AP Chem Take out HW to be checked Today: Bonding and Molecular Shapes Cont. Lab Write Up due Fri 10/5 Unit 2 Test ~Mon 10/8, Tues 10/9

Forming Bonds Bond= a region that forms when electrons from different atoms interact with each other The attraction between 2 or more atoms allows for the formation of a compound. Only valence electrons participate in bonding Octet Rule Atoms bond in order to get a full valence shell, or 8 valence electrons Exceptions: Hydrogen is happy with 2 valence electrons Boron is happy with 6 valence electrons

Covalent Bonds A covalent compound is one that is made up of 2 or more nonmetals A covalent bond involves the sharing of electrons Lewis Structures Show just the valence electrons

Remember: The group (vertical column) the element is in gives you information about the number of valence electrons it has!

“HONC Rule” Bonds each atom needs to form in order to achieve a full valence shell: Hydrogen: 1 bond Oxygen: 2 bonds Nitrogen: 3 bonds Carbon: 4 bonds Halogens: 1 bond 1 pair of electrons shared between atoms = single bond 2 pairs of electrons shared between atoms = double bond 3 pairs of electrons shared between atoms = triple bond

Exceptions to the Octet Rule: Expanded Octet Central atom ends up with more than an octet. Occurs when there are too many electrons to fit in. Central atom with an expanded octet MUST have an atomic number larger than 10 (beyond neon). In the discussion of hybridization, we see that when there are 5 or 6 “groups” on the central atom, d-orbitals are involved. Extra electrons should be first placed on the outside atoms. After the outside atoms have fulfilled the Octet Rule, and there are still extra electrons, start with placing them as lone pairs on the central atom. If the central atom has a positive charge should you move a lone pair from the outside atoms to share.

Hybrid Orbitals To explain molecular geometries, we can assume that the atomic orbitals on an atom (usually the central atom) mix to form new orbitals called hybrid orbitals.

Hybridization Hybridization: combining/mixing of electron orbitals when atoms bond. The hybridization is determined by the # things around the central atom The shape is determined by the lone pairs on the central atom

Common Hybridizations # Sets Orbital Hybridization 2 sp 3 sp2 4 sp3 5 sp3d 6 sp3d2 Note: “sets” refer to bonded atoms as well as lone pairs on the central atom

VSEPR Theory Valence Shell Electron Pair Repulsion A=Central Atom X=Bonded Atoms E=electron pairs (non-bonding) on the central atom

Bond Types Sigma Bond (σ) a bond formed between two elements orbitals come together head on

Bond Types Pi Bond (π) a bond formed above and below elements orbitals come together side by side

Bond Types Single bonds composed of one σ bond

Bond Types Double bonds composed of one σ bond and one π bond

Bond Types Triple Bond formed by one σ bond and two π bonds

Energy Associated with Bonding Atoms with a full valence shell are stable (“happy”) and are in a low energy state. Breaking Bonds (NOT spontaneous): Energy is absorbed Go from low energy (“happy” atoms) to high energy (“unhappy” atoms) Ripping two atoms apart requires energy= is endothermic

Energy Associated with Bonding SPONTANEOUS Bond formation = Energy is released Go from high energy (“unhappy” atoms) to low energy (“happy” atoms) Forming a bond is exothermic

Calculating Bond Energy

Covalent Bonds-Formation Atoms bonded together exist at the lowest energy possible

Covalent Bonds-Formation Bond Energy The energy needed to form/break a bond Dependent upon two atoms in bond as well as other atoms within the molecule, therefore average is used

Covalent Bonds-Formation Bond length Average distance between two bonded atoms to give the lowest energy possible The longer the bond length the weaker the energy The more pi bonds the stronger the bond (more energy) The stronger the bonds between atoms (ie single bond vs double bond) the shorter the bond length

Examples Draw the Lewis structures for C2H6 and C2H2. Indicate which has the longest C-C bond and which has the strongest C-C bond. 1. the average C–H bond energy is 413 kJ/mol considering that there are 4 C-H bonds, the total energy of CH4 molecule is 4*413 =1652 kJ/mol The average O-H bond energy is 467 kJ/mol there are 2 O-H bonds in H2O, the total energy is 2*467 = 934 kJ/mol