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The period 2 elements are the chemical elements in the second row (or period) of the periodic table. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behavior of the elements as their atomic number increases; a new row is started when chemical behavior begins to repeat, creating columns of elements with similar properties. The second period contains the elements lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, and neon. This situation can be explained by modern theories of atomic structure. In a quantum mechanical description of atomic structure, this period corresponds to the filling of the 2s and 2p orbitals. Period 2 elements obey the octet rule in that they need eight electrons to complete their valence shell. The maximum number of electrons that these elements can accommodate is ten, two in the 1s orbital, two in the 2s orbital and six in the 2p orbital. All of the elements in the period can form diatomic molecules except beryllium and neon. ==Periodic trends== Period 2 is the first period in the periodic table that periodic trends can be drawn from. Period 1, which only contains two elements (hydrogen and helium) is too small to draw any conclusive trends from it, especially because the two elements behave nothing like other s-block elements.〔(【引用サイトリンク】title=International Union of Pure and Applied Chemistry > Periodic Table of the Elements )〕 Period 2 has much more conclusive trends. For all elements in period 2, as the atomic number increases, the atomic radius of the elements decreases, the electronegativity increases, and the ionization energy increases. Period 2 only has two metals (lithium and beryllium), making it the least metallic period (period 1 has no metals, but as mentioned above, only has two elements, so it would be impossible to have more metals) and the most nonmetals, with four. The elements in period 2 often have the most extreme properties in their respective groups; for example, fluorine is the most reactive halogen, neon is the most inert noble gas, and lithium is the least reactive alkali metal. All period 2 elements completely obey the Madelung rule; in period 2s, lithium and beryllium fill the 2s subshell, and boron, carbon, nitrogen, oxygen, fluorine, and neon fill the 2p subshell. The period shares this trait with periods 1 and 3, none of which contain transition elements or inner transition elements, which often vary from the rule.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Period 2 element」の詳細全文を読む スポンサード リンク
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