Drawing Lewis Structures and VSEPR

OUTCOME QUESTION(S): C12-2-07 PERIODIC TRENDS Vocabulary & Concepts Identify factors and account for periodic trends among the properties of elements, and relate to electron configurations. Include: atomic / ionic radii, ionization energy, and electronegativity. Construct Lewis dot diagrams for various compounds and use VSEPR to predict bond shape from the electron arrangement. Vocabulary & Concepts  Line Bond

*Valence electrons determine atom reactivity* Aluminum Fluorine 3 F- Al+3

S Ca Electron Dot Diagrams (Lewis Dot) A quick way to show bonding electrons (valence) atomic symbol represents nucleus and inner shell electrons only valence electrons are used dots placed clockwise, ending in pairs S Ca Since valence electrons do all the work – the Lewis dot is only concerned with them

The central atom is supposed to have the lowest EN value Constructing a Lewis Dot Diagram: 1. Determine total valence electrons for all atoms. ~ watch for negative/positive ion SeCl2, CH2ClF, CO2, NH41+, PO43- 6 +7 + 7 = 20 2. Position first atom in the centre. ~ usually first in formula (unless H or F) The central atom is supposed to have the lowest EN value 3. Join other atoms with line bonds to the central. Cl – Se – Cl

4. Subtract 2 electrons for every bond formed. SeCl2 6 +7 + 7 = 20 - 4 = 16 remaining        Cl – Se – Cl          8 8 8 5. Place remaining e- around atoms to fill valence. Fill surrounding atoms BEFORE central atom 6. Check that each atom has a complete octet. Remember: a line bond counts as 2 “8 is great, but 2 will do” – Hydrogen is the exception

Draw the Lewis structure for CO2: CO2 : 4 + 6 + 6 = 16 - 4 = 12 Not “full”      – –  O – C – O       8 8 8 4 8 8 Too few electrons? - give central atom a complete octet by creating a double or triple bond. Create multiple bonds by moving an electron pair from a surrounding atom into a line bond with the central

Be sure to account for extra or lost electrons in a ionic compound Draw the Lewis structure for SO42- : SO42- : 6 + (4)6 + 2 = 32 - 8 = 24 Be sure to account for extra or lost electrons in a ionic compound 2-  S – O O –    Charged Compound? – put structure into brackets and write the charge on the right.

MgCl2 : 2 + 7 + 7 = 16 - 4 = 12 Ionic Bond Note: a complete octet for the metal is not necessary here – valence electrons are given away   Cl – Mg – Cl 2+ Mg 1- Cl  1- Cl  Ionic Compound? – put each atom into brackets and write the charge on the ions.

Valence Shell Electron Pair Repulsion Theory VSEPR Valence Shell Electron Pair Repulsion Theory Predicts 3D structures of molecules Minimizes the electrostatic repulsion between electrons in valence sublevels Shape calculated by comparing bonding sites and lone pair electrons around the central atom

A line bond counts as 1 pair of electrons SeCl2 : 6 +7 + 7 = 20 - 4 = 16  Cl – Se – Cl VSEPR shape is determined by counting bonding pairs and lone pairs around the CENTRAL atom only. A line bond counts as 1 pair of electrons A lone pair is a pair of non-bonding electrons

You are given this table – count bonding pairs and lone pairs and read the shape

This is just a different way of representing the same information AXE method: A: Central atom Xn: # of bonding sites En: # lone pairs This is just a different way of representing the same information

(Does not need a full octet) Central Atom with only bonding pairs: Draw the Lewis structure for BF3: BF3 : 3 + (3)7 = 24 - 6 = 18   *Exception (Does not need a full octet) – F F – B F –  3 – bonding sites 0 – lone pairs Trigonal Planar

Draw the Lewis structure for CH4: 4 – bonding sites (X4) 0 – lone pairs Tetrahedral Draw the Lewis structure for PCl5: 5 – bonding sites (X5) 0 – lone pairs Trigonal Bipyramidal Draw the Lewis structure for SF6: 6 – bonding sites (X6) 0 – lone pairs Octahedral

Its drawn linear but the lone electrons Central Atom with LONE pairs: Lone pairs have a greater repulsion force and distort the predicted bond angles. H2S : 1 + 1 + 6 = 8 - 4 – H H –  S 2 – bonding sites 2 – lone pairs Bent Its drawn linear but the lone electrons distort the shape

Notice again the distorted shape from the lone pair of electrons Draw the Lewis structure for PF3: PF3 : 5 + (3)7 = 26 - 6    – F F – P F – 3 – bonding sites 1 – lone pairs  Trigonal Pyramidal Notice again the distorted shape from the lone pair of electrons

Double and triple bonds still counted as ONE bonding site - 4   O – – C – O – Double Bonds   2 – bonding sites 0 – lone pairs Linear Double and triple bonds still counted as ONE bonding site

CAN YOU / HAVE YOU? C12-2-07 PERIODIC TRENDS Vocabulary & Concepts Identify factors and account for periodic trends among the properties of elements, and relate to electron configurations. Include: atomic / ionic radii, ionization energy, and electronegativity. Construct Lewis dot diagrams for various compounds and use VSEPR to predict bond shape from the electron arrangement. Vocabulary & Concepts  Line Bond