CHEM 120 Week 2 Ionic Compounds

IONIC COMPOUNDS In this lesson, you will be exploring ionic bonding. You have encountered many examples of molecules containing Ionic bonds including table salt and sodium fluoride which is found in toothpaste. We will ...

IONIC COMPOUNDS In this lesson, you will be exploring ionic bonding. You have encountered many examples of molecules containing Ionic bonds including table salt and sodium fluoride which is found in toothpaste. We will see in this lesson how ionic compounds are formed and how the formation of these compounds leads to the properties we observe. Which of the following would be considered an Ionic Compound? A. CH4 B. LiCl C. N2O4 D. SO In general, an ionic bond forms between atoms classified as: A. Metals and non-metals B. Transition metals C. More than one metal D. More than one nonmetal In an ionic bond electrons are: A. Shared B. Created C. Transferred D. Destroyed Which of the following would you classify as a polyatomic cation? A. NH + B. F- C. OH- D. O2- Ionic compounds are a class of molecules that form through the attraction of negative and positive charges. If you have had a pair of magnets, you have likely noticed that opposites attract. Positive and negative charges are attracted to one another. This is true not only with magnets, but also all the way down to the subatomic level. In an ionic compound, positively charged cations are attracted to negatively charged anions, holding tight to one another like a magnet and a metal surface. An example of this is sodium chloride, otherwise known as table salt. This compound is made of Na+ cations and Cl- anions held together due to a difference in charge. The term we use for this attraction is electrostatic attraction. FORMATION OF IONIC COMPOUNDS The outermost electrons of an atom, known as valence electrons, are transferred or shared in chemical reactions so that each atom has a full valence shell. This is typically 8 valence electrons, and this rule is known as the octet rule. In ionic compounds, the electrons are transferred, while in a molecular compound electrons are shared. In this lesson, we are focusing on Ionic compounds, so let us look into why these valence electrons are transferred. Consider the ionic compound NaCl. Metals tend to lose electrons while nonmetals tend to gain electrons. Na, a metal, has 1 valence electron while Cl, a nonmetal, has 7 valence electrons. As seen in the image below, when an atom of Na loses an electron, the atom now has a full valence shell, while the same is true if Cl gains one electron. For Na, this is like peeling an onion: when the outermost layer of an onion is pealed back, the layer underneath becomes the new outermost layer. This driving force, where the metal tends to lose electrons and the nonmetal tends to gain electrons, is why we see this transfer of electrons from the metal to the nonmetal as seen in the image below. When electrons are transferred, ions are formed. The atoms losing electrons become positively charged ions (cations) while atoms gaining electrons become negatively charged ions (anions). The new ions with opposite charges are now attracted to one another, and this new electrostatic attraction holds them together, forming an ionic bond. MOLECULE CATION ANION ELEMENT THAT LOST ONE OR MORE VALENCE ELECTRON ELEMENT THAT GAINED ONE OR MORE ELECTRONS KF K+ F- K F MgCl2 Mg2+ Cl- Mg Cl Li2O Li+ O- Li O Al2S3 Al3+ S- Al S FeCl3 Fe3+ Cl- Fe Cl IONIC COMPOUNDS: CHARGE BALANCE When ionic compounds form, electrons are transferred. Previously, you saw some examples of ionic compounds that did not have a one to one ratio, such as Li2O. You may have also noticed that in each of these compounds, the charges balanced out. For the example, in Li2O, the two Li+ ions balance out the one O2- ion. This is an important rule when looking at ionic compounds: The total charge in an ionic compound’s chemical formula always balances out to zero. This is the golden rule of ionic compounds. If the charges in an ionic compound are not balanced, the compound will not form, as it will be unstable. As another example, in CaO, the +2 charge of the Ca2+ cation balances out the -2 charge of the O2- anion, giving a net(total) charge of zero. MOLECULE NUMBER OF CATIONS CHARGE OF EACH CATION NUMBER OF ANIONS CHARGE OF EACH ANION Na2S 2 +1 1 -2 Mg3N2 3 +2 2 -3 AlBr3 1 +3 3 -1 NAMING SIMPLE IONIC COMPOUNDS Ionic compounds are actually easy to name! Element name of the cation + element name of the anion with the ~ide suffix. For example, let us take a look at LiBr. • Li is the metal cation, so we use the element name Lithium. • Br is the nonmetal anion, so we use the element name with the suffix replaced with ~ide. So bromine becomes bromide. • The name of the compound is then Lithium bromide. Because the charges always balance, we do not need to do anything with the numbers when naming an ionic compound. For example, CaBr2 would be Calcium Bromide. CaO Calcium oxide Mg3P2 Magnesium phosphide NaI Sodium iodide Na3P Sodium phosphide Li2S Lithium sulfide DETERMINING CHEMICAL FORMULA OF SIMPLE IONIC COMPOUNDS We will now explore how to determine the chemical formula of an ionic compound from the name of an ionic compound. We will be learning the crossover method, which depends on the fact that the charges on an ionic compound must cancel out. This method is illustrated for calcium chloride in the image. As in the image, we determine the charge of each ion from where it is located on the periodic table and write them down, cation followed by anion. Here we have Ca2+ for the cation and Cl1- for the anion. We then bring down the names of the elements and swap the exponents, turning them into subscripts