Ionic Bonds and Periodic Trends

Electron Configurations of Ions Chapter 6 z z Ionic Bonds and Periodic Trends z z z z z Anions: simply continue to add electrons in the Aufbau order Write the electron configuration of: FO2N31s2 2s2 2p6 for each These ions are called isoelectronic. 1 Electron Configurations of Ions z z z z z z z z 2 Electron Configurations of Ions z Transition metal cations: remove electrons from the s orbital first, then in the reverse Aufbau order Write the electron configurations for: Cr2+ z Ti3+ z Fe3+ z Cations: remove electrons in the reverse of the Aufbau order Write the electron configuration of: Al3+ Mg2+ Na+ 1s2 2s2 2p6 These ions are also isoelectronic. Problems 6.1, 6.2 z 3 Atomic Radius, Section 5.15 z z 4 Atomic Radius Atomic Radii What trends do you notice (left to right; top to bottom)? Why? Problem 5.21 Atomic radius can be predicted by looking at elements’ numbers of electrons Definition: one-half the distance between two nuclei l i iin ttwo adjacent dj t atoms t 5 6 1 Effective Nuclear Charge z z Trends in Atomic Radius Effective Nuclear Charge Electrons are all attracted to the nucleus, but electrons in inner shells shield protons and reduce attractive forces of valence electrons. Th effective The ff ti nuclear l charge h (Zeff) is i th the amount of positive charge from the nucleus that is perceived by an electron. z z z z z  In a row (or period) in the Periodic Table, the z number of protons increases, but the number of inner e- (shielding e-) stays the same. Atoms on the right side of the Table can pull e- in more tightly. Which element in each pair has a larger atomic radius? Why? F or Cl C or N Rb or Ca Na or Mg K or Na 7 Ionic Radius z z 8 Trends in Ionic Radii Radius of cation or anion Which gets larger as it goes from atom to ion: cation or anion? Why? Ionic Radii Zeff for Isoelectronic Series z z z In an isoelectronic series, all ions have the same number of electrons, but the number of protons increases from most negative to most positive ion ion. Therefore, the radius of the most positive ion is smallest and the most negative ion is largest. Place the following ions in order of increasing ionic radius: Cl1-, K1+, S2-, Ca2+, Al3+, P3- In each pair, which is larger? Why? z Be2+ or B3+ z Al3+ Ca2+ K or or or P3Ca Ca z z z z O2or FProblem 6.3, 6.4 12 2 Ionization Energy z z Periodicity of First Ionization Energy Equation representing ionization energy: X(g) → X+(g) + eDefine ionization energy Ionize Trends Mg Æ Mg+1 IE Defined 13 Trends in First Ionization Energy z z z z z z z 14 Electron Affinity Which member of each pair has the larger first ionization energy? Why? F or Cl N or C O or F Na or Mg K or Na Worked Ex. 6.1; Problem 6.5 z z z z z Equation representing electron affinity: X(g) + e- → X-(g) E- Affinity Define electron affinity Ionization energies are positive values (require input of energy). Electron affinities are negative for most atoms and for all cations. Greater attraction between atom and electron results in more negative EA (e.g., halogens). Worked Ex. 6.3; Problem 6.10 15 Electron Affinity (kJ/mol) 16 Octet Rule z EA Trends Elements react in order to obtain 8 valence (outermost) electrons. They do this by transferring electrons or sharing electrons.  Metals M t l tend t d to t lose l electrons l t (low (l II.E.), E ) nonmetals t l tend to gain electrons (high E.A.). Nonmetal atoms donate electrons to metals to make ionic bonds. Both elements achieve an octet.  Covalent bonds form when elements have to share electrons in order to get 8 valence electrons (an octet) (nonmetals + nonmetals). 17 3 Lattice Energy: F = k*(z1•z2 / d2) The Ionic Bond z What do the following have in common? z  LiF  CaO Lattice energy: energy required to completely separate one mole of a solid ionic compound into gaseous ions  Mg M 3 N2 z Look at Ionization Energies and Electron Affinities  Low ionization energy Æ cations  High electron affinity Æ anions 20 Relative Lattice Energies z z Relative Lattice Energies Charges and sizes of ions determine the value of the lattice energy Larger lattice energy Æ more stable crystal Æ stronger t ionic i i b bond d z Which compound in each pair is more stable? Is this determined by a high or low lattice energy? Z Z U = -A A +  NaCl N Cl or MgCl M Cl2 d±  MgO or Na2O  NaCl or KCl  NaBr or NaCl z Worked Ex. 6.5 Æ Problem 6.12, 6.13  Charges are greater, or  Sizes are smaller (lattice energy is inversely related to size) z Smaller radii Æ nuclei more strongly attracted to opposite electrons z 21 22 Metals versus Nonmetals z Metals tend to react with water to form bases:  2Na (s) + 2H2O Æ 2NaOH + H2  MgO (s) + H2O Æ Mg(OH)2 z CO2 (s) + H2O (l) Æ H2CO3 (aq) Alkali metals + water Nonmetals tend to react with water to form acids:  2F2 (g) + 2H2O Æ 4HF + O2  CO2 (g) + H2O Æ H2CO3 4