CHAPTER 7 - SUMMARY NOTES ON PERIODIC TRENDS VALENCE ELECTRONS (VE) = Electrons that reside in the outermost shell of the atom. Trend in Valence Electrons: # of VEs for a main group element = Group # on the periodic table Halogens: 7 VE Oxygen, Sulfur: 6 VE Sodium, Potassium: 1 VE Noble gases: 8 VE (except He: 2 VE) For Transition Metals, e- in the last s and d-subshells are typically considered VE's unless the d- subshell is full: V: [Ar] 4s2 3d3: 3d is not full, so count the 4s and 3d e- = 5 VE Cu: [Ar] 4s1 3d10: 3d is full, so only count the 4s e- = 1 VE EFFECTIVE NUCLEAR CHARGE (ZEFF ) = The net positive charge that an electron "sees" from the nucleus Zeff = Z-S, where Z= # of p in the nucleus and S is the # of shielding (core) e- Trends in Zeff experienced by valence electrons: Increase from left to right (across a row) - # of core electrons (and shielding) stays the same, but the number of protons increases Increase (slightly) from top to bottom (down a group)