Formal Charges

# valence e- - # covalent bonds - # e- in lone pairs Sometimes molecules can be drawn in more ways than one… which is the best way? What are formal charges? Look at the number of valence electrons an atom has Subtract # of covalent bonds it is touching Subtract # of e- in lone pair # valence e- - # covalent bonds - # e- in lone pairs THE MOST STABLE MOLECULE IS ONE WHERE THE FORMAL CHARGES ARE THE LOWEST!

Which HNO2 is more stable? + -

Sulfuric Acid – Which one is correct? TOO MANY FORMAL CHARGES! FORMAL CHARGE ON EACH ATOM IS ZERO!

Charged Molecules You need to add or subtract electrons, depending on the charge Be sure to add square brackets, and include the charge! Ex. NH4+ the + means you need to take one electron away 5 e- (from nitrogen) + 4 e- (from hydrogens) -1 e- (from the + charge) = 8 e- to use

Resonance Structures Sometimes molecules have fractional bonds as a result of electron movement among bonds To accurately represent the molecule, you need to show all possibilities! Each of these structures is called a RESONANCE STRUCTURE The real molecule is all of these structures! Each oxygen atom has 1.3 bonds Which is why you must draw all 3 structures!

Try to draw the Lewis Structure for HPO32-

Bond Energy The measure of BOND STRENGTH The stronger the bond, the HIGHER the bond energy It is expressed as heat required to break 1 mole of molecules

It takes energy to BREAK bonds (endothermic) Energy is released when bonds are FORMED (exothermic)

breaking required making released Example: The reaction turning 1 molecule of H2(g) into two H(g) atoms involves bond _______________. Energy is ______________. The reaction turning two oxygen atom into a molecule of O2 involves bond _______________. Energy is ______________. breaking required making released

https://www.youtube.com/watch?v=Dcq8MMu0ZlQ

c) Compare the bond energies of single, double, and triple bonds c) Compare the bond energies of single, double, and triple bonds. Explain the data.

c) Compare the bond energies of single, double, and triple bonds c) Compare the bond energies of single, double, and triple bonds. Explain the data. Triple > double > single More e- shared = more attractive forces between 2 nuclei

d) Compare the bond energies of C-N, C-O, C-F. Explain the data.

d) Compare the bond energies of C-N, C-O, C-F. Explain the data. C-F > C-O > C-N

What do you notice about atomic radius? C–N 305 C–O 360 C–F 484

d) Compare the bond energies of C-N, C-O, C-F. Explain the data. C-F > C-O > C-N Bond strength increases with increasing electronegativity and decreasing radius

Bond length: the distance between 2 nuclei of a molecule The shorter the bond length, the stronger the bond, the higher the bond energy Example: How do the bond lengths of single, double, and triple bonds compare? C C C C C C 0.120 nm 0.134 nm 0.154 nm

LiF LiCl LiBr Electronegativity: F > Cl > Br Example: Rank the following compounds in order of increasing melting point (increasing bond strength). LiF LiCl LiBr Electronegativity: F > Cl > Br bond strength LiF > LiCl > LiBr melting point LiF > LiCl > LiBr 845°C 605°C 552°C

Example: Which of the following compounds has the higher bond energy? NaCl MgO MgO is being held together by 2+ and 2- charge as opposed to 1+ and 1- in NaCl 801°C 2852°C

For ionic compounds, two things must be considered in order to determine bond strength. They are (in order of importance): The charge: the ___________ the charge on each ion in the ionic compound, the ______________ the ionic bond, the ___________ the bond energy. Electronegativity: the higher the electronegativity of the non-metal, the _______________ the attraction, the ______________ the bond energy. higher stronger higher higher higher

How do you know if a bond is strong? Covalent Compounds Number of bonds: more bonds = strong bond Atomic radii: short bond length = strong bond Electronegativity: strong electronegativity = strong bond Ionic Compounds Charge: high charge = strong bond Electronegativity of non-metal: high electronegativity = strong bond