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In the periodic table of the elements, elements are arranged in a series of rows (or periods) so that those with similar properties appear in vertical columns. Elements of the same period have the same number of electron shells; with each group across a period, the elements have one more proton and electron and become less metallic. This arrangement reflects the periodic recurrence of similar properties as the atomic number increases. For example, the alkaline metals lie in one group (group 1) and share similar properties, such as high reactivity and the tendency to lose one electron to arrive at a noble-gas electronic configuration.
Modern quantum mechanics explains these periodic trends in properties in terms of electron shells. As atomic number increases, shells fill with electrons in approximately the order shown below. The filling of each shell corresponds to a row in the table.
In the s-block and p-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 the d-block, trends across periods become significant, and in the f-block elements show a high degree of similarity across periods (particularly the lanthanides).
Seven periods of elements occur naturally on Earth. For period 8, which includes elements which may be synthesized after 2010, see the extended periodic table.
The first period contains fewer elements than any other, with only two, hydrogen and helium. They therefore do not follow the octet rule. Chemically, helium behaves as a noble gas, and thus is taken to be part of the group 18 elements. However, in terms of its nuclear structure it belongs to the s block, and is therefore sometimes classified as a group 2 element, or simultaneously both 2 and 18. Hydrogen readily loses and gains an electron, and so behaves chemically as both a group 1 and a group 17 element.
* Hydrogen (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.
Chemical elements in the second period
Period 2 elements involve the 2s and 2p orbitals. They include the biologically most essential elements besides hydrogen: carbon, nitrogen, and oxygen.
* Lithium 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.
Chemical elements in the third period
All period three elements occur in nature and have at least one stable isotope. All but the nobel gas argon are all essential to basic geology and biology.
* Sodium (symbol Na) is an alkali metal. It is present in Earth's oceans in large quantities in the form of sodium chloride.
Chemical elements in the fourth period
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, nickle, copper, and zinc. Almost all have biological roles.
Period 5 includes the important metals silver and tin and the biologically important element iodine. Also in period 5 is the lightest purely radioactive element, technetium, the first element to be artificially synthesized.
Period 6 is the first period to include the F block, with the lanthanides aka 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.
All elements of period 7 are radioactive. This period contains the heaviest element which occurs naturally on earth, uranium. Most of the subsequent elements in the period have been synthesized artificially. Whilst some of these (e.g. plutonium) are now available in tonne 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 that than the lanthanides. Initial studies suggest Group 14 element ununquadium appears to be a noble gas instead of a poor metal, and group 18 element ununoctium probably is not a noble gas. 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.