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Periodic table - Wikipedia, the free encyclopedia
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A diagram of the periodic table, highlighting the
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.
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.
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.
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
, named according to the
subshell in which the "last" electron resides.
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.
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,
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.