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O perioadă a tabelului periodic reprezintă un șir de elemente chimice, ce au același număr de învelișuri electronice. Fiecare element dintr-o perioadă conține un proton suplimentar și prezintă un caracter mai puțin metalic decât precedentul său. Aranjate în această manieră, elementele din aceeași grupă (coloană) prezintă aceleași proprietăși fizico-chimice, reflectând astfel legea periodică. De exemplu, halogenii se află in penultima grupă (grupa 17) și prezintă aceleași proprietăți, precum reactivitatea ridicată și tendința de dobândire a unui electron pentru a ajunge la configurația electronică a unui gaz nobil. 

Numărul straturilor electronice ale unui atom determină perioada de care aparține. De vreme ce ultimul strat electronic determină propietățiile chimice, acestea tind să fie asemănătoare în grupele tabelului periodic. 

Fiecare strat este împărțit în mai multe substraturi, care sunt umplute prin creșterile de număr atomic astfel:

 1s 
 2s           2p  
 3s           3p  
 4s        3d 4p  
 5s        4d 5p  
 6s     4f 5d 6p  
 7s     5f 6d 7p  
 8s  5g 6f 7d 8p  
 ...

Mecanica cuantică explică aceste tendințe periodice pe baza învelișurilor electronice. Pe măsură ce număru atomic crește, straturile electronice dobândesc electronic conform diagramei de mai josș fiecare strat constituie un rând din tabel. 

In blocurile s si p ale tabelului periodic, elementele din aceeași perioadă tind să nu prezinte tendințe și similarități ale proprietăților (tendințele fiind mai puternice de-a lungul unor grupe). Cu toate acestea, blocul d prezintă tendințe ce devin mai puternice, iar blocul f prezintă un grad mai mare al similaritățilo în cadrul perioadelor.

Periods
În prezent, tabelul periodic conține 7 perioade complete, cuprinzând cele 118 elemente chimice cunoscute. Orice nou element va fi plasat într-o a opta perioadă; vezi extended periodic table. Elementele sunt reprezentate prin culori, pe baza blocului periodic: roșu pentru blocul-s, galben pentru blocul-p, albastru pentru blocul-d și verde pentru blocul-f.

Perioada 1

Prima perioadă conține cele mai puține elemente: hidrogen și heliu. Datorită acestei apartenențe, cele 2 elemente nu urmează regula octetului, ci mai degrabă a dubletului. Chimic, heliul prezintă un caracter de gaz nobil și este considerat parte a grupei 18. Cu toate acestea, structura sa nucleară îl plasează în blocul-s, fiind așasat clasificat uneori ca un element al grupei 2 sau constituent simultan al grupelor 2 și 18. Hidrogenul prezintă un caracter chimic ce-l plasează ca un element al grupei 1 si grupa 17, datorită comportamentului său electronic. 

Hidrogenul (H) is the most abundant of the chemical elements, constituting roughly 75% of the universe's elemental mass. Ionized hydrogen is just a proton. Stars in the main sequence are mainly composed of hydrogen in its plasma state. Elemental hydrogen is relatively rare on Earth, and is industrially produced from hydrocarbons such as methane. Hydrogen can form compounds with most elements and is present in water and most organic compounds.
Heliu (He) exists only as a gas except in extreme conditions. It is the second-lightest element and is the second-most abundant in the universe. Most helium was formed during the Big Bang, but new helium is created through nuclear fusion of hydrogen in stars. On Earth, helium is relatively rare, only occurring as a byproduct of the natural decay of some radioactive elements. Such 'radiogenic' helium is trapped within natural gas in concentrations of up to seven percent by volume.

