Valence Electrons & Lewis Dot Diagrams

1. Valence Electrons & Lewis Dot Diagrams

2. Periodic Table Divisions • Main Group or “Representative Elements” • Elements belonging to the “s” and “p” blocks • Have predictable and similar characteristics as you move down their column or group on the periodic table • Transition Elements • Elements belonging to the “d” and “f” blocks • Characteristics are less predictable and conventional as you move down their column or group on the periodic table

3. Valence Electrons • The outermost electrons of an atom • Electrons located in the highest principle energy level (or highest principal quantum number) • By convention, valence electrons are only found in the “s” and “p” sub-shells • Situation is more complex for transition elements (will touch on this later)

4. Examples: • Calcium (20 e-) • 1s2 2s2 2p6 3s2 3p64s2 • 2 valence e- • Germanium (32 e-) • 1s2 2s2 2p6 3s2 3p64s2 3d104p2 • 4 valence e- • Krypton (36 e-) • 1s2 2s2 2p6 3s2 3p64s2 3d104p6 • 8 valence e-

5. Octet Rule – “Rule of 8” • States that the maximum possible amount of valence electrons for any atom is 8 • 2 electrons from the “s” sub-shell • 6 electrons from the “p” sub-shell • Electrons from the d & f sub-shells are not included • The only elements with a full valence shell are the Noble Gases • For this reason, Noble Gases are the most stable and least likely to react • Other atoms want to react in such a way as to change their number of valence electrons to that of a Noble Gas • i.e. – atoms want to gain or lose electrons to reach a full octet

6. Representing Valence Electrons – Lewis Diagrams • Lewis Theory • Proposed by American G.N. Lewis (1916-1919) • Theory had 4 major concepts: • Valence electrons play the most crucial role in bonding • Electrons may be transferred during bond formation • Electrons may be shared during bond formation • Electrons are transferred or shared in such a way that each atom acquires a stable valence shell or a stable “octet”

7. Lewis Dot Diagrams • Consists of two parts: • 1) Element symbol • Represent the nucleus and in inner-shell electrons of an atom • 2) Dots • Represent the valence electrons of an atom • Example: Calcium – [Ar] 4s2 .Ca. • Treat all valence electrons equally  do not distinguish between s & p electrons

8. Lewis Dot Diagrams - Steps • Remember maximum amount of valence electrons is 8 (s2 + p6) • Begin by writing the element symbol • Determine the amount of valence electrons • Place valence electrons around the element symbol • Think of the symbol as having 4 sides or “faces” • Each face can hold a maximum of 2 electrons • Place 1 electron on each side (or face) before doubling up

9. Examples • Calcium • Germanium • Krypton

10. Why transition elements are weird • If transition elements all played by the rules, they would all have the same number of valence electrons = 2 • Based on the full “s” shell • Since, by definition, valence electrons are those found in the outermost energy level, any of the electrons found in the “d” shell would not “count” since they belong to a lower principal quantum number • Unfortunately, due to some exceptions this is not always the case • If you notice on your periodic table, the transition elements may have 1 or 2 valence electrons • This is based on an exception to the order of filling rule • i.e. – Chromium  1 valence electron • Based on the fact that, in nature, only one electron enters the 4s orbital, and the additional electron is promoted into the 3d orbitals; giving it a non-conventional electron configuration of [Ar] 4s13d5 • Why does this happen? Out of the scope of this class, but a great idea for independent research or curiosity • For this reason, we will not consider transition elements for Lewis Structures and we will always use the traditional/conventional order of filling for electron configuration