
The ratio of a substance’s mass to the volume it occupies. Its value depends on conditions such as temperature, pressure, and physical state.
A covalent bond in which two atoms share two pairs of electrons. It is represented by two lines between their chemical symbols.
The net attraction exerted by the nucleus on an electron in an atom with multiple electrons, taking into account the effect of the other electrons.
A physical property responsible for electrical interactions. It can be positive or negative and is measured in coulombs, symbolized by C.
An elementary particle with a negative electric charge. In atoms, electrons occupy states described by orbitals around the nucleus.
The energy change associated with the addition of an electron to an isolated neutral atom in the gas phase, forming a negative ion. When the process releases energy, the system loses energy; when it requires energy, the system gains energy. The sign assigned to electron affinity depends on the convention used, so it is important to distinguish between the energy released and the change in the system’s energy.
The distribution of electrons in an atom or ion among its shells, sublevels, and orbitals.
A set of orbitals in an atom that share the same principal quantum number, denoted by n. Each shell contains one or more sublevels.
Another term for an electron sublevel. It groups together the orbitals of the same shell that belong to the same type: s, p, d, or f.
The ability of a bonded atom to attract toward itself the electron density shared with other atoms.
A set of elements with similar chemical properties. The term is typically used to refer to the members of the same group in the periodic table.
The relative amount of an element present in a particular environment, such as Earth’s crust, the oceans, the atmosphere, or the universe. It can be expressed in different ways, for example, as a mass fraction or as the proportion of atoms relative to the total number of atoms considered. The abundance of an element varies depending on the environment being studied.
An allowed energy value for a quantum system, such as an atom. Transitions between levels require the absorption or emission of a specific amount of energy.
A division of an electron shell consisting of orbitals with the same principal and angular quantum numbers. It is identified by combinations such as 2p or 3d.
A state with higher energy than the ground state. It can be reached, for example, through the absorption of light.
The lowest possible energy state of an atom, molecule, or other system.
Each of the 18 vertical columns in the periodic table. Elements in the same group generally have related valence electron configurations and similar chemical properties, although these similarities vary between groups and do not mean that all their elements behave in the same way. The modern numbering system assigns groups from 1 to 18.
The time required for half of the radioactive nuclei in a sample of a radionuclide to decay. It does not indicate when a specific atom will have decayed.
An element in Group 17 of the periodic table. Its atoms have seven electrons in the valence shell and typically form ions with a charge of −1.
A term of variable usage that is usually applied to metals with high density. It has no universal definition and is not necessarily equivalent to “toxic metal.”
A compound of hydrogen with another element. It can feature different types of bonds; in ionic hydrides, hydrogen appears as the H− ion.
An element belonging to the lanthanide or actinide series. These series are related to the filling of the 4f and 5f subshells, respectively.
An atom or group of atoms with a net electric charge. The charge is positive if there is a deficit of electrons and negative if there is an excess.
An electrostatic attraction between oppositely charged ions. In ionic solids, it holds the ions together in an extensive structure.
A measure of the size of an ion in a structure, typically crystalline. It depends on its charge and on the number and arrangement of the surrounding atoms or ions.
The energy required to remove an electron from an isolated atom or ion in the gaseous phase. The first ionization energy corresponds to the removal of an electron from a neutral atom in its ground state.
Any of the atoms of different elements that have the same mass number but different atomic numbers.
Atoms, ions, or molecules that have the same number of electrons and an equivalent electron configuration. For example, Ne and Na+ are isoelectric.
Any of the variants of an element whose atoms have the same number of protons but different numbers of neutrons and, therefore, different mass numbers. Stable isotopes do not undergo any known radioactive decay; radioactive isotopes, also called radioisotopes, have unstable nuclei that spontaneously transform and emit radiation. For example, carbon-12 and carbon-13 are stable, while carbon-14 is radioactive.
Each of the 15 elements between lanthanum and lutecium, with atomic numbers ranging from 57 to 71. They are metals with similar properties associated with the filling of the 4f subshell.
In a chemical reaction within a closed system, the total mass of the reactants is equal to that of the products, within the usual precision of chemical measurements.
A representation of the valence electrons of a chemical species using dots and lines. It shows bonds, unshared electron pairs, and formal charges.
The sum of the protons and neutrons in a nucleus. It is denoted by A, is an integer, and should not be confused with atomic mass.
An element that generally conducts heat and electricity well and exhibits luster, malleability, and ductility. These properties are related to its structure and metallic bonding.
A bond characteristic of metals, in which delocalized electrons hold the atomic centers of the structure together.
Elements with intermediate or combined properties of metals and nonmetals. Several are semiconductors; there is no single list accepted in all contexts.
The mass of a substance divided by its amount of substance; it is equivalent to the mass of one mole of the specified entities. Molar mass is usually expressed in grams per mole (g/mol) and allows for conversion between mass and amount of substance. For example, the molar mass of water is approximately 18.015 g/mol: one mole of water molecules has a mass of about 18.015 grams.
The International System unit for the amount of substance. One mole contains exactly 6.02214076 × 1023 specified entities, such as atoms, molecules, or ions.
The mass of a molecule, usually expressed in atomic mass units (u). It is obtained by adding the masses of the atoms that compose it.
A function that describes an electronic state in a molecule. It can span multiple atoms and contribute to strengthening or weakening bonds.
An electrically neutral entity consisting of two or more chemically bonded atoms. It may contain atoms of the same element, as in molecular oxygen (O2), or atoms of different elements, as in water (H2O). Not all substances consist of individual molecules: ionic compounds, for example, generally form extended structures of ions.
An ion with a charge of +1 or −1, expressed in units of elementary charge.
