# Writing Lewis Structures of Molecules 1.Determine the central atom (atom in the middle) - usually is the “single” atom - least electronegative element.

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Writing Lewis Structures of Molecules 1.Determine the central atom (atom in the middle) - usually is the “single” atom - least electronegative element - H never in the middle; C always in the middle 2.Count the total number of valence e - (group #) - add ion charge for “-” - subtract ion charge for “+” 3.Attach the atoms together with one pair of electrons

Writing Lewis Structures Cont. 4. All remaining e - = LONE PAIRS! - lone pairs are NOT involved in bonding - lone pairs are NOT involved in bonding 5. Place lone pairs around atoms to fulfill the “octet rule” - some elements may violate this octet - some elements may violate this octet rule – (H=2, Be=4, B=6) rule – (H=2, Be=4, B=6) 6. If more e - are still needed, create double or triple bonds around the central atom. or triple bonds around the central atom. - single = 1 pair of shared electrons (2 e - ) - single = 1 pair of shared electrons (2 e - ) - double = 2 pair of shared electrons (4 e - ) - double = 2 pair of shared electrons (4 e - ) - triple = 3 pair of shared electrons (6 e - ) - triple = 3 pair of shared electrons (6 e - )

Resonance When there is more than one Lewis structure for a molecule that differ only in the position of the electrons they are called resonance structures When there is more than one Lewis structure for a molecule that differ only in the position of the electrons they are called resonance structures –Lone Pairs and Multiple Bonds in different positions Resonance only occurs when there are double bonds and when the same atoms are attached to the central atom Resonance only occurs when there are double bonds and when the same atoms are attached to the central atom The actual molecule is a combination of all the resonance forms. The actual molecule is a combination of all the resonance forms. O S O

Coordinate Covalent Bond A covalent bond in which one atom contributes both bonding electrons. A covalent bond in which one atom contributes both bonding electrons.

Predicting Molecular Geometry VSEPR Theory VSEPR Theory –Valence Shell Electron Pair Repulsion The shape around the central atom(s) can be predicted by assuming that the areas of electrons on the central atom will repel each other The shape around the central atom(s) can be predicted by assuming that the areas of electrons on the central atom will repel each other Each Bond counts as 1 area of electrons Each Bond counts as 1 area of electrons –single, double or triple all count as 1 area Each Lone Pair counts as 1 area of electrons Each Lone Pair counts as 1 area of electrons –Even though lone pairs are not attached to other atoms, they do “occupy space” around the central atom –Lone pairs generally “push harder” than bonding electrons, affecting the bond angle

Shapes Linear Linear –2 atoms on opposite sides of central atom, no lone pairs around CA –180° bond angles Trigonal or Triangular Planar Trigonal or Triangular Planar –3 atoms form a triangle around the central atom, no lone pairs around CA –Planar –120° bond angles Tetrahedral Tetrahedral –4 surrounding atoms form a tetrahedron around the central atom, no lone pairs around the CA –109.5° bond angles 180° 120° 109.5°

Shapes Pyramidal or Trigonal Pyramidal Pyramidal or Trigonal Pyramidal –3 bonding areas and 1 lone pair around the CA –Bond angle = 107 0 V-shaped or Bent V-shaped or Bent –2 bonding areas and 2 lone pairs around the CA –bond angle = 104.5 0

Polarity of Molecules Molecule will be NONPOLAR if: Molecule will be NONPOLAR if: –the bonds are nonpolar (Br-Br, F-F) –there are no lone pairs around the central atom and all the atoms attached to the central atom are the same Molecule will be POLAR if: Molecule will be POLAR if: –the central atom has lone pairs

Sigma and Pi Bonds Sigma Bond – End to end overlap along the internuclear axis Sigma Bond – End to end overlap along the internuclear axis Pi Bond – Parallel (side by side) overlap (can only be done with p orbitals) Pi Bond – Parallel (side by side) overlap (can only be done with p orbitals) 2 “s” orbitals overlapping – sigma (σ) 2 “s” orbitals overlapping – sigma (σ) 1 “s” and 1 “p” orbital overlapping- sigma 1 “s” and 1 “p” orbital overlapping- sigma 2 “p” orbitals overlapping (same axis) - sigma 2 “p” orbitals overlapping (same axis) - sigma 2 “p” orbitals overlapping (parallel axes) - pi 2 “p” orbitals overlapping (parallel axes) - pi

Hybridization refers to a mixture or a blending refers to a mixture or a blending Biology – refers to genetic material Biology – refers to genetic material Chemistry – refers to blending of orbitals Chemistry – refers to blending of orbitals Remember, orbitals can only predict an area in space where an e - may be located. Remember, orbitals can only predict an area in space where an e - may be located. Sometimes blending orbitals can produce a lower, more stable bonding opportunity. Sometimes blending orbitals can produce a lower, more stable bonding opportunity. Orbital hybridization occurs through e - promotion in orbitals that have similar energies (i.e. same energy level). Orbital hybridization occurs through e - promotion in orbitals that have similar energies (i.e. same energy level).

Hybridization Cont. Hybridization occurs WITHIN the atom to enhance bonding possibilities. Hybridization occurs WITHIN the atom to enhance bonding possibilities. Do not confuse this concept with orbital overlap (bonding). Do not confuse this concept with orbital overlap (bonding). Hybridization is a concept used to explain observed phenomenon about bonding that can’t be explained by dot structures. Hybridization is a concept used to explain observed phenomenon about bonding that can’t be explained by dot structures. EXAMPLES – draw box diagrams for Be, B, and C (use noble gas core). EXAMPLES – draw box diagrams for Be, B, and C (use noble gas core).

How do I know if my CA is hybridized? If your CA is B, Be, C, Si, or Al then it is hybridized. If your CA is B, Be, C, Si, or Al then it is hybridized. If your molecule has multiple bonds in it then it is hybridized. If your molecule has multiple bonds in it then it is hybridized. –Double bonds – sp 2 hybridized –Triple bonds (or 2 double bonds) – sp hybridized

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