Chapter 10. Valence Electrons. Lewis dot symbols. Chemical Bonding

Transcription

1 Chapter 10 Chemical Bonding Valence Electrons Recall: the outer electrons in an atom are valence electrons. Valence electrons are related to stability Valence electrons can be represented with dots in the Lewis electron dot symbol. Each outer electron is represented by a dot around the atomic symbol: Lewis dot symbols Lithium has one valence electron, hence one dot: Calcium has two valence electrons, hence two dots: 1

2 Lewis dot and Valence electrons The number of valence electrons equal the number of dots around the symbol. The number of valence electrons can be derived from the periodic chart. Lewis dot symbols: across the chart Noble Gases and Stability Noble Gases are very stable The Stability of noble gases is associated with their electron configuration Most atoms 2

3 Atoms can become more stable by: 1. Gaining electrons 2. Losing electrons They become negativity charged This process is exothermic They become positively charge This process endothermic Usually happens to nonmetals Results in the formation of an anion Usually happens to metals Results in the formation of a cation Lewis dot symbols: Ions Atoms can become more stable: 3. Sharing electrons Both atoms share 1,2, or 3 pairs of electrons Called covalent bonding Results in the formation of a molecule or a polyatomic ion 3

4 Interactions, Bonding, Stability Ionic Bonding Covalent Bonding Metallic Bonding The Ionic Bond It is an electrostatic interaction Involves a cation and an anion Very strong Ionic compounds do not conduct electricity in solid form Ionic compounds do conduct electricity as a melt or in solution The Covalent Bond The covalent bond is the result of sharing electrons Molecules are neutral compounds with covalent bonds between the atoms Polyatomic ion are charged compounds with covalent bonds between the atoms A covalent bonds results from the overlap of electron clouds 4

5 Metallic Bonding Unique Nuclei in a sea of electrons Occurs in metal alloys Explain why metal conduct electricity well Lewis Dot Diagrams Simplest way to see what a molecule might look like. Can be used for ployatomic ions It will help predict the three dimensional shape. Making a Lewis Dot Diagram Count up all the valence electrons in the molecule (i.e. add the valence number of each atom in the molecule) If a charge is present add or subtract electrons 5

6 Making a Lewis Dot Diagram Construct a rough skeleton Locate the central atom Usually it occur once in the molecule It is never Hydrogen Carbon is a good central atom Distribute the remaining atoms around the center Connect the outside atoms to the center atom with bonds ( _ ) [ use single bonds at first] Making a Lewis Dot Diagram Count up the number of bonds and multiple by 2 (2 for the 2 electrons in each bond) = number of bonding electrons Making a Lewis Dot Diagram Subtract the number of bonding electrons from total number of valence electrons = left over electrons If there is 0 left then double check If some electron are left over these could become lone pairs 6

7 Making a Lewis Dot Diagram Distribute the left over electrons in pairs so as to give each atoms in the molecule 8 electrons Making a Lewis Dot Diagram Note: There are exception to the rule of eight you should be aware of two Hydrogen only need 2 electrons and it is stable Boron is stable with only 6 electrons Making a Lewis Dot Diagram If you have enough electrons to give all the atoms eight then do a double check If you do not have enough electrons to satisfy the rule of eight try double or maybe triple bonds. 7

8 Multiple Bonds A single bond is the result of shared pair of electrons A double bond is the result of shared pairs of electrons A triple bond is the result of shared pairs of electrons Double and triple bonds are defined as multiple bonds Double Check When doing a double check: Make sure each atom in the molecule has eight (except H and B) Make sure the total number of electrons in the molecule is the same as the total valence number calculated in step number 1 Exception To the Octet Rule Odd electron molecules (NO and NO 2 ) Indicate the molecule is unstable Molecules with more then 8 electrons around the central atom (PF 5 and SF 6 ) This can be seen in element in row 3 and higher Molecules with fewer then 8 electrons around the central atom (BeF 2 and BF 3 ) 8

9 2d to 3d via VSEPR A Lewis dot structure gives a 2-d picture of a molecule A 3-d picture can be obtained with VSEPR theory VSEPR Stand for: Valence Shell Electron Pair Repulsion Based on the idea that electron pair will repel and go to the maximum possible distinct apart VSEPR To determine a VSEPR shape Compare the number of electron regions to the number of bonding regions Note treat double or triple bond as one electron region and one bonding region 9

10 Four electron groups If you have four electron regions around the central atom and all four are bonding then shape is a tetrahedral Four electron groups If you have four electron regions around the central atom but only three are bonding then the shape is a Triangular Pyramid Four electron groups If you have four electron regions around the central atom but only two are bonding then the shape is Bent 10

11 Three electron groups If you have three electron regions around the central atom and all three are bonding then the shape is Triangular planar Two electron groups If you have two electron regions around the central atom and both are bonding then the shape is Linear Polarity The pair of electrons in a bond is not always shared equally A bond that has an unequal sharing of electrons is termed a polar bond A polar bond has partial charges The more electronegative atom has a partial negative The partial; charge are symbolized δ- and δ+ 11

12 Levels of Polarity Bond polarity The only true non-polar bond occur when two atoms of the same electronegativity are bonded together H 2, O 2, Cl 2, etc. Levels of Polarity Molecular polarity To determine the molecular polarity you must go through look at two things First: determine the bond polarity for each bond (only same/same is non-polar) Second: look at the structure (3-d) of the molecule If the polar bonds completely cancel each other out then the molecule is non-polar Molecular Polarity Canceling out bond polarity Linear will cancel if both groups are the same Tetrahedral will cancel if all groups are the same You must know the shape before you determine molecular polarity 12

13 Example of Molecular Polarity Example of Molecular Polarity 13