A chemical element found in nature, even if its abundance is extremely low. Some naturally occurring elements can also be produced artificially through nuclear reactions. Their natural occurrence may result from processes such as stellar nucleosynthesis, radioactive decay, or natural nuclear reactions.
Energy associated with the formation of an ionic solid from its gaseous ions, or with the reverse process of separating them. It must be specified which of the two processes is being used, because the sign changes.
An atom that has the same number of protons as electrons, so that its total electric charge is zero.
A particle with no net electric charge that, together with protons, forms atomic nuclei. Its mass is slightly greater than that of a proton.
An element in Group 18 of the periodic table. The electronic configuration of these elements explains their typically low reactivity, although some can form compounds.
A chemical element that generally exhibits properties different from those of metals, such as relatively low electrical and thermal conductivity. Nonmetals can be solid, liquid, or gaseous at room temperature and can form different types of bonds and compounds. They are located mainly in the upper-right region of the periodic table, along with hydrogen.
The splitting of a heavy nucleus into lighter nuclei, usually accompanied by the release of energy and neutrons.
The joining of light nuclei to form a heavier one. In the reactions that power stars, this process releases energy.
The condition of a nucleus that does not spontaneously transform through radioactive decay. It depends on its composition and the interactions between its protons and neutrons.
The number of electrons present in an atom or ion. In an electrically neutral atom, the number of electrons equals the number of protons, which is given by the atomic number. When an atom gains or loses electrons, it becomes an ion: negative ions have more electrons than protons, while positive ions have fewer.
A guideline according to which many atoms tend to form bonds until they are surrounded by eight valence electrons. It is useful for many elements in the main groups, but there are exceptions.
A chemical process in which the oxidation state of an atom or chemical species increases. It is commonly described as the loss of electrons, although in covalent bonds, changes in oxidation state are determined by formally assigning bonding electrons. Oxidation occurs alongside reduction in an oxidation-reduction (redox) reaction.
A number that expresses the charge an atom would have if the electrons in each bond between different elements were assigned to the more electronegative element. It is a tool for electronic accounting, not necessarily an actual charge.
A relative, unitless measure of electronegativity, developed by Linus Pauling based on the energies of chemical bonds.
Each of the seven horizontal rows in the periodic table. Within a period, the elements are arranged in order of increasing atomic number.
The principle according to which the properties of elements exhibit recurring patterns when they are arranged by atomic number.
A characteristic of elements that exhibits a pattern of variation in the periodic table, such as atomic radius, electronegativity, or ionization energy.
An arrangement of the elements in rows and columns in order of increasing atomic number. Its structure allows us to recognize relationships between the electronic configurations and the properties of the elements.
The general direction in which a property changes as one moves down a group or across a period. For example, atomic radius typically increases as one moves down a group.
The recurrence of patterns in the properties of elements as their atomic number increases. It arises from the repetition of similar valence electron configurations.
A group of two or more bonded atoms that has an overall electric charge, such as the sulfate ion, SO42−.
A term used for certain p-block metals, located to the right of the transition metals. Its definition is not entirely uniform.
A positively charged particle located in the atomic nucleus. The number of protons determines the identity of the element.
The spontaneous transformation of unstable nuclei, accompanied by the emission of particles or electromagnetic radiation.
A chemical process in which the oxidation state of an atom or chemical species decreases. It is commonly described as the gain of electrons, although in covalent bonds, changes in oxidation state are determined by formally assigning bonding electrons. Reduction occurs alongside oxidation in an oxidation-reduction (redox) reaction.
The ratio of the mean mass of the atoms of an element to the unified atomic mass unit, defined as one-twelfth of the mass of a carbon-12 atom at rest and in its ground state. It is a dimensionless quantity, meaning it has no units. Its value depends on the isotopic composition of the sample when isotope abundances vary.
A reduction in the nucleus’s attraction to an electron due to the presence of other electrons. It helps determine the effective nuclear charge.
A set of lines in a spectrum associated with transitions between energy levels that follow the same pattern. In hydrogen, each series corresponds to transitions linked to the same energy level.
An internationally recommended value for the relative atomic mass of an element in normal terrestrial materials, taking into account natural variations in isotopic abundances. For many elements, it is expressed as an interval because isotopic composition can vary between samples. Despite the traditional term “weight,” it refers to a relative quantity, not a gravitational force.
A chemical element produced artificially through nuclear reactions, for example, in nuclear reactors or particle accelerators. Many synthetic elements are radioactive and have unstable isotopes. Some of these elements can also occur naturally in very small amounts, so the terms “naturally occurring” and “synthetic” are not always mutually exclusive.
An element whose atom has an incomplete d subshell or that can form cations with an incomplete d subshell. Many transition metals exhibit multiple oxidation states, form complexes, and produce colored compounds, although these properties are not present in every case. They are found mainly in the d-block of the periodic table.
An element with an atomic number greater than 92, the atomic number of uranium. All known transuranic elements are radioactive.
A covalent bond in which two atoms share three pairs of electrons. It is represented by three lines connecting the atomic symbols.
An electron that occupies an orbital without another electron of opposite spin in that same orbital.
Electrons that can participate in chemical bonds and reactions. In main-group elements, these are typically the electrons in the outermost shell.
The outermost principal energy level occupied by electrons in an atom in its ground state. It corresponds to the valence shell and generally plays an important role in the element’s chemical properties. In transition elements, electrons from lower energy levels can also participate in chemical bonding.
The outermost occupied electron shell of an atom in its ground state. Its electrons generally play an important role in chemical bonding and the element’s reactivity. In some elements, particularly transition elements, electrons from inner shells can also participate in chemical bonding and reactions.
A measure of an atom’s size based on its proximity to other atoms without a direct chemical bond. For identical atoms, it is usually estimated as half their contact distance.