The periodic table, also called the periodic table of elements, is an organized arrangement of the 118 known chemical elements. The chemical elements are arranged from left to right and top to bottom in order of increasing atomic number, or the number of protons in an atom's nucleus, which generally coincides with increasing atomic mass.
The periodic table's organization provides important information about an element's structure and properties. Read on to learn more about how the periodic table was made over 150 years ago and why it's used today.
Related: Periodic table of elements quiz: How many elements can you name in 10 minutes?
Who created the periodic table?
Dmitri Mendeleev, a Russian chemist and inventor, is considered the "father" of the periodic table, according to the Royal Society of Chemistry. In the 1860s, Mendeleev was a popular lecturer at a university in St. Petersburg, Russia. At the time, no modern organic chemistry textbooks in the Russian language existed, so Mendeleev decided to write one. As he was working on that book, titled "Principles of Chemistry" (two volumes, 1868–1870), he simultaneously tackled the problem of the disordered elements, according to Khan Academy.
Putting the elements in order would prove quite difficult. At the time, there were 63 known chemical elements, each with an atomic weight calculated using Avogadro's hypothesis, which states that equal volumes of gases, when kept at the same temperature and pressure, hold the same number of molecules.
Just two strategies existed at the time to categorize these elements: separating them into metals and nonmetals or grouping them by an element's number of valence electrons (the electrons in the outermost shell). The first section of Mendeleev's book dealt with just eight of the known elements — carbon, hydrogen, oxygen, nitrogen, chlorine, fluorine, bromine and iodine — and those two strategies worked for those particular elements, according to Michael D. Gordin in his book "A Well-Ordered Thing: Dmitrii Mendeleev and the Shadow of the Periodic Table" (Princeton University Press, Revised Edition 2018). But they weren't enough to usefully sort the 55 additional chemical elements known at the time.
So, according to the Royal Society of Chemistry, Mendeleev wrote the properties of each element on cards and started ordering them by increasing atomic weight. This is when he noticed a correlation between atomic weight and chemical properties.
However, the exact "Eureka!" moment that led Mendeleev to the sorting strategy of the complete periodic table is still shrouded in mystery. "It is extremely difficult to reconstruct the process by which Mendeleev came to his periodic organization of elements in terms of their atomic weights," Gordin wrote of the full periodic table. "The problem from the historian's perspective is that while Mendeleev kept almost every document and draft that crossed his hands after he believed he would become famous, he did not do so before his formulation of the periodic law."
Gordin continued, "There are two basic ways that Mendeleev could have moved from a recognition of the importance of atomic weight as a good classifying tool to a draft of a periodic system: either he wrote out the elements in order of atomic weight in rows and noticed periodic repetition or he assembled several 'natural groups' of elements, like the halogens and the alkali metals, and noticed a pattern of increasing weight." The only known statement from Mendeleev that was related to his method came in April 1869; he wrote that he "gathered the bodies with the lowest atomic weights and placed them by order of their increase in atomic weight," according to Gordin's book.
Whatever his thought process, Mendeleev ultimately arranged the elements according to both atomic weight and valence electrons. Not only did he leave space for elements not yet discovered, but he predicted the properties of five of these elements and their compounds. In March 1869, he presented the findings to the Russian Chemical Society. Later that year, his new periodic system was published as an abstract in the German chemistry periodical Zeitschrift fϋr Chemie (Journal of Chemistry), according to the University of California, San Diego.
Reading the Periodic Table
The periodic table contains an enormous amount of information, but some of the most important are atomic numbers, atomic symbols, and atomic masses.
Atomic number: The number of protons in an atom's nucleus is referred to as the atomic number of that element. The number of protons defines what element it is and determines its chemical behavior. For example, carbon atoms always have six protons; hydrogen atoms always have one; and oxygen atoms always have eight. Different versions of the same element, called isotopes, can have a different number of neutrons. Elements can also gain or lose electrons to become charged, in which case they are called ions.
Atomic symbol: The atomic symbol (or element symbol) is an abbreviation chosen to represent an element ("C" for carbon, "H" for hydrogen and "O" for oxygen, etc.). These symbols are used internationally and are sometimes unexpected. For example, the symbol for tungsten is "W" because another name for that element is wolfram. The atomic symbol for gold is "Au" because the word for gold in Latin is "aurum."
Atomic mass: The standard atomic weight of an element is the average mass of the element written in atomic mass units (amu). Even though each atom has roughly a whole number of atomic mass units, you will notice that the atomic mass on the periodic table is a decimal; that's because the number is a weighted average of the various naturally-occurring isotopes of an element based on their abundance.
The atomic mass for certain manmade elements is a little more complicated. For elements 93-118, which are lab-created trans-uranium elements (elements beyond uranium, which has an atomic number of 92), there is no "natural" abundance, according to the Los Alamos National Laboratory (LANL). With these elements, the atomic weight of the longest-lived isotope gets listed on the periodic table, according to the International Union of Pure and Applied Chemistry (IUPAC). These atomic weights should be considered provisional, since a new isotope with a longer half-life (how long it takes 50% of that element to decompose) could be produced in the future, according to the LANL.
