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Chemical Formula Writing Guide

Learn how to write formulas for compounds, including molecular compounds, acids, and common ions using prefixes and charges. Understand the rules for naming and balancing charges in chemical formulas.

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Chemical Formula Writing Guide

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  1. Writing Formulas • The charges have to add up to zero. • Get charges on pieces. • Cations from name on table. • Anions from table or polyatomic. • Balance the charges by adding subscripts. • Put polyatomics in parenthesis.

  2. Writing Formulas • Write the formula for calcium chloride. • Calcium is Ca2+ • Chloride is Cl1- • Ca2+ Cl1- would have a 1+ charge. • Need another Cl1- • Ca2+ Cl21- (use criss-cross method)

  3. Write the formulas for these • Lithium sulfide • tin (II) oxide • tin (IV) oxide • Magnesium fluoride • Copper (II) sulfate • Iron (III) phosphide • gallium nitrate • Iron (III) sulfide

  4. Write the formulas for these • Ammonium chloride • ammonium sulfide • barium nitrate

  5. Things to look for • If cations have ( ), the number in parenthesis is their charge. • If anions end in -ide they are probably off the periodic table (Monoatomic) • If anion ends in -ate or -ite it is polyatomic

  6. Section 6.5Molecular Compounds and Acids • OBJECTIVES: • Apply the rules for naming and writing formulas for binary molecular compounds.

  7. Section 6.5Molecular Compounds and Acids • OBJECTIVES: • Name and write formulas for common acids.

  8. Molecular compounds • made of just nonmetals • smallest piece is a molecule • can’t be held together because of opposite charges. • can’t use charges to figure out how many of each atom

  9. Molecular are easier! • Ionic compounds use charges to determine how many of each. • Have to figure out charges. • Have to figure out numbers. • Molecular compounds name tellsyou the number of atoms. • Uses prefixes to tell you the number

  10. Prefixes (Table 6.5, p.159) • 1 = mono- • 2 = di- • 3 = tri- • 4 = tetra- • 5 = penta- • 6 = hexa- • 7 = hepta- • 8 = octa-

  11. Prefixes • 9 = nona- • 10 = deca- • To write the name, write two words:

  12. Prefixes • 9 = nona- • 10 = deca- • To write the name, write two words: Prefix name Prefix name -ide

  13. Prefixes • 9 = nona- • 10 = deca- • To write the name, write two words: • One exception is we don’t write mono- if there is only one of the first element. Prefix name Prefix name -ide

  14. Prefixes • 9 = nona- • 10 = deca- • To write the name, write two words: • One exception is we don’t write mono- if there is only one of the first element. • No double vowels when writing names (oa oo) Prefix name Prefix name -ide

  15. Name These • N2O • NO2 • Cl2O7 • CBr4 • CO2 • BaCl2

  16. Write formulas for these • diphosphorus pentoxide • tetraiodine nonoxide • sulfur hexafluoride • nitrogen trioxide • carbon tetrahydride • phosphorus trifluoride • aluminum chloride

  17. Acids Writing names and Formulas

  18. Acids • Compounds that give off hydrogen ions when dissolved in water. • Must have H in them. • will always be some H next to an anion. • The anion determines the name.

  19. Naming acids • If the anion attached to hydrogen ends in -ide, put the prefix hydro- and change -ide to -ic acid • HCl - hydrogen ion and chloride ion • hydrochloric acid • H2S hydrogen ion and sulfide ion • hydrosulfuric acid

  20. Naming Acids • If the anion has oxygen in it, then it ends in -ate of -ite • change the suffix -ate to -ic acid (use no prefix) • HNO3 Hydrogen and nitrate ions • Nitric acid • change the suffix -ite to -ous acid • HNO2 Hydrogen and nitrite ions • Nitrous acid

  21. Name these • HF • H3P • H2SO4 • H2SO3 • HCN • H2CrO4

  22. Writing Acid Formulas • Hydrogen will always be first • name will tell you the anion • make the charges cancel out. • Starts with hydro?- no oxygen, -ide • no hydro?, -ate comes from -ic, -ite comes from -ous

  23. Write formulas for these • hydroiodic acid • acetic acid • carbonic acid • phosphorous acid • hydrobromic acid

  24. Section 6.6Summary of Naming and Formula Writing • OBJECTIVES: • Use the flowchart in Figure 6.21 to write the name of a compound when given its chemical formula.

  25. Section 6.6Summary of Naming and Formula Writing • OBJECTIVES: • Use the flowchart in Figure 6.23 to write a chemical formula when given the name of a compound.

  26. Helpful to remember... 1. In an ionic compound, the net ionic charge is zero (criss-cross method) 2. An -ide ending generally indicates a binary compound 3. An -ite or -ate ending means there is a polyatomic ion that has oxygen 4. Prefixes generally mean molecular; they show the number of each atom

  27. Helpful to remember... 5. A Roman numeral after the name of a cation shows the ionic charge of the cation • Use the handout sheets provided by your teacher!

  28. Summary of Naming and Formula Writing • For naming, follow the flowchart- Fig. 6.21, page 161 • For writing formulas, follow the flowchart from Fig. 6.23, p. 162

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