Periodic table
Explore all 118 chemical elements. Select a symbol to open its element profile.
Find chemical properties, uses, health effects, environmental effects and discovery information in the element profiles.
Browse by name, atomic number, symbol or atomic mass. Download the periodic table of elements (PDF)
Rows are periods; columns are groups 1–18. The number above each symbol is its atomic number.
Colour key
Blue: metals · Green: other nonmetals and halogens · Purple: metalloids · Yellow: noble gases · Cyan: lanthanoids and actinoids.
Colours show broad categories rather than every individual family. Polonium is shown as a metal; astatine and tennessine are grouped with the halogens. The heaviest elements are grouped by their position in the table; their properties may be incompletely characterised.
La and Ac appear in the main table. Ce–Lu and Th–Lr continue periods 6 and 7 in the two rows below it. On a small screen, scroll within the table or use the element lists above.
Explore further
Read about chemical properties, health effects, environmental effects and the history of the periodic table.
Background information on the periodic table
The periodic table arranges the elements by atomic number and reveals repeating patterns in their chemistry. This guide explains those patterns for students, general readers and anyone using element data in water treatment.
How to read an element
Atomic number (Z) is the number of protons in the nucleus. It identifies the element: every carbon atom has six protons. A neutral atom has the same number of electrons as protons; an ion has gained or lost electrons. Isotopes of an element have different numbers of neutrons.
The number above each symbol in this chart is the atomic number. In element data, relative atomic mass usually reflects an average of isotope masses weighted by their abundance. It differs from the whole-number mass number of one isotope (protons plus neutrons). Where IUPAC shows a mass number in square brackets for an element without a standard atomic weight, it identifies a selected isotope rather than an average.
Definitions: Royal Society of Chemistry element-data glossary and IUPAC periodic table.
Periods, groups and electron blocks
- Periods (rows 1–7): atomic number increases from left to right. Rows follow the progression of electron energy levels. For main-group elements, the period number corresponds to the highest occupied principal shell; transition and inner transition elements also fill subshells within lower shells. “Row number equals the number of occupied shells” is a useful introductory guide, with exceptions.
- Groups (columns 1–18): elements often have related outer-electron configurations and similar chemical behaviour. Main-group elements in groups 1 and 2 generally have one and two outer-shell electrons; groups 13–18 generally have three to eight. Helium has two. Transition-metal valence is more complex, so the group number is not a universal electron count.
- Blocks (s, p, d and f): these describe the subshell being filled in the broad electron-configuration pattern. They help explain bonding and chemical similarities, with some configuration exceptions. Helium has an s-shell configuration (1s2) but belongs with the noble gases in group 18.
Two useful exceptions: hydrogen occupies group 1 but is a nonmetal, rather than an alkali metal. The detached rows belong to periods 6 and 7; they are placed below the main table to save width. In this chart, La and Ac remain in the main table, with Ce–Lu and Th–Lr below.
Periodic trends: patterns with exceptions
| Property | Across a period, left to right | Down a group | What it means |
|---|---|---|---|
| Atomic radius | Generally decreases | Generally increases | Increasing nuclear attraction draws electrons closer across a row; additional shells generally make atoms larger down a column. Radius depends on how it is defined. |
| Electronegativity | Generally increases | Generally decreases | The tendency to attract shared electrons in a bond. Fluorine is highest on the Pauling scale; values are not assigned to every element on every scale. |
| First ionization energy | Generally increases | Generally decreases | The energy needed to remove one electron from an isolated gaseous atom. Electron configuration produces exceptions to the general trend. |
Compare like with like: an ionic radius and a covalent atomic radius describe different situations. Trends help interpret properties, but do not replace measured data for a particular element or compound.
Further reading: OpenStax on periodic variations in element properties and electronegativity and bonding.
