Chemical Symbols, Formulas & Compounds 1) Symbols 2) Element names 3) Counting Atoms 4) Naming compounds 5) Combining capacity 6) Criss-cross rule 7) Naming compounds 8) Periods

Symbols From your work with the periodic table you know that each element is represented by a single symbol made up of 1, 2 or 3 letters (e.g. Oxygen = O, Sodium = Na, etc.) This is the case all around the world!

Element Names Many element names are Latin, e.g. Silver = Ag because the Latin name for Silver is Argentium Just as single symbols are used to represent single elements, these symbols can also be combined to form compounds

Counting Atoms in Compounds There are over 117 known elements. These elements can be combined in many different ways to make compounds. E.g. Carbon Dioxide  CO 2  Composed of C and O We use subscripts in a chemical formula to show how many atoms are in a compound.

Example: In this compound, there are two hydrogen atoms and one oxygen atom. H2OH2O subscript

Counting Atoms Each symbol represents an element e.g. NaCl  Na = Sodium, Cl = Chlorine If only one atom of an element is present there is no subscript e.g. H 2 O  only 1 O The number in front of the compound is called the coefficient and it tells us how many of that compound there is  E.g. 2H 2 O = H 2 O + H 2 O

Example con’t 2H 2 O  Normally we would say that there are: 2 Hydrogen's and 1 Oxygen. However, since there is a 2 in front, the correct answer is: 2 Hydrogen's and 1 Oxygen + 2 Hydrogen's and 1 Oxygen 2 x 2 Hydrogen’s = 4 Hydrogen’s 2 x 1 Oxygen = 2 Oxygen’s There are 6 atoms total (4 H + 2 O)

Learning Check: Counting Atoms - Worksheet

End of Day 1

Naming Compounds

Combining Capacity We need to know how atoms combine with each other in order to be able to write a chemical formula or draw a diagram Atoms can make a specific number of connections with other atoms The number of connections that an atom can make is called its combining capacity When atoms combine, it is called a bond

Example1: Building a Molecule The combining capacity of Hydrogen is 1.  This means that a Hydrogen atom can make only one bond The combining capacity of Oxygen is 2.  This means that oxygen can make two bonds. We simply switch the numbers and write them as subscripts on the other element (note that 1’s are not included) H2OH2O

Combining Capacity O H H one bond two bonds

All of the atoms involved in making the molecule are “happy” because they are making the correct number of connections. A diagram like the one above is called a structural diagram. O H H

Example 2: Building a Molecule The combining capacity of Calcium is 2.  This means that Calcium can make two bonds. The combining capacity of Chlorine is 1.  This means that Chlorine can make one bond. Ca = 2Cl = 1  Ca 1 Cl 2 What is wrong?

Ca 1 Cl 2  What is wrong with this? We NEVER write the 1’s Therefore,  CaCl 2 is the correct answer You will be given the combining capacities!!

More Examples Silver and Bromine - Ag = 1, Br = 1  AgBr Calcium and Iodine - Ca = 2, I = 1  CaI 2

**You must always bring the subscripts down to lowest possible form** E.g. Mg 2 O 2  divide both subscripts by the greatest number you can (2) = Mg 1 O 1 = MgO Be 2 Si 4  divide both subscripts by 2 = Be 1 Si 2 = BeSi 2 Mg 2 S 2  Mg 1 S 1 = MgS

Common Combining Capacities These will be provided for you on your test

The Criss-Cross Rule – How to do this easily

Example: Aluminum Chloride Step 1: Step 2: Step 3: 13 Step 4: AlCl 3 Criss-Cross Rule Al Cl 3 1 write out name with space write symbols & combining capacity above criss-cross combining capacity as subscripts combine as formula unit (“1” is never shown) Aluminum Chloride

Example: Aluminum Oxide Step 1: Aluminum Oxide Step 2: Al 3 O 2 Step 3: Al O 23 Step 4: Al 2 O 3 Criss-Cross Rule

Example: Magnesium Oxide Step 1: Magnesium Oxide Step 2: Mg 2 O 2 Step 3: Mg O 22 Step 4: Mg 2 O 2 Step 5: MgO Criss-Cross Rule

Naming Compounds Rules 1) Metals combine with nonmetals in many compounds -Metals will be to the left of non-metals on the periodic table -If in doubt, the one to the left goes first 2) Write the name of the metal first then the nonmetal 3) Change the ending of the nonmetal to “ide”

Naming Compound Concluded E.g. Aluminum (metal) and Oxygen (nonmetal)  Aluminum Oxide E.g. Chlorine (metal or nonmetal) and Magnesium (metal or nonmetal)  Magnesium is the metal therefore,  Magnesium Chloride

Naming Simple Compounds FormulaName 1BaO 2NaBr 3MgI 2 4SrF 2 5KCl 6CsF Magnesium Iodide Cesium Fluoride Potassium Chloride Strontium Fluoride Barium oxide Sodium Bromide