Method of visualizing the relationship between elements
In theperiodic table of the elements, each numbered row is a period.
Aperiod on theperiodic table is a row ofchemical elements. Allelements in a row have the same number ofelectron shells. Each next element in a period has one moreproton and is lessmetallic than its predecessor. Arranged this way, elements in the samegroup (column) have similarchemical andphysical properties, reflecting theperiodic law. For example, the halogens lie in the second-to-last group (group 17) and share similar properties, such as high reactivity and the tendency to gain one electron to arrive at a noble-gas electronic configuration. As of 2022[update], a total of 118 elements have been discovered and confirmed.
The Madelung energy ordering rule describes the order in which orbitals are arranged by increasing energy according to the Madelung rule. Each diagonal corresponds to a different value of n + l.
Modernquantum mechanics explains theseperiodic trends in properties in terms ofelectron shells. As atomic number increases, shells fill with electrons in approximately the order shown in the ordering rule diagram. The filling of each shell corresponds to a row in the table.
In thef-block andp-block of the periodic table, elements within the same period generally do not exhibit trends and similarities in properties (vertical trends down groups are more significant). However, in thed-block, trends across periods become significant, and in thef-block elements show a high degree of similarity across periods.
There are currently seven complete periods in the periodic table, comprising the 118 known elements. Any new elements will be placed into an eighth period; seeextended periodic table. The elements are colour-coded below by theirblock: red for the s-block, yellow for the p-block, blue for the d-block, and green for the f-block.
The first period contains fewer elements than any other, with only two,hydrogen andhelium. They therefore do not follow theoctet rule, but rather aduplet rule. Chemically, helium behaves like anoble gas, and thus is taken to be part of thegroup 18 elements. However, in terms of its nuclear structure it belongs to thes-block, and is therefore sometimes classified as agroup 2 element, or simultaneously both 2 and 18. Hydrogen readily loses and gains an electron, and so behaves chemically as both agroup 1 and agroup 17 element.
Hydrogen (H) is the mostabundant of the chemical elements, constituting roughly 75% of the universe's elemental mass.[1] Ionized hydrogen is just aproton.Stars in themain sequence are mainly composed of hydrogen in itsplasma state. Elemental hydrogen is relatively rare onEarth, and is industrially produced fromhydrocarbons such asmethane. Hydrogen can form compounds with most elements and is present inwater and mostorganic compounds.[2]
Helium (He) exists only as agas except in extreme conditions.[3] It is the second-lightest element and is the second-most abundant in the universe.[4] Most helium was formed during theBig Bang, but new helium is created throughnuclear fusion of hydrogen in stars.[5] OnEarth, helium is relatively rare, only occurring as a byproduct of the naturaldecay of some radioactive elements.[6] Such 'radiogenic' helium is trapped withinnatural gas in concentrations of up to seven percent by volume.[7]
Period 2 elements involve the2s and2porbitals. They include the biologically most essential elements besides hydrogen: carbon, nitrogen, and oxygen.
Lithium (Li) is the lightest metal and the least dense solid element.[8] In its non-ionized state it is one of the most reactive elements, and so is only ever found naturally incompounds. It is the heaviestprimordial element forged in large quantities during theBig Bang.
Beryllium (Be) has one of the highestmelting points of all thelight metals. Small amounts of beryllium weresynthesised during the Big Bang, although most of itdecayed or reacted further within stars to create larger nuclei, like carbon, nitrogen or oxygen. Beryllium is classified by theInternational Agency for Research on Cancer as agroup 1 carcinogen.[9] Between 1% and 15% of people are sensitive to beryllium and may develop an inflammatory reaction in theirrespiratory system andskin, called chronic beryllium disease.[10] The primary mirrors on the James Webb Space Telescope (JWST) are made of beryllium.
Boron (B) does not occur naturally as a free element, but in compounds such asborates. It is an essential plantmicronutrient, required for cell wall strength and development, cell division, seed and fruit development, sugar transport and hormone development,[11][12] though high levels are toxic.
Carbon (C) is the fourth-most abundant element in the universe by mass afterhydrogen,helium and oxygen[13] and is thesecond-most abundant element in the human body by mass after oxygen,[14] the third-most abundant by number of atoms.[15] There are an almost infinite number of compounds that contain carbon due to carbon's ability to form long stable chains of C—C bonds.[16][17] Allorganic compounds, those essential for life, contain at least one atom of carbon;[16][17] combined with hydrogen, oxygen, nitrogen, sulfur, and phosphorus, carbon is the basis of every important biological compound.[17]
Nitrogen (N) is found mainly as mostlyinertdiatomic gas, N2, which makes up 78% of the Earth's atmosphere by volume. It is an essential component ofproteins and therefore of life.
Oxygen (O) comprising 21% of the atmosphere by volume and is required forrespiration by all (or nearly all) animals, as well as being the principal component ofwater. Oxygen is the third-most abundant element in the universe, and oxygen compounds dominate the Earth's crust.
Fluorine (F) is the most reactive element in its non-ionized state, and so is never found that way in nature.
Chlorine (Cl) is ahalogen. Since it is one of the most reactive elements, it is often found on the Earth's surface as sodium chloride. Its compounds used as a disinfectant, especially inswimming pools.
Argon (Ar) is anoble gas, making it almost entirely nonreactive.Incandescent lamps are often filled with noble gases such as argon in order to preserve the filaments at high temperatures.
Period 4 includes the biologically essential elementspotassium andcalcium, and is the first period in thed-block with the lightertransition metals. These includeiron, the heaviest element forged inmain-sequence stars and a principal component of the Earth, as well as other important metals such ascobalt,nickel, andcopper. Almost all have biological roles.
Period 5 has the same number of elements as period 4 and follows the same general structure but with one more post transition metal and one fewer nonmetal. Of the three heaviest elements with biological roles, two (molybdenum andiodine) are in this period;tungsten, in period 6, is heavier, along with several of the earlylanthanides. Period 5 also includestechnetium, the lightest exclusivelyradioactive element.
Period 6 is the first period to include thef-block, with thelanthanides (also known as therare earth elements), and includes the heaviest stable elements. Many of theseheavy metals are toxic and some are radioactive, butplatinum andgold are largely inert.
All elements of period 7 areradioactive. This period contains the heaviest element which occurs naturally on Earth,plutonium. All of the subsequent elements in the period have been synthesized artificially. Whilst five of these (fromamericium toeinsteinium) are now available in macroscopic quantities, most are extremely rare, having only been prepared in microgram amounts or less. Some of the later elements have only ever been identified in laboratories in quantities of a few atoms at a time.
Although the rarity of many of these elements means that experimental results are not very extensive, periodic and group trends in behaviour appear to be less well defined for period 7 than for other periods. Whilstfrancium andradium do show typical properties of groups 1 and 2, respectively, theactinides display a much greater variety of behaviour and oxidation states than thelanthanides. These peculiarities of period 7 may be due to a variety of factors, including a large degree ofspin–orbit coupling and relativistic effects, ultimately caused by the very high positive electrical charge from their massiveatomic nuclei.
No element of the eighth period has yet been synthesized. Ag-block is predicted. It is not clear if all elements predicted for the eighth period are in fact physically possible. Therefore, there may not be a ninth period.
^Blevins, Dale G.; Lukaszewski, Krystyna M. (1998). "Functions of Boron in Plant Nutrition".Annual Review of Plant Physiology and Plant Molecular Biology.49:481–500.doi:10.1146/annurev.arplant.49.1.481.PMID15012243.