The superheavy elements, or those with atomic numbers above 104, also fit into this non-natural category. The larger the atom's nucleus — which increases with the number of protons inside — the more unstable that element is, generally. As such, these outsized elements are fleeting, lasting mere milliseconds before decaying into lighter elements, according to the IUPAC. For instance, superheavy elements 113, 115, 117 and 118 were verified by the IUPAC in December 2015, completing the seventh row, or period, on the table. Several different labs produced the superheavy elements. The atomic numbers, temporary names and official names are:
- 113: ununtrium (Uut), nihonium (Nh)
- 115: ununpentium (Uup), moscovium (Mc)
- 117: ununseptium (Uus), tennessine (Ts)
- 118: ununoctium (Uuo), oganesson (Og)
How is the Periodic Table arranged?
The periodic table is arranged by atomic weight and valence electrons. These variables allowed Mendeleev to place each element in a certain row (called a period) and column (called a group). The table comprises seven rows and eighteen columns.
The number of each row, or period, indicates the number of orbitals for the elements in that row, according to Los Alamos National Laboratory. (Atoms have protons and neutrons in their nucleus, and surrounding that, they have their electrons arranged in orbitals., Orbitals describe the location of an electron as well as its wave-like behavior.)That means all of the elements in the third period — sodium, magnesium, aluminum, silicon, phosphorus, sulfur, chlorine and argon — have three atomic orbitals where their electrons reside.
The column, or group, signifies the number of electrons in the atom's outermost shell; these are called the valence electrons, and they are the electrons that can chemically bond with valence electrons of other elements. The valence electrons can be either shared with another element, in covalent bonding, or exchanged via ionic bonding, according to Purdue University.
As an example, elements in Group 8A (or VIIIA) all have a full set of eight electrons in the highest-energy orbital, chemist William Reusch wrote on his webpage at Michigan State University. Elements that occupy the same column on the periodic table (called a "group") have identical valence electron configurations and consequently behave in a similar fashion chemically. For instance, all the group 18 elements are inert gases, meaning they don't react with any other elements.
There are some exceptions to this rule in the transition elements, which fill the shorter columns at the center of the periodic table. These transition elements have partially filled d-orbitals, which contribute to their unique properties. This distinguishes them from the main group elements that primarily fill s- and p-orbitals.
Let's try an example: We can choose selenium, which has an atomic number of 34, meaning there are 34 total electrons in a neutral atom of selenium. This non-metal resides in Period 4, Group 6A. That means selenium keeps its electrons in four atomic orbitals, and has six valence electrons, or six electrons in its outermost orbital. You can also figure out how many electrons are in its first, second and third orbitals: The first orbital can hold a maximum of two electrons, while the second has four suborbitals and so can hold a total of eight electrons. The third shell of an atom, which consists of nine suborbitals, can hold a maximum of 18 electrons, according to Florida State University's Department of Chemistry and Biochemistry. That means selenium has 2, 8, 18 and 6 electrons in its first, second, third and fourth atomic orbital, respectively.
Evolution of the Periodic Table
When Mendeleev first published the original periodic table in 1869, it contained only 63 elements — about half as many as there are today. His table had gaps to leave room for elements that had not been yet discovered, and Mendeleev used the position of these gaps to predict the properties of potential new elements. One example of this is what Mendeleev called "eka-aluminium," a proposed element with properties similar to aluminum. A few years later, this element was identified as gallium, and its properties were confirmed to be very similar to Mendeleev's predictions.
Since the 19th century, the number of known elements has nearly doubled. Those newly discovered elements have slowly filled in the gaps on Mendeleev's original design and extended the table to even higher atomic numbers. All of the gaps were officially filled on the periodic table in the 2010's after the synthesis of element 117, Tennessine.
It's theoretically possible to extend the periodic table even further as chemists synthesize increasingly heavier elements, so the periodic table may never be complete. These manmade elements are created using particle accelerators that smash atoms and subatomic particles together, generating nuclei with extra protons and neutrons. However, these ultra-heavy elements are highly unstable and difficult to make. “We really do not know what is the heaviest element that could exist,” Witold Nazarewicz, a nuclear physicist at Michigan State University, told Smithsonian Magazine.
How is the Periodic Table used today?
By knowing that certain elements that are lumped together on the table have certain characteristics and behaviors, scientists can figure out which ones would be best for certain industries and processes. For instance, engineers use different combinations of elements in Groups III and V of the table to create new semiconductor alloys, such as gallium nitride (GaN) and Indium nitride (InN), according to the National Institute of Standards and Technology (NIST).
In general, chemists and other scientists can use the table to predict how certain elements will react with one another. The alkali metals, for instance, are in the first column or group of the table and tend to have one valence electron and so carry a charge of +1. This charge means they "react vigorously with water, and combine readily with nonmetals," chemist Anne Marie Helmenstine wrote on ThoughtCo. Magnesium, which is in the same group on the table as calcium, is becoming useful as part of alloys for bone implants, NIST said. Since these alloys are biodegradable, they serve as a scaffolding and then disappear after natural bone grows on the structures.
Over a century after its creation, the periodic table remains one of the most critical tools in science, providing a framework for understanding the building blocks of matter. It not only organizes all known elements into a coherent structure, but also serves as a roadmap for predicting and synthesizing new elements and guiding chemical innovation.
Additional reporting by Traci Pedersen, Live Science contributor
