Ionic Formula Writing Video Notes
In this lesson, you will: Learn to write formulas for a variety of ionic compounds.
Chemical Formula Tells the relative number of atoms of each element in a compound. Molecular (covalent bonding) formula represents the number of atoms of each element in a single molecule. Ionic (ionic bonding) formula represent the simplest ratio of positive and negative ions in one formula unit. (remember only covalent bonding produces molecules)
Ionic Bond The attraction that binds oppositely charged ions together One or more electrons are transferred Metals give away electrons Positively charged Cations Nonmetals Take electrons Negatively charged Anions Tries to get a noble gas formulation to increase stability.
How to interpret ions Ex: AlCl 3 Aluminum chloride With no subscript, the assumption is there is there is one ion of that element. With a subscript, there are the number of ions present as the subscript represents. 1 aluminum cation 3 chloride anions (notice it is chloride, not chlorine- this is because IDE is added to the end of anions)
Aluminum Chloride How do we know? Oxidation number- speaks of whether an element tends to give up, share, or gain electrons. Metals- positive oxidation numbers Gasses or non metals- negative oxidation number Ionic compounds- no net charge as the positive and negative charges in the compound balance each other out. If there is a charge, the compound is not stable and will continue to react until it is stable (no charge)
Rules for Ionic Compounds Writing Ionic Formulas 1. Write the symbols for the elements. The cation is written first. The anion is written second. 1. Determine the charge on each ion (use the periodic table to determine charge) 2. Select subscripts that will make the total positive charge equal to the total negative charge. (overall charge of our ionic formula must be zero)
Example- Sodium Chloride NaCl Since Na is in group 1 of the periodic table it needs to lose or give away one electron (gives it a positive charge) Since Cl is in group 17 of the periodic table, it needs to gain one electron (has a negative charge. (1+) + (1-) = 0 Notice the number precedes the charge.
Magnesium Chloride Mg is in group 2. Needs to lose 2 electrons Cl is in group 17, needs to gain 1 electron MgCl 2 (2+) + (2 x 1+) = 0 There needed to be 2 chlorine atoms to make the net charge zero.
Criss-Cross Method Barium and Iodine 1. Write the symbols for your cation and anion (how do you know? Barium is a metal and gives up electrons. Iodine is a non-metal and gains electrons) 2. Ba is in group 2 so it has a 2+ charge, I is in group 17, so it has a 1- charge. Write it as a superscript 3. Criss-cross the charges and make it the subscripts for the ions.
Criss-Cross Method Continued Ba 2+ I 1- Ba 1- I 2+ You now don t need the charges or the 1 in the subscript, it is understood. Take out the top charges as well. New formula-
Calcium Sulfide Ca 2+ S 2- Ca 2 S 2 But the ionic formula is the simplest ratio possible, so the formula is CaS.
Criss-Cross Method for Formula Writing Write symbol of cation followed by symbol of anion along with their ionic charges. Use the absolute value of the charge of each ion as the subscript for the other. If a subscript is one omit it. If the subscripts are the same omit them.
Polyatomic Ions In the polyatomic ion, you still write the cation first, then write the anion Ex. Mg(NO 3 ) 2 (the 2 outside the parenthesis denotes that there are 2 nitrates for every magnesium ion) NO 3-1 Mg +2 NO 3-1
What is the formula? Calcium Nitrate- Ca 2+ NO 3 1- (don t forget to put the polyatomic nitrate ion in parenthesis before criss-crossing the charges!) Ca(NO 3 ) 2
You Try 1. Ca 2+ O 2-2. K 1+ ClO 3 1-3. Al 3+ SO 4 2-
How did you do? 1. CaO 2. KClO 3 3. Al 2 (SO 4 ) 3
Chemistry Quiz CR 1. The orbital shaped like a dumb-bell is: a. s b. p c. d d. f
Chemistry Quiz CR2. What is the volume of a cube if each side measures 2.0 cm? a. 8.0 cm b. 8.0 cm 3 c. 2.0 cm d. 6.0 cm 3
Chemistry Quiz 1. In an ionic compound, the chemical formula represents. a. One molecule of the compound. b. One formula unit of the compound. c. The simplest ration of positive to negative ions in the compound. d. Both B and C are correct.
Chemistry Quiz 2. Are electrons lost or gained during the formation of a negatively charged ion? a. Lost b. Gained
Chemistry Quiz 3. When writing the formula of an ionic compound: a. The net charge must be zero. b. The total positive charge equals the total negative charge. c. The cation is written first followed by the anion. d. All of the above are true.
Chemistry Quiz 4. Choose the formula that correctly represents the compound formed from the ions Sr 2+ and Se 2-. a. SrSe b. Sr 2 Se 2 c. Se 2 Sr 2 d. SeSr
Chemistry Quiz 5. When writing the formula of a compound containing at least one polyatomic ion, parenthesis. a. Must always be placed around the polyatomic ion. b. Must be placed around the polyatomic ion only if a subscript is to be placed outside the parentheses in order to balance charge. c. May or may not be used according to the discretion of the chemist.