Element families at a glance
| Family or category | Typical characteristics | Examples |
|---|---|---|
| Alkali metals, group 1 excluding H | Soft, reactive metals; their reactions with water generally become more vigorous down the group. | Lithium, sodium, potassium |
| Alkaline earth metals, group 2 | Commonly form 2+ ions. Their hardness, density and reaction with water vary; they should not all be described as soft or strongly water-reactive. | Magnesium, calcium |
| Transition metals | Often good conductors, with several oxidation states and coloured compounds. Many are hard and have high melting points, but these are not universal properties of the central metallic elements. | Iron, copper, gold |
| Lanthanoids and actinoids | The two series associated with the detached rows. Most lanthanoids have stable isotopes, but promethium does not. All actinoids are radioactive; many heavier members are produced artificially. | Neodymium; uranium and plutonium |
| Metalloids | Have properties between those of metals and nonmetals. Some are important semiconductors. The exact membership of this category varies between references. | Silicon, germanium |
| Other nonmetals | Diverse elements that commonly form covalent bonds and many compounds essential to life. | Carbon, nitrogen, oxygen, sulfur |
| Halogens, group 17 | Reactive nonmetals that commonly form halide salts with metals. Hazards depend on the element, chemical form and exposure; a halide ion is not the same substance as the elemental halogen. | Fluorine, chlorine, iodine |
| Noble gases, group 18 | Filled outer shells give the familiar noble gases low reactivity. They are not completely inert: xenon forms compounds. Properties of the superheavy member oganesson remain incompletely characterised. | Helium, neon, xenon |
The chart's colour key combines some of these families into broader categories. Colours indicate classification, rather than an element's appearance or a universal measure of reactivity. See the RSC periodic table for individual element data, including xenon compounds.
How the modern table developed
1869 — Dmitri Mendeleev: arranged elements using atomic mass and recurring chemical properties. He left gaps and predicted properties of undiscovered elements, including the elements later identified as gallium and germanium.
1913 — Henry Moseley: used characteristic X-ray measurements to establish atomic number as the correct ordering principle. Ordering by nuclear charge resolved inconsistencies in a table based on atomic mass.
The modern table contains 118 recognised elements. Its present seventh period was completed with the naming of nihonium, moscovium, tennessine and oganesson in 2016.
Sources: RSC history of the periodic table and IUPAC announcement of elements 113, 115, 117 and 118.
From stars to living organisms
- Cosmic origins: the early universe produced mainly hydrogen and helium, with small amounts of lithium. Fusion in stars forms elements such as carbon and oxygen and, in massive stars, nuclei up to the iron group. Heavier nuclei also form through neutron capture: slowly in evolved stars and rapidly in environments such as neutron-star mergers and some stellar explosions. Elements beyond iron therefore do not all require a supernova or merger.
- A rare natural element: astatine is among Earth's rarest naturally occurring elements. All its isotopes are radioactive and short-lived, so it occurs only in tiny, continually renewed quantities. Estimates of its total abundance depend on what part of Earth is considered.
- The elements of life: CHNOPS stands for carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. These are central to biological molecules. Oxygen, carbon, hydrogen and nitrogen alone account for about 96% of human body mass; calcium and other elements also have essential roles.
Sources: NASA on the first elements, NASA on element formation in evolved stars, US Department of Energy on nucleosynthesis, RSC astatine profile and RSC human-body element resource (PDF).
Why the periodic table matters in water treatment
Element families and electron behaviour help explain why dissolved substances react differently. Calcium and magnesium ions contribute to hardness; iron and manganese can occur in several oxidation states; silicon occurs in forms relevant to silica scaling. These applications require information about the actual dissolved species and water conditions.
An element is not a chemical species. Chlorine gas (Cl2), chloride (Cl−) and hypochlorite (OCl−) contain the same element but behave very differently. Use the element profiles as a starting point, then consider concentration, pH, oxidation state and chemical form when assessing water-treatment behaviour.
Select Periodic table above to return to the chart and open any element profile.
Background guide last reviewed: by Lenntech.