In the late 19th century, Russian chemist Dmitri Mendeleev published his first attempt at agroup chemical elements according to their atomic weights. There were only about 60 known elements at the time.
However, Mendeleev realized that when elements were arranged by weight, certain types of elements occurred at regular intervals or periods.
Today, 150 years later, chemists officially recognize 118 elements (after adding four newcomers in 2016) and still use the periodic table of Mendeleev elements to organize them.
The table starts with the simplest atom, hydrogen, and 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 to the increasing mass of each atom.
Index
- Table
- alkali metals
- alkaline earth metals
- Lanthanides
- actinides
- transition metals
- Post-Transition Metals
- Nonmetals
- non-metals
- Halogens
- noble gases
Table
The table has seven rows and 18 columns. Each line represents a period. An element's period number indicates how many of its energy levels harbor electrons. Sodium, for example, is in the third period.
This means that a sodium atom normally has electrons in the first three energy levels. Moving across the table, the periods are longer because more electrons are needed to fill the larger, more complex outer levels.
Table columns represent groups or families of elements. The elements of a group generally look and behave similarly because they have the same number of electrons in their outermost shell. It's 'the face' they show the world.
The elements in group 18 on the right side of the table, for example, have completely filled outer shells and rarely participate in chemical reactions.
Elements are typically classified as metals or non-metals, but the dividing line between the two is blurred. Metallic elements are generally good conductors of electricity and heat.
Subgroups within 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
Alkali metals make up the bulk of Group 1, the first column in the table. Bright and soft enough to cut with a knife, these metals start with the lithium (li) and end with the francium (Fr).
They are also extremely reactive and burst into flames or even explode on contact with water. In this way, chemists store them in oils or inert gases.
O hydrogen, with its single electron, also lives in Group 1, but the gas is considered a nonmetal.
alkaline earth metals
The alkaline earth metals form group 2 of the periodic table, from the beryllium (Be) to radio (Ra). Each of these elements has two electrons at its outermost energy level. This makes alkaline earths reactive enough that they are rarely found alone in nature.
However, they are not as reactive as alkali metals. Their chemical reactions generally proceed more slowly and produce less heat compared to alkali metals.
Lanthanides
The third group is too long to fit in the third column, so it floats at the bottom of the table. These are the lanthanides, elements from 57 to 71 - lanthanum (La) to lutetium (Lu). The elements of this group have a silvery white color and tarnish on contact with air.
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actinides
Actinides comprise the 89 elements, actinium (Ac), up to 103, laurencium (Lr). Of these elements, only the thorium (Th) and uranium (U) occur naturally on Earth in substantial amounts. All are radioactive.
Actinides and lanthanides together form a group called the internal transition metals.
transition metals
Returning to the main body of the table, the remainder of Groups 3 through 12 represents the remainder of the transition metals. Tough but malleable, shiny and with good conductivity, these elements are what you normally think of when you hear the word metal.
Many of the most famous metals – including gold, silver, iron and platinum – live here.
Post-Transition Metals
Post-transition metals are the aluminum (Al), gallium (Ga), Indian (In), thallium (Tl), tin (Sn), lead (Pb) and bismuth (Bi), and cover Group 13 to Group 17.
These elements have some of the classic characteristics of transition metals, but tend to be softer and have lower conductivity than other transition metals.
Nonmetals
The metalloids are the boron (B), silicon (Yes), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Dust). They form the ladder that represents the gradual transition from metals to non-metals.
These elements sometimes behave like semiconductors (B, Si, Ge) rather than conductors. Metalloids are also called "semimetals" or "poor metals".
non-metals
Everything else on the right side of the ladder – plus the hydrogen (H), stranded in Group 1 – are nonmetals. These include carbon (Ç), nitrogen (N), phosphor (P), oxygen (O), sulfur (S) and selenium (If).
Halogens
The four main elements of Group 17, from fluorine (Fan astatine (At), represent one of two subsets of nonmetals. Halogens are chemically reactive and tend to form alkali metal pairs to produce various types of salts.
The table salt in your kitchen, for example, is a marriage between the alkali metal sodium and chlorine, a halogen.
noble gases
Colorless, odorless and almost completely non-reactive, inert or noble gases complete the Group 18 table. Many chemists hope that oganesson, one of the four newly named elements, will share these characteristics.
However, since this element has a half-life of milliseconds, no one has been able to test it directly.
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.
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