Lewis Dot Structures Dots are arranged around an element’s symbol to indicate the valence electrons Reminder: valence shell only has s & p orbitals  maximum of 8 e − s Place the electrons around the four sides of the symbol, in pairs.

Lewis Dot Structures - Atoms example for Cl Cl 2 “s” electrons 5 “p” electrons

Lewis Dot Structures - Atoms example for B B 2 “s” electrons 1 “p” electron

Lewis Dot Structures - Compounds Octet Rule (only for each atom in a compound) –Each atom in a compound must see eight electrons. –These electrons will be on the outside of the atom OR in the bond to another atom (shared)

Lewis Dot Structures - Compounds Compound Structures –must have ONE central atom – must look like spokes on a wheel –NO rings (unless told to do so) S O O O O O O O S

Lewis Dot Structures - Compounds Bonds are a pair of electrons: We represent the pair of electrons in a bond with a short line segment:

Lewis Dot Structures - Compounds For compounds, count the total number of valence electrons for the atoms in the compound –ex: PCl 3 1 P  5 valence electrons=5 3 Cl  3(7 valence e − s)=21 Total = 26 electrons to share These electrons DO NOT belong to each atom anymore. They are shared to make the compound

LDS – Compounds (cont.) Place either the single atom of an element in compound at the center OR the least electronegative element (if not one single) For PCl 3 : P Cl

LDS – Compounds (cont.) Connect each terminal (outside) atom to the central (center) atom with a single bond (shared pair of electrons) P Cl

LDS – Compounds (cont.) Subtract number of electrons used in bonds from total and distribute around the atoms, in pairs make sure the terminal atoms have 8 electrons, including the 2 atoms in the bond

LDS – Compounds (cont.) (26 total e − s) – (6 bonding e − s) = 20 e − s 20 e − s go around the atoms, terminal first P Cl

LDS – Compounds (cont.) Count to make sure you have: –8 e − s around each atom including all e − s in the bonds to that atom (shared e − s) –each atom has a bond (pair of e − s) to another atom –there are no single e − s anywhere in the structure If all checks have passed, you have a proper lewis dot structure!

Lewis Dot Structures - Ions Count the number of electrons in the original valence shell AFTER adding or taking e − s to make the ion. Place them around the symbol (or not, if there are none) Then, place square brackets ([ ]) around the LDS for the ion, with the charge outside the brackets

Lewis Dot Structures - Ions Ex: Cl 1− Cl Neutral chlorine has 7 valence electrons Cl Chloride has 8 valence electrons the extra electron added to make charge of 1− ] [ 1−

Lewis Dot Structures - Ions Ex: Mg 2+ Mg Neutral magnesium has 2 valence electrons Mg The magnesium ion has no valence electrons in its original valence shell 2 electrons removed to make charge of 2+ ] [ 2+

LDS – Polyatomic Ions Count valence e − s form atoms Add electrons that make charge construct the LDS the same way as for compounds, using the extra e − s for the charge Place brackets around entire compound and place charge outside

LDS – Polyatomic Ions Example: SO 4 2− S O O O O 2− S: 6 electrons O: 46 = 24 electrons Neutral atom valence electrons = 30 2 more electrons for the 2− charge = 32 total electrons Note: sulfate cannot be made without adding 2 electrons to its structure. Try it!

LDS Exceptions Hydrogen and Helium –Their valence shell will only hold 2 e ─ s –So, they may only have 2 electrons around them in a LDS for a compound –This mean all of their electrons will be shared in the bond

LDS Exceptions Multiple bonds –You can ONLY have single bonds, double bonds, or triple bonds in a compound Sometimes, after distributing the electrons around the compound’s structure, not all of the atoms see 8 electrons To fix this: take a pair of e ─ s from the TERMINAL atom and place them in the bond – now the terminal atom and the central atom see both e ─ s

Multiple Bonds –do not overbond – this means do not put more multiple bonds than is necessary in an atom Ex SO 2 : First distribution of e ─ s Fixed structure: All atoms, through sharing, see 8 e ─ s LDS Exceptions S O O S O O