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The nuclear charge; oxidation number; The screening effect of the inner shells; The extent to which the outermost electron penetrates into the charge cloud set up by the inner lying electron; In the periodic table, effective nuclear charge decreases down a group and increases left to right across a period.
The energy needed to remove the second electron from the neutral atom is called the second ionization energy and so on. [10] [11] As one moves from left-to-right across a period in the modern periodic table, the ionization energy increases as the nuclear charge increases and the atomic size decreases.
On moving downward within a given group, the electrons are held in higher-energy shells with higher principal quantum number n, further from the nucleus and therefore are more loosely bound so that the ionization energy decreases. The effective nuclear charge increases only slowly so that its effect is outweighed by the increase in n. [11]
An electron in the s-sublevel is capable of shielding electrons in the p-sublevel of the same principal energy level. The size of the shielding effect is difficult to calculate precisely due to effects from quantum mechanics. As an approximation, we can estimate the effective nuclear charge on each electron by the following:
An example provided in Slater's original paper is for the iron atom which has nuclear charge 26 and electronic configuration 1s 2 2s 2 2p 6 3s 2 3p 6 3d 6 4s 2.The screening constant, and subsequently the shielded (or effective) nuclear charge for each electron is deduced as: [1]
Care must be taken in interpreting the ionization potentials for indium and thallium, since other effects, e.g. the inert-pair effect, become increasingly important for the heavier members of the group. The cause of the d-block contraction is the poor shielding of the nuclear charge by the electrons in the d orbitals.
The lanthanide contraction is the greater-than-expected decrease in atomic radii and ionic radii of the elements in the lanthanide series, from left to right. It is caused by the poor shielding effect of nuclear charge by the 4f electrons along with the expected periodic trend of increasing electronegativity and nuclear charge on moving from left to right.
The first of these quantities is used in atomic physics, the second in chemistry, but both refer to the same basic property of the element. To convert from "value of ionization energy" to the corresponding "value of molar ionization energy", the conversion is: 1 eV = 96.48534 kJ/mol 1 kJ/mol = 0.0103642688 eV [12]