13B Chemical Formulas

Transcription

1 13B Chemical Formulas Why do atoms combine in certain ratios? Chemists have long noticed that groups of elements behave similarly. The periodic table is an arrangement of the elements grouped according to similar behavior. In this investigation, you will discover how the arrangement of electrons in atoms is related to groups on the periodic table. You will also learn why atoms form chemical bonds with other atoms in certain ratios. Materials Periodic Table Tiles Periodic table with oxidation numbers Special Bonds card A Oxidation numbers and ions An element s oxidation number indicates how many electrons are lost or gained when chemical bonding occurs. The oxidation number is equal to the charge an atom has when it ionizes, that is, gains or loses electrons to become an ion. The partial periodic table below shows the most common oxidation numbers of the elements. The oxidation numbers are written above the group number above each column on the table. The most common oxidation numbers for the main group elements are shown. 1

2 Investigation 13B Chemical Formulas B Stop and think a. Describe the groups on the periodic table according to their valence electrons. b. Why do elements in group 2 have an oxidation number of 2+? c. Why do elements in group 17 have an oxidation number of 1? d. Why do the oxidation numbers in the first two groups tend to be positive? C Predicting chemical formulas A binary compound is composed of two different elements. Predict the chemical formulas for the binary compounds that are made up of the pairs of elements in the table below. Use the following steps: 1. Using the periodic table on the previous page, determine the ion formed by each element. 2. Figure out how many periodic table tiles of each element will be needed to make the compound electrically neutral. 2

3 3. Form the compound with your tiles and write the chemical formula for each compound based on the number of tiles of each element. Table 1: Writing chemical formulas for binary compounds Element 1 Element 2 Oxidation number 1 Oxidation number 2 Number of tiles of element 1 Number of tiles of element 2 Chemical formula hydrogen magnesium calcium aluminum potassium lithium rubidium fluorine sulfur bromine oxygen chlorine argon sulfur D Naming compounds Naming binary ionic compounds is very simple if you follow these rules: 1. Write the name of the element with a positive oxidation number first. 2. Write the root name of the element with a negative oxidation number second. For example, chlor- is the root name of chlorine. Subtract the -ine ending. 3. Add the ending -ide to the root name. Chlor- becomes chloride. Using these rules, write the name of each of the compounds in Table 1. 3

4 Investigation 13B Chemical Formulas E Playing Compound Crossword Now that you understand how elements combine to form compounds, you are ready to play Compound Crossword. In this game, you will score points by forming stable compounds, crossword style. Players use the oxidation numbers of the elements to form correct compounds. Points are determined by adding up the atomic numbers of each atom in the compound. Play Sample game after four turns: CH 3 OH scores 18 points Fe 2 O 3 scores 76 points SiO 2 scores 30 points H 2 SO 4 scores 50 points Starting the game Each player starts with ten randomly selected tiles. The remaining tiles should be placed in a box or paper bag so that additional tiles may be drawn without being seen. The playing surface can be any flat table (or floor) with minimum dimensions of 75-by-75 centimeters. Each player draws a tile from the bag; the highest atomic number goes first. Once the starting player has been determined, those tiles are returned to the bag. The play continues to the starting player s right. 1. Players take turns adding a compound to the crossword by using tiles from their set of ten. Either side of a tile may be used. 2. The elements in a compound may be arranged in any order, as long as they are in a single horizontal or vertical row. 3. Each new compound must be shown by the player to have oxidation numbers that add up to zero. Otherwise the player must take back the compound and wait until the next turn. 4

5 4. If the compound is correct, the player adds up the atomic numbers of all the atoms in the compound to determine the points and then draws new tiles to restore a set of ten. 5. Play continues until all the tiles in the bag are used and one player is out of tiles, or until all players are unable to make a compound with their remaining tiles. 6. The winner is the player with the highest score at the end of the game. Determining correct compounds The oxidation numbers found directly above each element on the periodic table provided are used to determine whether a molecule is correct. The oxidation numbers of all the elements in the compound must add up to zero. In some cases there is more than one oxidation number for an element. Iron (Fe), for example, has oxidation numbers of +2 and +3. The player may choose either oxidation number to add to the total. The Special Bonds card included with the periodic table tiles gives some additional possibilities for forming compounds. Examples of correct and incorrect compounds: The oxidation number for each element in a compound is counted once for every atom present. The two iron atoms contribute a total of +6, which is balanced by the -6 from the three oxygen atoms. Note: The Special Bonds card has some extra ways compounds can form when two atoms of the same element bond together, and when hydrogen is involved in unusual ways. The example of CH 3 OH (methanol) above uses one of the special bonds that hydrogen can make with carbon. Miscellaneous rules and strategies: The noble gases do not form bonds; therefore they can t be played. Players may decide at the start of the game to remove these tiles. As another option, players can choose to turn the noble gas tiles over, revealing the + symbol. This symbol could be used as a wildcard in which the player chooses which element the + should represent. The symbol scores no points but allows the player to complete a molecule. Players must agree on the use of the noble gas tiles before the game begins. 5