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Ionization EnergyThe ionization energy (IE) of an atom is the energy required to strip it of an electron. More generally, the nth ionization energy of an atom is the energy required to strip it of an nth electron after the first have already been removed. It is centrally significant in physical chemistry as a measure of the "reluctance" of an atom to surrender an electron, or the "strength" by which the electron is bound. Values and trends Generally speaking, ionization energies decrease down a group of the Periodic Table, and increase left-to-right across a period. Ionization energy exhibits a strong negative correlation with atomic radius. Successive ionization energies of any given element increase markedly. Particularly dramatic increases occur after any given block of atomic orbitals is exhausted. Some values for elements of the third period are given in the following table: Successive ionization energies in kJ/mol | Element | First | Second | Third | Fourth | Fifth | Sixth | Seventh | | Na | 496 | 4,560 | | Mg | 738 | 1,450 | 7,730 | | Al | 577 | 1,816 | 2,744 | 11,600 | | Si | 786 | 1,577 | 3,228 | 4,354 | 16,100 | | P | 1,060 | 1,890 | 2,905 | 4,950 | 6,270 | 21,200 | | S | 999 | 2,260 | 3,375 | 4,565 | 6,950 | 8,490 | 11,000 | | Cl | 1,256 | 2,295 | 3,850 | 5,160 | 6,560 | 9,360 | 11,000 | | Ar | 1,520 | 2,665 | 3,945 | 5,770 | 7,230 | 8,780 | 12,000 | Electrostatic explanation Ionization energy can be predicted by a simple analysis using electrostatic potential and the Bohr model of the atom, as follows. Consider an electron of charge -e, and an ion with charge +ne, where n is the number of electrons missing from the ion. According to the Bohr model, were the electron to approach and bind with the atom, it would come to rest at a certain radius a. The electrostatic potential at distance a from the ionic nucleus, referenced to a point infinitely far away, is: . Since the electron is negatively charged, it is drawn to this positive potential. (The value of this potential is called the ionization potential). The energy required for it to "climb out" and leave the atom is: . This simple analysis is incomplete, as it leaves the distance a as an unknown. It can be made more rigorous by assigning to each electron of every chemical element a characteristic distance, chosen so that this relation agrees with experimental data. Quantum-mechanics explanation According to the more sophisticated theory of quantum mechanics, though, it is not reasonable even to speak of a characteristic distance, since the location of an electron is best described as a "cloud" of likely locations (specifically, an electron orbital) that ranges near and far from the nucleus. This orbital is described by a probability density function. The energy of the orbital is then described by the Schrdinger equation. Except in the simple case where the ion in question is stripped of all of its electrons, leaving a bare nucleus, this equation cannot be solved analytically. Related topics
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