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Ionic Compounds: Writing Formulas

Ionic Compounds: Writing Formulas. Empirical Formulas. formulas with smallest whole-number ratio of elements in compound ionic compounds are only written as empirical formulas. Formula Unit. simplest ratio of ions in ionic compound examples: KBr is a formula unit

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Ionic Compounds: Writing Formulas

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  1. Ionic Compounds:Writing Formulas

  2. Empirical Formulas formulas with smallest whole-number ratio of elements in compound ionic compounds are only written as empirical formulas

  3. Formula Unit simplest ratio of ions in ionic compound examples: • KBr is a formula unit • potassium ions & bromide ions are in a 1-to-1 ratio • AlCl3 is a formula unit • aluminum & chloride ions in 1-to-3 ratio

  4. Na+1 Ca+2 O-2 Al+3 Ag+1 Fe+2 Fe+3 Oxidation Number or State • charge of monatomic ion (right superscript) • some elements form only 1 ion • some elements can form more than 1 ion

  5. Writing Formulas for Binary Ionic Compounds • Write positive ion first • Equal but opposite charges cancel out (no subscripts) • Unlike charges use criss-cross method to determine subscripts

  6. reminder: compounds are electrically neutral therefore the sum of all charges in the compound MUST be 0 (+) and (-) charges must = each other

  7. Equal but Opposite Charges • Na+1 and Cl-1: NaCl +1 + (-1) = 0 • Mg+2 and O-2: MgO +2 + (-2) = 0 • Al+3 and P-3: AlP +3 + (-3) = 0 • Formula: write symbols, (+) FIRST!

  8. try a few formulas: • Li+1 and I-1 • Ca+2 and O-2 • Al+3 and N-3 • K+1 and F-1 • Ba+2 and S-2 LiI CaO AlN KF BaS

  9. Criss-Cross Method charges opposite but NOT equal: • Mg+2 and Cl-1, CROSS and DROP! (# only, forget signs!) • Mg1Cl2 (if subscript is 1, don’t write it!) MgCl2

  10. Check the Math • MgCl2 means 1 Mg+2 and 2 Cl-1 (1)(+2) + (2)(-1) = +2 + -2 = 0 • charges MUST add up to zero!

  11. try these: CaCl2 • Ca+2 + Cl-1 • Na+1 + O-2 • Cs+1 + S-2 • Al+3 + Cl-1 • Al+3 + Se-2 • Mg+2 + F-1 • K+1 + N-3 Na2O Cs2S AlCl3 Al2Se3 MgF2 K3N

  12. of course, it gets more difficult KF • potassium (K) and fluorine (F) • zinc (Zn) and iodine (I) • sodium (Na) and oxygen (O) • magnesium (Mg) and oxygen (O) • aluminum (Al) and oxygen (O) • calcium (Ca) and bromine (Br) • cesium (Cs) and iodine (I) • silver (Ag) and sulfur (S) ZnI2 Na2O MgO Al2O3 CaBr2 CsI Ag2S

  13. and more challenging AuF • gold +1 and fluorine • gold +3 and fluorine • titanium +2 and oxygen • titanium +3 and oxygen • titanium +4 and oxygen • manganese +4 and bromine • manganese +7 and bromine AuF3 TiO Ti2O3 Ti2O4  Ti O2 MnBr4 MnBr7

  14. SO42- CO32- PO43- OH1- POLYATOMIC IONS • Table E: groups of covalently bonded atoms that have a charge (+) or (–) • polyatomic ions have “names” • (-) polyatomic ions can form ionic bonds with (+) metal ions

  15. Ternary Compounds • contain 3 or more elements • usually contain a polyatomic ion: • ifpolyatomic ion is (+) it’s bonded toa (-) non-metal ion or a (-) polyatomic ion • ifpolyatomic ion is (–) it’s bonded to a (+) metal ion or a (+) polyatomic ion

  16. Formulas with Polyatomics • What’s the formula for the compound formed from Al+3 and SCN-1? • charges must add up to zero • write symbols, positive first! • figure out how many of each ion needed Al(SCN)3 **You must use parentheses for more than 1 polyatomic ion**

  17. easy ones with equal but opposite charges: • Na+1 and OH-1 • K +1 and HCO3-1 • Mg+2 and CO3-2 • Li +1 and NO3-1 • Ca+2 and SO4-2 NaOH KHCO3 MgCO3 LiNO3 CaSO4

  18. these are more challenging: Mg3(PO4)2 • Mg+2 and (PO4)-3 • Al+3 and (NO3)-1 • Fe+2 and OH-1 • Hg2+2 and SCN-1 • Mg+2 and HCO3-1 • Al+3 and C2O4-2 Al(NO3)3 Fe(OH)2 Hg2(SCN)2 Mg(HCO3)2 Al2(C2O4)3

  19. more challenging Zn(NO3)2 • zinc + nitrate ion • magnesium + hydroxide ion • lithium + carbonate ion • potassium + sulfate ion • calcium + phosphate ion • beryllium + chlorate ion Mg(OH)2 Li2CO3 K2SO4 Ca3(PO4)2 Be (ClO3)2

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