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16 6213 1144 am periodic table wikipedia the free

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6/2/13 11:44 AM Periodic table - Wikipedia, the free encyclopedia Page 5 of 17 http://en.wikipedia.org/wiki/Periodic_table A diagram of the periodic table, highlighting the different blocks Elements in the same period show trends in atomic radius, ionization energy, electron affinity, and electronegativity. Moving left to right across a period, atomic radius usually decreases. This occurs because each successive element has an added proton and electron which causes the electron to be drawn closer to the nucleus. [17] This decrease in atomic radius also causes the ionization energy to increase when moving from left to right across a period. The more tightly bound an element is, the more energy is required to remove an electron. Electronegativity increases in the same manner as ionization energy because of the pull exerted on the electrons by the nucleus. [14] Electron affinity also shows a slight trend across a period. Metals (left side of a period) generally have a lower electron affinity than nonmetals (right side of a period), with the exception of the noble gases. [18] Blocks Main article: Block (periodic table) Because of the importance of the outermost electron shell, the different regions of the periodic table are sometimes referred to as blocks , named according to the subshell in which the "last" electron resides. [19] The s-block comprises the first two groups (alkali metals and alkaline earth metals) as well as hydrogen and helium. The p-block comprises the last six groups which are groups 13 to 18 in IUPAC (3A to 8A in American) and contains, among other elements, all of the metalloids. The d-block comprises groups 3 to 12 in IUPAC (or 3B to 2B in American group numbering) and contains all of the transition metals. The f-block, usually offset below the rest of the periodic table, comprises the lanthanides and actinides. [20] Other conventions and variations In presentations of the periodic table, the lanthanides and the actinides are customarily shown as two additional rows below the main body of the table, [21] with placeholders or else a selected single element of each series (either lanthanum or lutetium, and either actinium or lawrencium, respectively) shown in a single cell of the main table, between barium and hafnium, and radium and rutherfordium, respectively. This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table inserts the lanthanide and actinide series in their proper places, as parts of the table's sixth and seventh rows (periods). Periodic table with f-block separated (left) and inline (right) Some periodic tables include a dividing line, or equivalent, between metals and nonmetals. [22]
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