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Writing Formulas for Ionic Compounds

Writing Formulas for Ionic Compounds. A Few Points about Ionic Cds. Ionic Compounds, as you know, are composed of ions (charged atoms). Every ionic compound has 1 type of cation (+) and 1 type of anion (-). In formulas, the cation is ALWAYS written first and the anion is second.

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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 Cds • Ionic Compounds, as you know, are composed of ions (charged atoms). • Every ionic compound has 1 type of cation (+) and 1 type of anion (-). • In formulas, the cation is ALWAYS written first and the anion is second.

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

  4. Na+ (sodium ion) has a charge of “+1” meaning it “wants” to or has given away 1 electron • Cl- (chloride ion) has a charge of “-1” meaning it “wants to” or has stolen 1 electron

  5. NaCl • Well-known ionic compound (metal + nonmetal, opposite sides of the chart) is NaCl, sodium chloride. • It is formed from the Na+ (sodium ion) and the Cl- (chloride ion), hence 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 happen when the magnesium ion combines with the sulfide ion?

  9. MgS • If you said, it would form MgS, you’re right! Mg gives up 2 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 the aluminum ion combines with the nitride ion?

  11. AlN • Good Job! You’re getting the hang of this now! • 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. The 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.

  17. More than 1 polyatomic ion • What happens when you are writing a formula with polyatomic ions and you need more than one of them to balance out the charges? • For example for the compound that combines Ba+2 and NO3- we would need one barium ion and two nitrate ions to balance out to a zero overall charge

  18. You can’t just add subscripts like we did with the monatomic ions or it would get too confusing. Ex: Ba+2 and NO3- would then be BaNO32, which looks like 32 O’s!

  19. You can’t add coefficients (or big numbers) in the middle of a formula Ex: Ba+2 and NO3- would then be Ba2NO3, which isn’t right, either!

  20. You can’t distribute the subscripts or you would totally change up the formula of the polyatomic ion and then not be able to recognize it anymore! Ex: Ba+2 and NO3- would then be BaN2O6, which is wrong because you can’t tell that it is nitrate now!

  21. Use parentheses! • When you have more than one polyatomic ion, you use parentheses to show it is a set of which you have more than one Ex: Ba+2 and NO3- when combined becomes Ba(NO3)2

  22. Be selective when using parentheses… • Only use parentheses with polyatomic ions; it is not necessary with monatomic ions (of single elements) • And, only use parentheses when there is more than one polyatomic ion needed Ex: incorrect (Na)2SO4 incorrect Na2(SO4) correct Na2SO4

  23. Naming Ionic Compounds • Name cation; drop the word “ion.” • Remember that transition metals and Pb and Sn need Roman numerals to match their charges (Ag, Zn, and Cd do not need Roman numerals); work backwards to determine the charge • Name anion.

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