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Published byBernard Stone Modified over 9 years ago
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Ionic bonds and main group chemistry
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Towards the noble gas configuration Noble gases are unreactive – they have filled shells Shells of reactive elements are unfilled Achieve noble gas configuration by gaining or losing electrons Metals lose electrons – form positive ions Metals lose electrons – form positive ions Nonmetals gain electrons – form negative ions Nonmetals gain electrons – form negative ions
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Lewis dot model The nucleus and all of the core electrons are represented by the element symbol The valence electrons are represented by dots – one for each Number of dots in Lewis model is equal to group number (in 1 – 8 system)
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The Octet Rule All elements strive to become a noble gas, at least as far as the electrons are concerned. Filling the outer shell – 8 electrons Achieve this by adding electrons Or taking them away
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Predicting ion charges s and p block elements are easy: charge = group number for cations charge = group number for cations charge = -(8 – group number) for anions charge = -(8 – group number) for anions
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Less predictable for transition metals Occurrence of variable ionic charge Cr 2+, Cr 3+, Cr 4+, Cr 6+ etc. Cr 2+, Cr 3+, Cr 4+, Cr 6+ etc. 4s electrons are lost first and then the 3d Desirable configurations coincide with empty, half-filled or filled 3d orbitals Fe 2+ ([Ar]3d 6 ) is less stable than Fe 3+ ([Ar]3d 5 ) Fe 2+ ([Ar]3d 6 ) is less stable than Fe 3+ ([Ar]3d 5 )
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Ionic size and charge Loss of electrons increases the effective nuclear charge – ion shrinks Gain of electrons decreases the effective nuclear charge – ion expands
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Ionization energy Energy required to remove an electron from a neutral gaseous atom Always positive Follows periodic trend Increases across period Increases across period Decreases down group Decreases down group Removal of electrons from filled or half-filled shells is not as favourable [He]2s 2 [He]2s 2 2p 3 [He]2s 2 2p 4 [He]2s 2 2p 1
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Higher ionization energies Depend on group number Much harder to remove electrons from a filled shell Stepwise trend below illustrates this Partially filled – valence electrons Completely filled – core electrons
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Electron affinity Energy released on adding an electron to a neutral gaseous atom Values are either negative – energy released, meaning negative ion formation is favourable negative – energy released, meaning negative ion formation is favourable Or zero – meaning can’t be measured and negative ions are not formed Or zero – meaning can’t be measured and negative ions are not formed Addition of electrons to filled or half-filled shells is not favoured (e.g. He, N) It is easier to add an electron to Na (3s 1 ) than to Mg (3s 2 )
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Ionic bonding Reaction between elements that form positive and negative ions Metals (positive ions) and nonmetals (negative ions) Metals (positive ions) and nonmetals (negative ions) Neutral Na + Cl → ionic Na + Cl - [Ne]3s 1 + [Ne]3s 2 3p 5 = [Ne] + + [Ar] - [Ne]3s 1 + [Ne]3s 2 3p 5 = [Ne] + + [Ar] -
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Stability of the ionic lattice Simply forming ions does not give an energy payout: E i (Na) = 496 kJ/mol E i (Na) = 496 kJ/mol E a (Cl) = -349 kJ/mol E a (Cl) = -349 kJ/mol Net energy investment required Formation of a crystal lattice releases energy The lattice energy is the energy released on bringing ions from the gas phase into the solid lattice Depends on coulombic attraction between ions -U = κz 1 z 2 /d (κ = 8.99x10 9 JmC -2
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Born-Haber cycle for calculating energy The lattice energy can be obtained using other experimentally determined quantities and the energy cycle
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Lattice energies follow simple trends As ionic charge increases, U increases (U z 1 z 2 ) As ion size decreases, U increases (U 1/d) U(LiF) > U(LiCl) > U(LiBr) U(LiF) > U(LiCl) > U(LiBr) U(NaI) < U(MgI 2 ) < U(AlI 3 ) U(NaI) < U(MgI 2 ) < U(AlI 3 )
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The Octet Rule Main-group elements undergo reactions which leave them with eight valence electrons Group 1 (ns 1 ) M + Group 1 (ns 1 ) M + Group 2 (ns 2 ) M 2+ Group 2 (ns 2 ) M 2+ Group 6 (ns 2 np 4 ) X 2- Group 6 (ns 2 np 4 ) X 2- Group 7 (ns 2 np 5 ) X - Group 7 (ns 2 np 5 ) X - Works very well for second row (Li – F) Many violations in heavier p-block elements (Pb 2+, Tl +, Sb 3+ )
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