Periodic Properties Chapter 7

Overview  Periodic Table  Electron Shells & Sizes of Atoms  Ionization Energy  Electron Affinities  Metals, Nonmetals & Metalloids and Trends

Periodic Table  Dmitri Mendeleev & Lothar Meyer  published classification schemes of elements  based on order of increasing elemental atomic weight  which follows atomic number  some missing elements were “discovered” based upon prediction that they should be there  Henry Moseley  related energy of nuclear x-rays to atomic number

Sizes of Atoms  Electron Shells  quantum mechanical model predicts shells of electron density  as number of protons increases, inner shells are held more tightly, closer to nucleus  Sizes of Atoms  atom boundaries are difficult to assess  define atom size as ½ the distance between nuclei of two bound atoms

++ Cl Atomic radii = ½ distance between the nuclei r

General Trends of Physical Properties  most trends are related to Z eff, effective nuclear charge  due to increase in distance from nucleus and shielding, Z eff decreases going down a group  due to ineffective shielding by electrons in the same shell, Z eff increases going across a row Z eff increases decreases

 Compare valence e - ‘s of Na and Cs  which will be held more tightly by nucleus? the e - that is closest to the nucleus  which will be closest to the nucleus? Na’s 3s electron  Compare last electron of C and F  which has more protons interacting? Z F = 9 while Z C = 4 do e - in the same subshell shield their fellow e - ‘s very well? –no so which last e - will experience the greatest Z eff ? –F–F

General Trends in Atomic Size  radius increases on going down a group ( Z eff decreases )  radius tends to decrease on going left to right across a row ( Z eff increases ) size decreases increases

 We know that Z eff decreases going down a group, how does that affect size?  As Z eff (on a valence e - ) decreases, it is less tightly held, hence can expand somewhat  We know that Z eff increases going across a row, how does that affect size?  As Z eff (on a last e - ) increases, it is more tightly held, hence contraction occurs and atom is smaller

 Compare valence e - ‘s of Na and Cs  which experiences the greatest Z eff ? Na’s 3s electron (bec. it is closest to nucleus)  which atom will be smaller? Na < Cs  Compare last electron of C and F  which has more protons interacting? Z F = 9 while Z C = 4 do e - in the same subshell shield their fellow e - ‘s very well? –no so which last e - will experience the greatest Z eff ? –F–F which atom will experience greatest contraction? –F will be more contracted, hence smaller

Trends in Ionization Energy  ionization energy decreases going down ( Z eff decreases )  ionization energy increases going across ( Z eff increases ) I.E. increases decreases

 Ionization Energy -- endothermic process  first ionization energy, I 1 -- to remove first electron M  M + + 1e -  second ionization energy, I 2 -- to remove second electron M +  M e -  third ionization energy, I 3 -- to remove third electron M 2+  M e -  I 1 < I 2 < I 3

 Note: removal of inner core electrons is very energy expensive Element I 1 I 2 I 3 I 4 Na Mg Al ,600

 The more tightly an e - is held, the more energy it takes to remove it  in other words, greater Z eff = greater I.E.  Compare valence e - ‘s of Na and Cs  which has greater Z eff ? Na  which has greater I 1 ? Na (Cs’s 6s electron is further away, less tightly held and easier to remove than Na’s 3s electron)  Compare last e - of C and F  which has greater Z eff ? F  which has greater I 1 ? F (more tightly held, more energy to remove)

 transition metals and f-block metals show small variation in ionization energies across rows  representative elements show a larger range of values for I 1  irregularities in ionization energies occur due to shielding  I 1 for Al < I 1 for Mg -- removal of p versus s electron  I 1 for O < I 1 for N -- removal of p electron to produce stable configuration

Electron Affinities  energy change associated with adding an electron to gaseous atom  measures attraction of atom for the added electron Cl (g) + 1e -  Cl -  E = -349 kJ/mol  exothermic process  most electron affinities are exothermic  some exceptions & irregularities nobel gases Be & Mg Group III elements

