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Writing Formulas for Ionic Compounds

Writing Formulas for Ionic Compounds. A Few Points about Ionic Compounds. Ionic Compounds:are made of ions ( charged atoms). Every ionic compound has 1 cation (+) & 1 anion (-). In formulas, the cation is ALWAYS written 1st and the anion is 2nd. GOAL: balance out the charges

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Writing Formulas for Ionic Compounds

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  1. Writing Formulas for Ionic Compounds

  2. A Few Points about Ionic Compounds • Ionic Compounds:are made of ions (charged atoms). • Every ionic compound has • 1 cation (+) & 1 anion (-). • In formulas, the cation is ALWAYS written 1st and the anion is 2nd.

  3. GOAL:balance out the charges • FINAL PRODUCT:overall charge = 0 • Though ions have charges, ionic compounds do not.

  4. Na+ (sodium ion) = charge of “+1” • (it “wants” to or has given away 1 electron or lost one e-.) • Cl- (chloride ion) = charge of “-1” • “wants to” or has stolen 1 electron or gains

  5. NaCl = Sodium Chloride • ionic compound (metal + nonmetal, opposite sides of the chart) • Formed from the Na+ (sodium ion) and the Cl- (chloride ion), so the formula NaCl (no charges anymore because they balanced out.)

  6. What would the formula be for the compound formed between a potassium ion and a bromide ion?

  7. K+ (potassium ion) • Br- (bromide ion) • K+ gives up 1 electron to Br- which steals that 1 electron • +1 – 1 = 0 • KBr

  8. Now, what do you think will happenwhen the magnesium ion (Mg2+) combines with the sulfide ion (S2- ) ?

  9. MgS • Mg gives up2 electrons to S which takes two. • So the Mg2+ and the S2- ions’ charges cancel out, giving the desired 0 (or neutral) charge.

  10. Now, what do you think will happen when thealuminum ion (Al3+) combines with thenitride ion (N3- ) ?

  11. AlN • The +3 of Al3+ and the –3 of N3-cancel out and the neutral compound formed is AlN (aluminum nitride).

  12. But, what do you think will happen when the aluminum ion(Al3+) combines with the bromide ion (Br- ) ? They do not have equal charges…

  13. Aluminum ion contributes a +3 charge and the bromide ion has a –1 charge. • In order to get a neutral charge, you need to have three bromide ions to give an overall –3 charge. • +3 (1 ion) = +3 -1 (3 ions) = -3 • + 3 – 3 = 0

  14. To show 3 bromide ions, we use a subscript after the Br. • Thus, the formula for the ionic compound formed between Al3+ ion and Br- ion is AlBr3

  15. Compounds with Polyatomic Ions • What do you think would happen if I combine a polyatomic ion, such as ammonium (NH4+) with the chloride ion (Cl-)?

  16. NH4Cl • The polyatomic ions “work” in just the same way as the monatomic ions do when it comes to contributing a charge towards forming a compound.

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