Lewis Dot Structures and their Molecular Geometry

Lewis Dot Structures What are Lewis Dot Structures? They are drawings representing how the VALANCE electrons of atoms are arranged within a molecule. How to draw Lewis Dot Structures of atoms and mono ions. It is the symbol of element + its valence electrons. Valence electrons are represented by (x) or dots. One dot (x) = one electron. x Carbon: xCx x Lewis Dot Structures Guidelines Octet: non-metal elements in the second row that are trying to achieve “noble gas electronic configuration” via bonding must have and can only have EIGHT electrons surrounding each atom in the compound Duet: non-metal elements in the first row need only TWO electrons to achieve noble look. They can only have a total of two electrons around it. Expanded octet: if the center element a third row element and there are valance electrons left over after all attached atoms have satisfied the octet or duet requirements then place the extra valance electrons around the center atom. Steps to Draw a Lewis Dot Structure of a Compound or Polyatomic Ion 1) Decide which atoms in the molecule are bonded to each other – determine the center atom to which all the atoms are bonded to it. 2) Determine the number of valance electrons each atom has and will be bring to bond. Total up the number of valance electrons that must be distributive throughout the molecule. 3) Place TWO electrons for a single bond FOUR electrons for a double bond* SIX electrons for a triple bond* * Elements, which can form double bonds with each other and themselves: C, S, N, O, & P * Elements, which can form triple bonds with each other or themselves: C, S, and N & O NOTE: when halogens Cl, Br, and I or noble gases Xe, and Kr are the CENTER atom and oxygen is bond to them (oxyacids) then these atoms can also form double and or triple bonds with oxygen only. 4) Complete octets of atoms – all attaching atoms MUST have eight electrons. Exceptions: HYDROGEN - it can only have two Metals with nonmetals (ionic compounds): alkali (two) alkaline earths (four), Al and B (six) 6) If the central atom does not have an octet via single bond and it is an element that can form multiple bonds then use doubles and triples to complete an octet. SUMMARY: The number of atoms and lone pairs surrounding the center atom determines the molecular shape and geometric family. The geometric family determines the type of hybridization that the center atom COULD undergo when bonding with other atoms. Electron Geometric (family) and Molecular Geometry (shapes) VSEPR Theory Predicted Hybridization Angle0 Attachments To center atom Lone Pairs Family Molecular shape 2 0 linear linear 3 0 trigonal planar trigonal planar 2 1 trigonal planar bent (angular) sp2 >120.0 4 0 tetrahedral tetrahedral sp3 109.5 3 1 tetrahedral trigonal pyramidal sp3 ~107.0 sp 180.0 (triple bond) ____________________________________________________________________________________ sp2 (double bond) 120.0 ~104.5 2 2 tetrahedral bent or angular sp3 ____________________________________________________________________________________ 5 0 trigonal bipyramidal trigonal bipyramidal dsp3 120 (plane) 180 (vertical) 4 1 trigonal bipyramidal sawhorse (see saw) dsp3 120 (plane) 180 (vertical) 3 2 trigonal bipyramidal T - shape dsp3 120 (plane) 180 (vertical) 2 3 trigonal bipyramidal linear dsp3 120 (plane) 180 (vertical) 6 0 octahedral octahedral d2sp3 all 90.0 5 1 octahedral square pyramidal d2sp3 4 2 octahedral square planar d2sp3 3 3 octahedral T-shape d2sp3 2 4 octahedral linear d2sp3 Hybridization, Sigma & Pi Bonds: A Very Quick Overview • If you know the electron-pair geometry on a molecules CENTER atom then you can determine the HYBRIDIZATION on the center atom IF IT OCCURRED. Electron Pair Geometry o o o o o Hybridization Linear Trigonal Planar Tetrahedral Trigonal Bipyramid Octahedral sp sp2 sp3 dsp3 d2sp3 Hybridized Orbitals Required 2 3 4 5 6 • Every bond has a sigma (σ) component. o A sigma component is the electron density (bond) between two atoms within the plane of the two atoms. o When two atoms contact (bond) there is ALWAYS one sigma bond.  There is NEVER two sigma bonds between two atoms. o Sigma bonds use hybridized orbitals. o Lone pairs use hybridized orbitals • Pi bonds occur when atoms multiple bond to each other (double or triple). o Double bonds contain ONE pi (Π) component and ONE sigma (σ) component. o Triple bonds contain TWO pi (Π) component and one ONE sigma (σ) component. o Pi bonds NEVER use hybridizes orbitals.  Pi bonds are formed from electrons that occupy un hybridized “p” and/or “d” orbital. • o Empty orbitals are never hybridized.