PolyatomicIonsWritingFormulas

Polyatomic Ions: 

Polyatomic Ions What do they “look” like? What about their Lewis structure? How do we write chemical formulas with them?

What is a polyatomic ion?: 

What is a polyatomic ion? It is a covalently bonded particle It's just like a covalently bonded molecule Except it has an electric charge It has either gained or lost electrons to get that charge Example: hydroxide ion ( OH - ) -

What is a polyatomic ion?: 

What is a polyatomic ion? Polyatomic ions are held together by strong covalent bonds Except in oxidation-reduction reactions, when electrons are gained and lost, they stick together in chemical reactions Example: hydroxide ion ( OH - ) -

What is a polyatomic ion?: 

What is a polyatomic ion? Sticking together—example When Barium nitrate solution is added to sodium hydroxide, a precipitate forms The precipitate is Barium hydroxide The hydroxide does not come apart - - Ba 2+ Before the reaction the hydroxide is in solution, surrounded by water molecules

What is a polyatomic ion?: 

What is a polyatomic ion? Sticking together—example When Barium nitrate solution is added to sodium hydroxide, a precipitate forms The precipitate is Barium hydroxide The hydroxide does not come apart - - Ba 2+ After the reaction the hydroxide, still bonded oxygen to hydrogen, is associated with the Barium ion as a solid Compounds with hydroxide are BASES

Lewis Structure—polyatomic ion: 

Lewis Structure—polyatomic ion Count the total electrons Write the skeleton structure Covalently bond central atom to surrounding atoms Complete shells of surrounding atoms after adding or subtracting the electron charge Complete shell of central atom to make octet—however you can

Lewis Structure—hydroxide ion: 

Lewis Structure—hydroxide ion Total electrons: O=6, H=1, plus one e - =8 Skeleton: O H Covalently bond central atom to surrounding atoms O : H Complete H shell O : H (needs 2) Complete central atom Eight electrons!

Lewis Structure: oxypolyanion: 

Lewis Structure: oxypolyanion What is the sulfate ion Lewis structure? SO 4 2- Total valence electrons? Oxygen has 6 (6 x 4 =24) Sulfur has 6, like oxygen (6 x 1 = 6) And there's two from the -2 charge (2) 24 + 6 + 2 = 32

Lewis Structure: oxypolyanion: 

Lewis Structure: oxypolyanion What is the sulfate ion Lewis structure? SO 4 2- (32 electrons) Set up skeleton structure

Lewis Structure: oxypolyanion: 

Lewis Structure: oxypolyanion What is the sulfate ion Lewis structure? SO 4 2- (32 electrons) Put complete shells around outside atoms We have no electrons left, but the sulfur also has a complete octet. We have done it!

Writing formulas with polyatomic ions: 

Writing formulas with polyatomic ions We'll do it just as with the binary compounds Determine what ions are in the compound From the PT and ion chart, determine the charge on the ion Set up metal first, then anion, with charges Apply the cris-cross rule Reduce to lowest whole # ratio

Writing formulas with polyatomic ions: 

Writing formulas with polyatomic ions Our example compound will be sodium sulfate Ions: Na ion and sulfate Charts: Na + and SO 4 2- Skeleton formula Na + SO 4 2- Cris-cross: This one already at whole # ratio 2:1 Na 2 SO 4

Practice: 

Practice Write the formula for Aluminum nitrate Determine what ions are in the compound From the PT and ion chart, determine the charge on the ions Set up metal first, then anion, with charges Apply the cris-cross rule Reduce to lowest whole # ratio

Practice: 

Practice Write the formula for Aluminum nitrate Ions: aluminum and nitrate PT/chart: Al 3+ and NO 3 - Al 3+ NO 3 - Cris-cross: Reduce & add ( ) Al(NO 3 ) 3