Chapter 7: Periodic Properties of the Elements

Dimitri Mendeleev ( ) ordered known elements according to atomic weight elements with similar properties fell into vertical groups => in the modern version, elements are ordered according to atomic number

Chapter 7: Periodic Properties of the Elements Properties (e.g. reactivity) of atoms depends on… … electron configuration … number of electrons … how tightly electrons are bound to nucleus (and many more properties)

Chapter 7: Periodic Properties of the Elements How tightly electrons are bound to nucleus… … is measured by effective nuclear charge, Z eff Core electrons shield (mask) nuclear charge core 7 N: 1s 2 2s 2 2p 3 core electron configuration: [He] = 1s 2 Z eff = +7 – 2 = 5 + Effective Nuclear Charge: valence electrons #protons # core electrons

Chapter 7: Periodic Properties of the Elements What happens to Z eff along a period in the periodic table? 11 Na 12 Mg 13 Al 14 Si ….

Chapter 7: Periodic Properties of the Elements What happens to the atomic radius along a period ? The effective nuclear charge increases along a period and draws in the surrounding electrons, making the atom more compact Lithium Fluorine

Chapter 7: Periodic Properties of the Elements What happens to the atomic radius as we go down a group ? => but - more main shells are added => Z eff essentially remains constant 11 Na 19 K 37 Rb atomic radius increases as we go down a group

Chapter 7: Periodic Properties of the Elements atomic radius increases

Chapter 7: Periodic Properties of the Elements which radius is largest? which radius is smallest?

Chapter 7: Periodic Properties of the Elements

How about the radii of ions? cations are always smaller than their parent ions anions are always larger than their parent ions

Chapter 7: Periodic Properties of the Elements Which of the following has the largest radius? S 2-, S, O 2- Why does the ionic radius decrease along the following series? O 2- F - Na + Mg 2+ Al 3+

Chapter 7: Periodic Properties of the Elements Ionization Energies measure the energy required to remove electrons from an atom in the gas phase 1 st ionization energy: energy required to remove 1 st electron K (g) K + (g) + e - I 1 = 419 kJ/mol Mg (g) Mg + (g) + e - I 1 = 738 kJ/mol 2 nd ionization energy: energy required to remove 2 nd electron 3 rd ionization energy: energy required to remove 3 rd electron …

Chapter 7: Periodic Properties of the Elements Trends in 1 st Ionization Energies 1 st ionization energy

Chapter 7: Periodic Properties of the Elements Why does the ionization energy increase sharply after removal of the 5 th electron for Nitrogen? What would a similar graph look like for beryllium (Be)? Nitrogen

Chapter 7: Periodic Properties of the Elements Some Group trends: Reactivity of alkali metals increases with increasing atomic number Melting point of alkali metals decreases with increasing atomic number:

Chapter 7: Periodic Properties of the Elements General Properties of Metals: compounds made of metals and non-metals tend to be … …ionic substances 2 Ni (s) + O 2 (g) → 2 NiO (s) Metal oxides dissolve in water to make metal hydroxides: Na 2 O (s) + H 2 O (l) → 2 NaOH (aq) strong base!

Chapter 7: Periodic Properties of the Elements Metal oxides tend to make basic solutions in water Na 2 O (s) + H 2 O (l) → 2 NaOH (aq) Non-metal oxides tend to make acidic solutions in water CO 2 (g) + H 2 O (l) → H 2 CO 3 (aq) SO 3 (g) + H 2 O (l) → H 2 SO 4 (aq) SO 2 (g) + H 2 O (l) → H 2 SO 3 (aq) natural water is always slightly acidic ! contribute to acid rain!