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Writing Chemical Formulas

Writing Chemical Formulas. SNC2P. Warm Up – Putting it together. What is the definition of a compound? Draw a Bohr-Rutherford diagram of a Hydrogen and Oxygen atom How do you think the compound H 2 O is formed? . Learning Goals. By the end of class, I will be able to:

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Writing Chemical Formulas

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  1. Writing Chemical Formulas SNC2P

  2. Warm Up – Putting it together • What is the definition of a compound? • Draw a Bohr-Rutherford diagram of a Hydrogen and Oxygen atom • How do you think the compound H2O is formed?

  3. Learning Goals • By the end of class, I will be able to: • Understand how ionic compounds are formed • Write the chemical formula for ionic compounds

  4. Ionic Compounds • An ionic compound is formed from one or more positively charged ions and one or more negatively charged ions • Eg: Sodium + Chlorine = Sodium Chloride (NaCl)- Table Salt!

  5. Chemical Formulas • A chemical formula is a combination of symbols and subscripts that represent the ratio of elements in a compound. • To indicate which atoms or ions are in a compound, we use the symbols for the elements found on the periodic table • The subscripts are used to show how many atoms or ions of each element combine to form the compound • Example: CaF2

  6. Example: CaF2 Element Symbols CaF2 If only one atom of an element is used, we do not include a subscript Subscript

  7. Writing Formulas for Ionic Compounds • The metal ion is always written first. It has a positive ion charge • The non-metal ion is written second. It has a negative ion charge • The total positive charge and the total negative charge must be equal in order for the overall charge of the compound to be balanced (or 0).

  8. Crisscross Rule for Writing Formulas Step 1: Write the symbols of each element (remember to write the symbol for the metal first) Step 2: Write the ion charge above each corresponding symbol. Remember that the ion charge is based on the number of electrons gained (negative) OR lost (positive) to become an ion. Step 3: Crisscross the ion charges, erase the signs of each ion charge, and write them as subscripts

  9. Examples of Writing Formulas • Write the correct formula for a compound of aluminum and sulphur • (a) 3+ 2- Al S = Al2S3

  10. Examples of Writing Formulas • Write the correct formula for a compound of barium and oxygen • (b) 2+ 2- Ba O = Ba2O2 Note: If you need the same number of each ion, you can reduce the formula to one of each ion, as shown here. = BaO

  11. Questions to Try Yourself • Write the chemical formula for the compound formed by each of the following combination of elements • (a) Lithium and iodine • (b) Calcium and bromine • (c) Aluminum and chlorine

  12. Rules for Naming Chemical Formulas • Eg: LiCl • Name the metal first, using the name that appears on the periodic table • Eg: Lithium Name the non-metal second. The ending of its name gets changed to “ide”. Eg: Lithium Chloride

  13. Practice Naming Ionic Compounds • MgS • NaCl • Al2O3 • CaS • Zn3P2 • CaI2 • KF

  14. Practice • Write the formulas for these ionic compounds: • Potassium iodide • Magnesium phosphide • Silver sulphide • Ionic Compounds Worksheet

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