2.7: Chemical Symbols and Formulas

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1 Science 9 2.7: Chemical Symbols and Formulas Chemical Symbols On the periodic table all the elements are represented by different symbols letters which are an abbreviation of the element s name. These same symbols are used in every country in the world to represent the different elements. Sometimes the letters may be similar to our spelling of the word but most of the symbols come from the element s Latin names. Examples: Mercury Hg comes from the Latin word hydrargyrum which means liquid silver. Sodium Na comes from the Latin word natrium. If a chemical s symbol has only one letter it is always capitalized. If the symbol has two letters the first one is capitalized and the second one is not. Chemical Formulas Combinations of chemical symbols are used to represent compounds. This combination of symbols is called a chemical formula. The chemical formula indicates which elements are in a particular compound and also their proportion or ratio. A chemical formula is like a recipe. If there is only one atom of an element in the compound no number is included, if there is more than one atom then the number of atoms is written in subscript after the symbol for that particular atom. Examples: - H 2 O has 2 atoms of hydrogen and one atom of oxygen - NaHCO 3 has one atom each of sodium, hydrogen and carbon and 3 atoms of oxygen.

2 Science : Names and Formulas for Compounds How do you determine how many atoms of each type of element will combine to form a molecule? For example why is water H 2 O instead of HO or H 3 O? The key is knowing how atoms combine and this has to do with their combining capacity. Combining capacity the ability to combine with other elements shows how many connections an atom can make. For example, you have two arms so you can connect with 2 other people. If you only had one arm, you could only connect with one other person. In order for a compound to be stable, all the atoms in it must have made all the possible connections. Also, if an element has a positive charge (mostly metals) it must connect with an element which has a negative charge (mostly nonmetals) to be stable. Sometimes more than one positive may band together to connect with a negative or visa versa. On the periodic table the elements in columns 1, 2, and 13 are considered positive. (the metals in the middle have varying combining capacities and we will talk about them later). Column 1 has a combining capacity of +1. Column 2 has a combining capacity of +2. Column 13 has a combining capacity of +3. Column 14 has a combining capacity of + or 4. The elements in columns 15,16 and 17 have negative combining capacities. Column 17 has a combining capacity of -1. Column 16 has a combining capacity of -2. Column 15 has a combining capacity of -3. Column 18 is the noble gases and they do not combine so have a capacity of 0. Give the combining capacity of the following elements. Li N C O Mg H Ne Cl B Metals with More Than One Combining Capacity The metals in the middle of the periodic table do not follow the same rules as the outside columns. Some of their combining capacities can vary depending on the compound that they are forming. Lead, copper, iron, and tin all have 2 different combining capacities. When you name compounds containing these elements, you must state which type is in the compound by using a Roman numeral after the element s name. Example: FeO - iron (II) oxide or Fe 2 O 3 - iron (III) oxide

3 Science 9 Naming Compounds When naming a compound the first element in the compound (the metal) maintains the same name (as seen on the periodic table); the second element (the nonmetal) has it s ending replaced with ide. (See Table 3 in your notes) If the compound is made of two nonmetals, which both have negative charges, you add a prefix to the second element as follows: 2 = di- 3 = tri- 4 = tetra- Eg. Carbon dioxide CO 2 Name the following compounds: LiBr K 2 S Mg 3 P 2 CaCl 2 CaO N 2 S 3 SO 3

4 Science 9 Hook-and-Ball Diagrams One way of representing elements bonding together is with hook-and-ball diagrams. Example: calcium chloride Write the formula for each of the following compounds. Draw hook-and-ball diagrams. hydrogen chloride sodium bromide magnesium chloride calcium oxide

5 potassium iodide water sodium oxide aluminum oxide zinc chloride silver sulfide

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