Valence Electrons & Lewis Dot Diagrams

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

Valence Electrons The outermost electrons of an atom The outermost electrons of an atom Electrons located in the highest principle energy level (or highest principal quantum number) 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 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) Situation is more complex for transition elements (will touch on this later)

Examples: Calcium (20 e-) Calcium (20 e-) 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 2 valence e- 2 valence e- Germanium (32 e-) Germanium (32 e-) 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 2 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 2 4 valence e- 4 valence e- Krypton (36 e-) Krypton (36 e-) 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 8 valence e- 8 valence e-

Octet Rule – “Rule of 8” States that the maximum possible amount of valence electrons for any atom is 8 States that the maximum possible amount of valence electrons for any atom is 8 2 electrons from the “s” sub-shell 2 electrons from the “s” sub-shell 6 electrons from the “p” sub-shell 6 electrons from the “p” sub-shell Electrons from the d & f sub-shells are not included Electrons from the d & f sub-shells are not included The only elements with a full valence shell are the Noble Gases 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 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 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 i.e. – atoms want to gain or lose electrons to reach a full octet

Representing Valence Electrons – Lewis Diagrams Lewis Theory Lewis Theory Proposed by American G.N. Lewis ( ) Proposed by American G.N. Lewis ( ) Theory had 4 major concepts: Theory had 4 major concepts: 1. Valence electrons play the most crucial role in bonding 2. Electrons may be transferred during bond formation 3. Electrons may be shared during bond formation 4. Electrons are transferred or shared in such a way that each atom acquires a stable valence shell or a stable “octet”

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

Lewis Dot Diagrams - Steps Remember maximum amount of valence electrons is 8 (s 2 + p 6 ) Remember maximum amount of valence electrons is 8 (s 2 + p 6 ) 1. Begin by writing the element symbol 2. Determine the amount of valence electrons 3. Place valence electrons around the element symbol I. Think of the symbol as having 4 sides or “faces” II. Each face can hold a maximum of 2 electrons III. Place 1 electron on each side (or face) before doubling up

Examples Calcium Calcium Germanium Germanium Krypton Krypton

Why transition elements are weird If transition elements all played by the rules, they would all have the same number of valence electrons = 2 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 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 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 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 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 This is based on an exception to the order of filling rule i.e. – Chromium  1 valence electron 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] 4s 1 3d 5 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] 4s 1 3d 5 Why does this happen? Out of the scope of this class, but a great idea for independent research or curiosity 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 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