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The ionization energy (IE) is qualitatively defined as the amount of energy required to remove the most loosely bound electron of an isolated gaseous atom to form a cation. It is quantitatively expressed in symbols as: ::X + energy → X+ + e− where X is any atom or molecule capable of being ionized, X+ is that atom or molecule with an electron removed, and e− is the removed electron. This is an endothermic process. Comparison of IE's of atoms in the Periodic table reveals two patterns: 1)IE's generally increase as one moves from left to right within a given row (period); and 2) IE's decrease as one moves from row (period) 1 to period 7 down any given column. The latter is due to the outer electron shell being progressively further away from the nucleus with the addition of one inner shell per row as one moves down the column. The units for ionization energy are different in physics and chemistry. In physics, the unit is the amount of energy required to remove a single electron from a single atom or molecule:expressed as an electron volt. In chemistry, the units are the amount of energy it takes for all the atoms in a mole of substance to lose one electron each: molar ionization energy or enthalpy, expressed as kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol). 〔http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/Periodic_Table_of_the_Elements/Ionization_Energy〕 The ''n''th ionization energy refers to the amount of energy required to remove an electron from the species with a charge of (''n''-1). For example, the first three ionization energies are defined as follows: ::1st ionization energy :::X → X+ + e− ::2nd ionization energy :::X+ → X2+ + e− ::3rd ionization energy :::X2+ → X3+ + e− The term ionization potential is an older name for ionization energy,〔F. Albert Cotton and Geoffrey Wilkinson, ''Advanced Inorganic Chemistry'' (5th ed., John Wiley 1988) p.1381 ISBN 0-471-84997-9〕 because the oldest method of measuring ionization energies was based on ionizing a sample and accelerating the electron removed using an electrostatic potential. However this term is now considered obsolete.〔(IUPAC Gold Book )〕 Some factors affecting the ionization potential include: # Nuclear charge. # Number of electron shells. # Screening effect. # Type of orbital ionized. # Occupancy of the ionized orbital: completely filled or half filled. == Values and trends == Generally the (''n''+1)th ionization energy is larger than the ''n''th ionization energy. When the next ionization energy involves removing an electron from the same electron shell, the increase in ionization energy is primarily due to the increased net charge of the ion from which the electron is being removed. Electrons removed from more highly charged ions of a particular element experience greater forces of electrostatic attraction; thus, their removal requires more energy. In addition, when the next ionization energy involves removing an electron from a lower electron shell, the greatly decreased distance between the nucleus and the electron also increases both the electrostatic force and the distance over which that force must be overcome to remove the electron. Both of these factors further increase the ionization energy. Some values for elements of the third period are given in the following table: Large jumps in the successive molar ionization energies occur when passing noble gas configurations. For example, as can be seen in the table above, the first two molar ionization energies of magnesium (stripping the two 3s electrons from a magnesium atom) are much smaller than the third, which requires stripping off a 2p electron from the neon configuration of Mg2+. That electron is much closer to the nucleus than the previous 3s electron. Ionization energy is also a periodic trend within the periodic table organization. Moving left to right within a period or upward within a group, the first ionization energy generally increases with a few discrepancies (aluminum and sulphur). As the nuclear charge of the nucleus increases across the period, the atomic radius decreases and the electron cloud becomes closer towards the nucleus. Ionization energy increases from left to right in a period and decreases from top to bottom in a group. 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「Ionization energy」の詳細全文を読む スポンサード リンク
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