In the late 19th century, Russian chemist Dmitri Mendeleev published his first attempt at grouping chemical elements according to their atomic weights. There were only about 60 elements known at the time, but Mendeleev realized that when the elements were organized by weight, certain types of elements occurred in regular intervals, or periods.
Today, 150 years later, chemists officially recognize 118 elements (after the addition of four newcomers in 2016) and still use Mendeleev's periodic table of elements to organize them. The table starts with the simplest atom, hydrogen, and then organizes the rest of the elements by atomic number, which is the number of protons each contains. With a handful of exceptions, the order of the elements corresponds with the increasing mass of each atom.
The table has seven rows and 18 columns. Each row represents one period; the period number of an element indicates how many of its energy levels house electrons. Sodium, for instance, sits in the third period, which means a sodium atom typically has electrons in the first three energy levels. Moving down the table, periods are longer because it takes more electrons to fill the larger and more complex outer levels.
The columns of the table represent groups, or families, of elements. The elements in a group often look and behave similarly, because they have the same number of electrons in their outermost shell — the face they show to the world. Group 18 elements, on the far right side of the table, for example, have completely full outer shells and rarely participate in chemical reactions.
Elements are typically classified as either a metal or nonmetal, but the dividing line between the two is fuzzy. Metal elements are usually good conductors of electricity and heat. The subgroups within the metals are based on the similar characteristics and chemical properties of these collections. Our description of the periodic table uses commonly accepted groupings of elements, according to the Los Alamos National Laboratory.
Alkali metals: The alkali metals make up most of Group 1, the table's first column. Shiny and soft enough to cut with a knife, these metals start with lithium (Li) and end with francium (Fr). They are also extremely reactive and will burst into flame or even explode on contact with water, so chemists store them in oils or inert gases. Hydrogen, with its single electron, also lives in Group 1, but the gas is considered a nonmetal.
Alkaline-earth metals: The alkaline-earth metals make up Group 2 of the periodic table, from beryllium (Be) through radium (Ra). Each of these elements has two electrons in its outermost energy level, which makes the alkaline earths reactive enough that they're rarely found alone in nature. But they're not as reactive as the alkali metals. Their chemical reactions typically occur more slowly and produce less heat compared to the alkali metals.
Lanthanides: The third group is much too long to fit into the third column, so it is broken out and flipped sideways to become the top row of the island that floats at the bottom of the table. This is the lanthanides, elements 57 through 71 — lanthanum (La) to lutetium (Lu). The elements in this group have a silvery white color and tarnish on contact with air.
Actinides: The actinides line the bottom row of the island and comprise elements 89, actinium (Ac), through 103, lawrencium (Lr). Of these elements, only thorium (Th) and uranium (U) occur naturally on Earth in substantial amounts. All are radioactive. The actinides and the lanthanides together form a group called the inner transition metals.
Transition metals: Returning to the main body of the table, the remainder of Groups 3 through 12 represent the rest of the transition metals. Hard but malleable, shiny, and possessing good conductivity, these elements are what you typically think of when you hear the word metal. Many of the greatest hits of the metal world — including gold, silver, iron and platinum — live here.
Post-transition metals: Ahead of the jump into the nonmetal world, shared characteristics aren't neatly divided along vertical group lines. The post-transition metals are aluminum (Al), gallium (Ga), indium (In), thallium (Tl), tin (Sn), lead (Pb) and bismuth (Bi), and they span Group 13 to Group 17. These elements have some of the classic characteristics of the transition metals, but they tend to be softer and conduct more poorly than other transition metals. Many periodic tables will feature a bolded "staircase" line below the diagonal connecting boron with astatine. The post-transition metals cluster to the lower left of this line.
Metalloids: The metalloids are boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po). They form the staircase that represents the gradual transition from metals to nonmetals. These elements sometimes behave as semiconductors (B, Si, Ge) rather than as conductors. Metalloids are also called "semimetals" or "poor metals."
Nonmetals: Everything else to the upper right of the staircase — plus hydrogen (H), stranded way back in Group 1 — is a nonmetal. These include carbon (C), nitrogen (N), phosphorus (P), oxygen (O), sulfur (S) and selenium (Se).
Halogens: The top four elements of Group 17, from fluorine (F) through astatine (At), represent one of two subsets of the nonmetals. The halogens are quite chemically reactive and tend to pair up with alkali metals to produce various types of salt. The table salt in your kitchen, for example, is a marriage between the alkali metal sodium and the halogen chlorine.
Noble gases: Colorless, odorless and almost completely nonreactive, the inert, or noble gases round out the table in Group 18. Many chemists expect oganesson, one of the four newly named elements, to share these characteristics; however, because this element has a half-life measuring in the milliseconds, no one has been able to test it directly. Oganesson completes the seventh period of the periodic table, so if anyone manages to synthesize element 119 (and the race to do so is already underway), it will loop around to start row eight in the alkali metal column.
Because of the cyclical nature created by the periodicity that gives the table its name, some chemists prefer to visualize Mendeleev's table as a circle.
- Watch this brief video about the periodic table and element groups, from Crash Course.
- Flip through this interactive periodic table of elements.
- Check out this free, online educational resource for understanding elemental groups from CK-12.