Valence Electrons & Lewis Dot Valence Electrons & Lewis Dot DiagramsDiagrams

Periodic Table DivisionsPeriodic Table Divisions Main Group or “Representative Elements”Main Group or “Representative Elements”

Elements belonging to the Elements belonging to the “s” and “p” blocks“s” and “p” blocks Have Have predictable and similar characteristics predictable and similar characteristics as you move down as you move down

their column or group on the periodic tabletheir column or group on the periodic table Transition ElementsTransition Elements

Elements belonging to the Elements belonging to the “d” and “f” blocks“d” and “f” blocks Characteristics are Characteristics are less predictable and conventional less predictable and conventional as you as you

move down their column or group on the periodic tablemove down their column or group on the periodic table

Valence ElectronsValence Electrons

The The outermostoutermost electrons of an atom electrons of an atomElectrons located in the Electrons located in the highest principle highest principle

energy levelenergy level (or highest principal quantum (or highest principal quantum number)number)

By convention, valence electrons are only By convention, valence electrons are only found in the found in the “s” and “p” sub-shells“s” and “p” sub-shellsSituation is more complex for transition Situation is more complex for transition

elements (will touch on this later)elements (will touch on this later)

Examples:Examples:

Calcium (20 e-)Calcium (20 e-)1s1s22 2s 2s22 2p 2p66 3s 3s22 3p 3p66 4s4s22 2 valence e-2 valence e-

Germanium (32 e-)Germanium (32 e-)1s1s22 2s 2s22 2p 2p66 3s 3s22 3p 3p66 4s4s22 3d 3d1010 4p4p22 4 valence e- 4 valence e-

Krypton (36 e-) Krypton (36 e-) 1s1s22 2s 2s22 2p 2p66 3s 3s22 3p 3p66 4s4s22 3d 3d1010 4p4p66 8 valence e- 8 valence e-

Octet Rule – “Rule of 8”Octet Rule – “Rule of 8” States that the States that the maximummaximum possible amount of valence possible amount of valence

electrons for any atom is 8electrons for any atom is 8 2 electrons from the “s” sub-shell2 electrons from the “s” sub-shell 6 electrons from the “p” sub-shell6 electrons from the “p” sub-shell

Electrons from the d & f sub-shells are not includedElectrons from the d & f sub-shells are not included The only elements with a full valence shell are the The only elements with a full valence shell are the

Noble GasesNoble Gases For this reason, Noble Gases are the For this reason, Noble Gases are the most stable and most stable and

least likely to reactleast likely to react Other atoms want to react in such a way as to Other atoms want to react in such a way as to

change their number of valence electrons to that of a change their number of valence electrons to that of a Noble GasNoble Gas i.e. – atoms want to gain or lose electrons to reach a full i.e. – atoms want to gain or lose electrons to reach a full

octetoctet

Representing Valence Electrons – Lewis DiagramsRepresenting Valence Electrons – Lewis Diagrams

Lewis TheoryLewis Theory Proposed by American G.N. Lewis (1916-1919)Proposed by American G.N. Lewis (1916-1919) Theory had 4 major concepts:Theory had 4 major concepts:

1.1. Valence electrons Valence electrons play the most crucial role in play the most crucial role in bondingbonding

2.2. Electrons may be Electrons may be transferred during bond formationtransferred during bond formation

3.3. Electrons may be Electrons may be shared during bond formationshared during bond formation

4.4. Electrons are transferred or shared in such a way Electrons are transferred or shared in such a way that each atom that each atom acquires a stable valence shell or a acquires a stable valence shell or a stable “octet”stable “octet”

Lewis Dot DiagramsLewis Dot Diagrams

Consists of two parts:Consists of two parts: 1) Element symbol1) Element symbol

Represent the Represent the nucleus and in inner-shell electrons nucleus and in inner-shell electrons of an of an atomatom

2) Dots2) Dots Represent the Represent the valence electrons valence electrons of an atomof an atom

Example: Calcium – [Ar] 4sExample: Calcium – [Ar] 4s22

.Ca..Ca.

Treat all valence electrons equally Treat all valence electrons equally do not do not distinguish between s & p electronsdistinguish between s & p electrons

Lewis Dot Diagrams - StepsLewis Dot Diagrams - Steps

Remember maximum amount of valence Remember maximum amount of valence electrons is 8 (selectrons is 8 (s22 + p + p66))

1.1. Begin by writing the Begin by writing the element symbolelement symbol2.2. Determine the amount of Determine the amount of valence electronsvalence electrons3.3. Place valence electrons around the element Place valence electrons around the element

symbolsymbolI.I. Think of the symbol as having Think of the symbol as having 4 sides or “faces”4 sides or “faces”II.II. Each face can hold a Each face can hold a maximum of 2 electronsmaximum of 2 electronsIII.III. Place 1 electron on each side (or face) Place 1 electron on each side (or face) before before

doubling updoubling up

ExamplesExamples

CalciumCalcium

GermaniumGermanium

KryptonKrypton

Why transition elements are weirdWhy transition elements are weird

If transition elements all played by the rules, they would all have the If transition elements all played by the rules, they would all have the same number of valence electrons = 2same number of valence electrons = 2 Based on the full “s” shellBased on the full “s” shell Since, by definition, valence electrons are those found in the outermost 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” energy level, any of the electrons found in the “d” shell would not “count” since they belong to a lower principal quantum numbersince they belong to a lower principal quantum number

Unfortunately, due to some exceptions this is not always the caseUnfortunately, due to some exceptions this is not always the case If you notice on your periodic table, the transition elements may have 1 If you notice on your periodic table, the transition elements may have 1

or 2 valence electronsor 2 valence electrons This is based on an exception to the order of filling ruleThis is based on an exception to the order of filling rule

i.e. – Chromium i.e. – Chromium 1 valence electron 1 valence electron Based on the fact that, in nature, only one electron enters the 4s orbital, and 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-the additional electron is promoted into the 3d orbitals; giving it a non-conventional electron configuration of [Ar] 4sconventional electron configuration of [Ar] 4s113d3d55

Why does this happen? Out of the scope of this class, but a great idea for Why does this happen? Out of the scope of this class, but a great idea for independent research or curiosityindependent research or curiosity

For this reason, we will not consider transition elements for Lewis For this reason, we will not consider transition elements for Lewis Structures and we will always use the traditional/conventional order Structures and we will always use the traditional/conventional order of filling for electron configurationof filling for electron configuration