1. Ionic, Covalent, Metallic

2. What is a chemical bond?  A strong attractive force between atoms in a compound  Ionic: attraction of oppositely charged ions  Covalent: attraction of the nucleus of each atom to a shared pair of valence electrons  Metallic: attraction of positively charged metal ions to the mobile valence electrons in the metal

3. The Octet Rule  The Octet Rule:  Atoms will lose, gain, or share valence electrons until each atom achieves…  an octet of valence electrons  a total of 8 valence electrons  a noble gas electron configuration  The Octet Rule is our guiding principle in the formation of chemical bonds between atoms

4. Types:  Ionic: atoms lose or gain valence electrons, forming + and – ions, which are then attracted to one another because they are oppositely charged  Covalent: atoms share valence electrons, the nuclei of both atoms are strongly attracted to the shared valence electrons, forming the bond  May be nonpolar (equal sharing) or polar (unequal sharing)  Metallic: the valence electrons of atoms in the metal are mobile, leaving the metal atoms with a + charge, the + charged metal ions are attracted to the free- floating electrons

5. What determines the type of bond formed between two atoms?   The “TUG OF WAR” for electrons!

6. Tug of War  Atoms play “tug of war” with electrons  The electronegativity of the atoms determines their “strength” in the tug of war  Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons.  The Pauling scale is the most commonly used. Fluorine (the most electronegative element) is assigned a value of 4.0, and values range down from there

7. Consider two atoms…  Atom A and Atom B form a bond…

8. What if atom B is a lot more electronegative than atom A?  Sometimes one of the atoms is much stronger (more electronegative) than the other, and “wins” the electron- this results in a transfer of electron(s) to form ions, and thus an ionic bond  Consider a bond between two atoms, A and B.  In this case, the electron pair is dragged right over to B's end of the bond. A has lost control of its electron, and B has complete control over both electrons. Ions have been formed. A +1 --------:B -1

9. What happens if B is slightly more electronegative than A?  Sometimes one of the atoms is a little stronger than the other, but not strong enough to “win” the electron  This results in unequal sharing of electrons, which is a polar covalent bond

10. What happens if B is slightly more electronegative than A?  B will attract the electron pair more than A does.  That means that the B end of the bond has more than its fair share of electron density and so becomes slightly negative. At the same time, the A end (rather short of electrons) becomes slightly positive. δ+ A--------:---B δ- (note that δ is pronounced “delta” means “slightly”, so δ+ means slightly positive and δ- means slightly negative. The δ- will always be on the more electronegative of the two atoms)

11.  δ+ A--------:---B δ-  This is described as a polar bond (or polar covalent)- a covalent bond in which there is a separation of charge between one end and the other  in other words one end is slightly positive and the other slightly negative.  The electron density lies closer to one atom than the other  Examples include most covalent bonds. The hydrogen- chlorine bond in HCl or the hydrogen-oxygen bonds in water are typical

12. What happens if two atoms of equal electronegativity bond together?  Sometimes the atoms are equal or nearly equal in their strength (electronegativity), and neither wins  This results in equal sharing of the electron(s), and thus a nonpolar covalent bond  If the atoms are equally electronegative, both have the same tendency to attract the bonding pair of electrons, and so it will be found on average half way between the two atoms. A-----:-----B  To get a bond like this, A and B would usually have to be the same atom. You will find this sort of bond in, for example, H 2 or Cl 2 molecules.

13. How to determine bond type  The difference in electronegativity between the two atoms is what determines whether a bond is ionic, polar covalent, or nonpolar covalent  To determine the bond type-  Look up the electronegativity values for the two atoms in the bond  Subtract to find the difference  Use the chart (next slides) to determine the type of bond

14.  Difference in electronegativityType of Bond 0- 0.3nonpolar covalent 0.4- 1.0moderately polar covalent 1.1- 1.7very polar covalent > 1.7ionic

15.  Electronegativity and bond type Electronegativity and bond type

16. % Ionic Character  Bonds can also be described based on their % ionic character, which is another way to express the degree of polarity in the bond.  100% ionic character would represent a purely ionic bond  0% ionic character would represent a purely covalent bond

17. Let’s compare ionic and covalent IONIC: Transfer of electrons to form ions due to large electronegativity difference between the two atoms Oppositely charged ions are attracted The ions build up in a 3 dimensional crystal lattice; there are not individual molecules The chemical formula represents a formula unit: the lowest whole number ratio of ions that is neutral POLAR COVALENT: Unequal sharing of electrons due to some electronegativity difference between the two atoms Electron clouds overlap and electrons are shared unequally to form bond Individual molecules are formed The chemical formula represents a molecule NONPOLAR COVALENT: Equal sharing of electrons since the two atoms have equal or nearly equal electronegativities Electron clouds overlap and electrons are shared equally to form the bond Individual molecules are formed The chemical formula represents a molecule