without charges. The subscripts show us how many of each atom is in the compound. A few points to note about this: • The reason this works is that the charges must balance out to zero. • If the charges on your anions balance before you cross, do not use this method. For example calcium oxide, made of Ca2+ and O2-, is CaO and not Ca2O2. This is a quick way to determine the formula of an ionic compound from the name. Once you have this down, you will have down the foundations of ionic compound naming. magnesium phosphide  Mg3P2 aluminum sulfide  Al2S3 calcium nitride  Ca3N2 POLYATOMIC IONS A group of nonmetal atoms bound together with a charge are known as polyatomic ions. A series of these is found in the table. Each of these compounds has a charge and acts as an ion in an ionic compound. For example, Ammonium Chloride is NH4Cl. When in an ionic compound, the charges balance out. The polyatomic ion examples in the table are the common ones we will be using in this lesson, so be sure to make flashcards to ensure you memorize each of these. To name ionic compounds with polyatomic ions as part of the structure, the key concept to remember is that polyatomic ions keep their name. For example, in MgSO4, the sulfate, bolded, keeps the name sulfate, even when it is the anion. The name of this compound would be magnesium sulfate. As a few more examples: • NaNO3 is sodium nitrate (a preservative found in processed meats) • KOH is potassium hydroxide • NH4Br is ammonium bromide Li3PO4 Lithium Phosphate CaSO4 Calcium Sulfate Al(OH)3 Aluminum Hydroxide IONIC COMPOUNDS CONTAINING POLYATOMIC IONS Let’s explore how to use polyatomic ions in ionic compounds. To start us off, two key concepts to keep in mind are: • Polyatomic ions are generally treated as one group for the purposes of chemical formulas. • If a subscript should go after a polyatomic ion, use parenthesis around the polyatomic ion. For example, the formula for calcium nitrate is Ca(NO3)2. As we can see in the illustration, we treat the NO3 as one group and use parenthesis to make clear that there are two NO3 ions in the final atom. Aluminum Nitrite Al (NO2)3 Ammonium Sulfate (NH4)2 SO4 Calcium Phosphate Ca3(PO4)2 Ammonium Nitrate NH4NO3 PROPERTIES OF IONIC COMPOUNDS The properties of ionic compounds are a result of their structure. As we have seen, ionic compounds are made of cations and anions. As we know, positive and negative charges attract, so the structures of these compounds tend to consist of cations surrounded by anions and vice versa. As seen in the image below, the positively charged Na+ ions (small grey spheres) are surrounded by Cl- anions. This structure is known as crystalline structure. Crystalline structures are well ordered as seen in the image. The key properties of ionic compounds are: • High melting points • Crystalline structure • Dissolves well in water • Are electrolytes (When dissolved in water, conducts electricity) • Hard texture • Brittle Ionic compounds are based on ions, and typically involves the interaction of metals and nonmetals. Polyatomic compounds can act as either the cation or anion in an ionic compound. How are each of these properties explained by the structure of ionic compounds? Write down your explanations and then click on the property to expose the explanation: High melting points The strong electrostatic interactions take a lot of heat to break apart Crystalline Structure The positive to negative bonding creates a well-ordered structure Dissolves well in water The positive and negative ions dissolve well in polar solutions Ionic compounds are electrolytes The charged ions help the charged electrons move through solution Hard texture Strong electrostatic interactions hold the compound together in an ordered structure Brittle Disruption of the ordered structure can cause repulsion instead of attraction Select the correct chemical formula for each compound listed and select the type of product you would expect to find it in. Ionic Compound Formula Product Sodium Phosphate Na3PO4 Contact solution Sodium Chloride NaCl Skincare products Sodium Bicarbonate NaHCO3 Deodorant 1. Which of the following would be considered an ionic compound? Select all that apply. a. CaSO4 b. CaO c. N3O6 d. Mg3P2 e. CF4 2. What would be the name of the compound with the name Na2O? a. Sodium oxygen b. Sodium Hydroxide c. Sodium oxide d. Sodium dioxide 3. Na3PO4 would be named as a. Trisodium phosphate b. Sodium phosphide c. Trisodium phosphide d. Sodium phosphate 4. I would expect the chemical NaBr to: a. Dissolve in oil b. Have a crystalline structure c. Dissolve in water d. Conduct electricity if dissolved in water 5. Match the name of the compound with the chemical formula a. Sodium nitride  Na3N b. Sodium nitrate  NaNO3 c. Sodium nitrite  NaNO2 d. Sodium bromide  NaBr e. Sodium carbonate  Na2CO3 6. What would be the chemical formula for calcium phosphate? a. Ca3P2 b. CaPO4 c. Ca3(PO4)2 d. CaPO42 7. In the bond(s) within ionic compounds, what is holding the atoms together? a. Attraction between multiple metals b. Electrostatic attraction c. Sharing of electrons d. Hydrogen bonding 8. In a reaction forming an ionic bond, which of the following would NOT be expected to occur? a. The transfer of two valence electrons from a metal to a nonmetal b. The transfer of a valance electron from a metal to a nonmetal c. The transfer of a valance electron from a metal to a polyatomic ion d. The sharing of two valance electrons between two nonmetals 9. If an ionic bond forms between an atom of Al and an atom of N, how many valance electrons are transferred? a. 2 b. 4 c. 1 d. 3 10. In an ionic compound, the total charge of the compound must balance out to: a. 0 b. -1 c. -2 d. +1