Metals  general trends and properties  to the left on periodic table  ability to lose electrons (low ionization energies) -- what kind of ions?  have a metallic or shiny luster, ductile  form crystalline solids  have good thermal and electrical conductivity  transition metals vary in charge most have +2 in addition to other charges (+1 to +5)  form ionic compounds with non-metals especially with oxides or halides

 metal oxides are called basic oxides react with water to form basic hydroxides Na 2 O (s) + H 2 O (l)  2NaOH (aq) react with acid to form water and salt

Nonmetals  general trends and properties  to the right on periodic table  ability to gain electrons -- what kind of ions?  non-conductors and thermal insulators  have lower melting points – are molecular solids rather than ionic solids  seven exist as diatomic molecules –H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2

 form molecular compounds with other nonmetals such as oxides, halides & hydrides  nonmetal oxides are called acidic oxides react with water to form acids CO 2(g) + H 2 O (l)  H 2 CO 3(aq) react with bases to form salt and water CO 2(g) + 2NaOH (aq)  Na 2 CO 3(aq) + H 2 O (l)  Metalloids  have a mixture of properties some metallic, some not

Group Trends for Active Metals  Group 1A -- Alkali Metals  very reactive, only found in nature in oxidized form (compounds)  metals produced by electrolysis of molten compounds  reactivity increases down the group  react rapidly with water 2K (s) + 2H 2 O (l)  2KOH (aq) + H 2(g)  react with acid: 2K (s) + 2HCl (aq)  2KCl (aq) + H 2(g)

 react with hydrogen to form hydrides 2K (s) + H 2(g)  2KH (s)  react with oxygen in different ways 4Li (s) + O 2(g)  2Li 2 O (s) containing O 2- this is the more common reaction  other alkali metals form peroxides, O Na (s) + O 2(g)  Na 2 O 2(s)  K, Rb, Cs form superoxides, O 2 - K (s) + O 2(g)  KO 2(s) peroxides and superoxides are not as common  most alkali metal compounds are colorless

 Group 2A -- Alkaline Earth Metals  reactive – less than alkali metals (ionization energies higher)  reactivity increases down the group  heavier metals react with water to form bases Ca (s) + 2H 2 O (l)  Ca(OH) 2(aq) + H 2(g) lighter metals react similarly with hot water or steam  react with acids: Mg (s) + 2HCl (aq)  MgCl 2(aq) + H 2(g)  react with O to produce oxides 2Mg (s) + O 2(g)  2MgO (s)

Trends for Nonmetals  Hydrogen  nonmetal, diatomic, colorless gas  high ionization energy (no shielding)  typically forms molecular compounds with other nonmetals these reactions are fairly exothermic 2H 2(g) + O 2(g)  2H 2 O (l)  H o = kJ/mol  forms hydrides (H - ) with metals 2Na (s) + H 2(g)  2NaH (s)

 Group 6A: Oxygen Group  density increases, metallic character increases down group  oxygen is diatomic, & the only gas in the group  allotrope (different form) is ozone, O 3 3O 2(g)  2O 3(g)  H o = kJ/mol is ozone more or less stable than O 2 ?  ions are in different forms O 2-, oxide O 2 2-, peroxide O 2 -, superoxide less

 Group 7A: Halogens  increased nonmetallic character  lighter elements are diatomic gases & more reactive than heavier elements  have very exothermic electron affinities and gain electron easily  F has highest electron affinity 2Na (s) + F 2(g)  2NaF (s)  H o = kJ 2H 2 O (l) + 2F 2(g)  4HF (aq)  H o = kJ  Cl reacts somewhat more slowly Cl 2(g) + H 2 O (l)  HCl (aq) + HOCl (aq)

 Group 8A: Noble Gases  all are monotomic, quite stable and unreactive  Xe was the first noble gas compound identified XeF 6  Kr is less reactive & has one known compound KrF 2  no compounds of He, Ne or Ar are known however, there have been implications of a possible Ar compound