Period 2

Period 2 elements involve the 2s and 2p orbitals. 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. In its non-ionized state it is one of the most reactive elements, and so is only ever found naturally in compounds. It is the heaviest primordial element forged in large quantities during the Big Bang .  
Beryllium (Be) has one of the highest melting points of all the light metals. Small amounts of beryllium were synthesised during the Big Bang, although most of it decayed or reacted further within stars to create larger nuclei, like carbon, nitrogen or oxygen. Beryllium is classified by the International Agency for Research on Cancer as a group 1 carcinogen. Between 1% and 15% of people are sensitive to beryllium and may develop an inflammatory reaction in their respiratory system and skin, called chronic beryllium disease.
Boron (B) does not occur naturally as a free element, but in compounds such as borates. It is an essential plant micronutrient, required for cell wall strength and development, cell division, seed and fruit development, sugar transport and hormone development, though high levels are toxic.
Carbon (C) is the fourth-most abundant element in the universe by mass after hydrogen, helium and oxygen and is the second-most abundant element in the human body by mass after oxygen, the third-most abundant by number of atoms. 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. All organic compounds, those essential for life, contain at least one atom of carbon; combined with hydrogen, oxygen, nitrogen, sulfur, and phosphorus, carbon is the basis of every important biological compound.
Nitrogen (N) is found mainly as mostly inert diatomic gas, N2, which makes up 78% of the Earth's atmosphere by volume. It is an essential component of proteins and therefore of life.
Oxygen (O) comprising 21% of the atmosphere by volume and is required for respiration by all (or nearly all) animals, as well as being the principal component of water. 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.
Neon (Ne) is a noble gas used in neon lighting.

Period 3

All period three elements occur in nature and have at least one stable isotope. All but the noble gas argon are essential to basic geology and biology.
Sodium (Na) is an alkali metal. It is present in Earth's oceans in large quantities in the form of sodium chloride (table salt).
Magnesium (Mg) is an alkaline earth metal. Magnesium ions are found in chlorophyll.
Aluminium (Al) is a post-transition metal. It is the most abundant metal in the Earth's crust.
Silicon (Si) is a metalloid. It is a semiconductor, making it the principal component in many integrated circuits. Silicon dioxide is the principal constituent of sand. As Carbon is to Biology, Silicon is to Geology.
Phosphorus (P) is a nonmetal essential to DNA. It is highly reactive, and as such is never found in nature as a free element.
Sulfur (S) is a nonmetal. It is found in two amino acids: cysteine and methionine.
Chlorine (Cl) is a halogen. 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 in swimming pools.
Argon (Ar) is a noble 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

Period 4 includes the biologically essential elements potassium and calcium, and is the first period in the d-block with the lighter transition metals. These include iron, the heaviest element forged in main-sequence stars and a principal component of the Earth, as well as other important metals such as cobalt, nickel, and copper. Almost all have biological roles.

Completing the fourth period are six p-block elements: gallium, germanium, arsenic, selenium, bromine, and krypton.

Period 5

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 and iodine) are in this period; tungsten, in period 6, is heavier, along with several of the early lanthanides. Period 5 also includes technetium, the lightest exclusively radioactive element.

Period 6

Period 6 is the first period to include the f-block, with the lanthanides (also known as the rare earth elements), and includes the heaviest stable elements. Many of these heavy metals are toxic and some are radioactive, but platinum and gold are largely inert.

Period 7

All elements of period 7 are radioactive. 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 (from americium to einsteinium) 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. Whilst francium and radium do show typical properties of groups 1 and 2, respectively, the actinides display a much greater variety of behaviour and oxidation states than the lanthanides. These peculiarities of period 7 may be due to a variety of factors, including a large degree of spin–orbit coupling and relativistic effects, ultimately caused by the very high positive electrical charge from their massive atomic nuclei.

Period 8

No element of the eighth period has yet been synthesized. A g-block is predicted. It is not clear if all elements predicted for the eighth period are in fact physically possible. There may therefore be no ninth period.

See also
 Group (periodic table)

References

Periodic table
 Period (periodic table)

Vezi și 

 Grupă (tabelul periodic al elementelor)

Tabelul periodic al elementelor