O these electrons in the same shell are at

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o These electrons in the same shell are at approximately the same distance from the nucleus, and relatively ineffective at shielding one another. At the same time , though, the nuclear charge Z increases from Na to Ar. Thus, the effective nuclear charge for the valence- shell electrons increases across the period from 2.20 for Na to 6.75 for Ar. o This draws all the valence-shell electrons closer to the nucleus and progressively shrinking the atomic radii.
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Periodic Trends: (AR) Summary
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Periodic Trends: Ionic Radii The cation that results when an electron is removed from a neutral atom is smaller than the atom o The electron is removed from a large, valence shell orbital and there is an increase in Z eff E.g. On going from a neutral Na atom to a Na + cation: o Na : 1s 2 2s 2 2p 6 3s 1 vs Na + : 1s 2 2s 2 2p 6 o Going from n = 3 to n = 2 o Z eff increases as 11 p + attracted 11 e - in Na vs 11 p + and 10 e - in Na + o Less electrons also translate into less shielding o In a given row, the greater the charge on the cation, the smaller the radius e.g. Li + < Be 2+
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Just as atoms shrink when converted to cations by loss of an electron, they expand when converted to anions by gain of an electron. The expansion is dramatic. o Chlorine, for e.g. goes from 99 pm for the neutral atom to 184 pm for the chloride anion. The expansion can’t be accounted for by a change in the quantum number of the valence shell, o Because the added electron simply completes an already occupied p subshell: [Ne] 3s 2 3p 5 for a Cl atom becomes [Ne] 3s 2 3p 6 for a Cl - anion, for example. The expansion is due entirely to the decrease in Z eff and the increase in electron electron repulsions that occurs when extra electrons are added. In a given row, the greater the charge on the anion, the larger the radius e.g. F - < O 2-
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Isoelectronic species An Isoelectronic Series is a group of atoms/ions that have the same number of electrons N 3- , O 2- , F - , Ne, Na + , Mg 2+ , Al 3+ o This series each have 10 electrons. P 3- , S 2- , Cl - , Ar, K + , Ca 2+ , Sc 3+ o This series each have 18 electrons Since the number of electrons are the same, size is determined by the number of protons . Al 3+ has 10 protons, therefore greatest Z eff of its series, thus is the smallest. The N 3- has 7 protons the smallest Z eff of its series, thus it is the largest.
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Periodic Trends: Ionization Energies The absorption of light or electrical energy by an atom leads to a change in electron configuration. A valence-shell electron is promoted from a lower-energy orbital to a higher-energy one (with a larger value of n) If enough energy is absorbed, the electron can be removed completely from the atom, leaving behind an ion. The amount of energy necessary to remove the highest- energy electron from an isolated neutral atom in the gaseous state is called the atom’s ionization energy , abbreviated E i .
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Note that all the values of E i are positive , meaning that energy is always required to remove an electron from an atom .
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