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Chapter 3: Ions, Ionic Compounds, and Nomenclature. An ion is a small particle having an electrical charge. Ions are either single, charged atoms ( simple ions ), or small charged “molecules” ( polyatomic ions ). Examples of simple ions are Na+, Ca+2, Cl-‐, and S-‐^2. Examples of polyatomic ions are NH^ + 4 , CO 3!^2 , and OH!^. Positively charged ions are called cations (pronounced cat-‐ eye-‐ons) while negatively charged ions are called anions (pronounced an-‐eye-‐ons). In many cases, the elements position on the periodic table will help you determine the kind of ion formed (anion or cation) and the size of the ionic charge. Look at the periodic table (PeriodicTable.pdf or the equivalent). Starting with boron (atomic number 5) and continuing downwards and to the right to astatine (atomic number 85), there is a bold “staircase” shaped red line. Elements to the left of this line are metals, and commonly form cations. Elements to the right of this line are non-‐metals, and commonly form anions. Some of the elements adjacent to the line are called “metalloids”, but this distinction doesn’t help us, so don’t concern yourself with it. Starting on the left of the periodic table, we see groups labeled “I A”, “II A”, III B”, and so on. On some newer versions of the periodic table, the groups are numbered 1 -‐ 18. I will use the older version, and show the new group numbers in parenthesis. The elements in any particular group tend to form ions with characteristic charges. Please note, some elements are capable of having a range of electrical charges, and therefore form different ions. Group I A (1) elements form cations with +1 charge. Group II A (2) elements form cations with +2 charge. Groups III B through II B (3 – 12) are transition metals. These elements form cations having varying amounts of charge. Charges of +2 or +3 are common, but charges from +1 to + 6 can be found. There is no simple way of accurately predicting the charges for these elements. Group III A (13) metals form cations with +3 charge. Please note that the first element in this group, boron (B) is a non-‐metal and typically doesn’t form a cation. Group IV A ( 14 ) metals form cations with +4 charge, although tin (Sn) and lead (Pb) can form cations having +2 charge. Generally, metals in this group are treated similarly to the “B” elements. The non-‐metals carbon (C) and silicon (Si) generally don’t form cations.
Group V A ( 15 ) non-‐metals (nitrogen (N), phosphorous (P)) form anions with a – 3 charge. Arsenic (As) anion has -‐3 charge, but also forms cations with +3 or + 5 charge. Bismuth (Bi) behaves similarly to arsenic. Group VI ( 16 ) non-‐metals form anions having – 2 charge. Polonium (atomic number
- forms only cations. Group VII A ( 17 ) elements form anions having – 1 charge. Group VIII A ( 18 ) elements are normally uncharged. Figure 3.1 below summarizes the typical charges on anions and cations. Figure 3.1. Pattern of Ionic Charges. Nomenclature of simple ions There are only two rules for naming simple ions. Rule 1: Cations. Naming the element and adding the word “ion” forms the cation name. So, Na+^ is “sodium ion”. Ca+2^ is “calcium ion”. Al+3^ is “aluminum ion”. For transition metal cations from groups III B – II B ( 3 -‐ 12 ), the cation name is the name of the element, plus the size of the ionic charge. For example, Fe+3^ is “iron three ion”. Pb+2^ is “lead two ion”. When writing these names, we commonly use Roman numerals in parenthesis to indicate charge size. “Iron three ion” is written
Polyatomic anions: Formula Name Formula Name OH!^ hydroxide ion^ NO 3! NO 2! nitrate ion nitrite ion CO 3!^2 HCO 3! carbonate ion hydrogen carbonate (or bicarbonate)
SO^! 42
HSO^! 4
sulfate ion hydrogen sulfate ion (bisulfate) PO 4!^3 HPO^! 42 H 2 PO^! 4 phosphate ion monohydrogen phosphate dihydrogen phosphate
SO 3!^2
HSO 3!
sulfite ion hydrogen sulfite ion (bisulfite) ClO ! ClO^! 2 ClO 3! ClO 4 ! hypochlorite ion chlorite ion chlorate ion perchlorate ion CrO^! 42 Cr 2 O 7!^2 chromate ion dichromate ion MnO 4!^ permanganate ion^ C 2 H 3 O 2!^ acetate ion CN!^ cyanide^ ion^ C 2 O 4!^2 oxalate ion Hydrogen Hydrogen is a special case. Hydrogen can form a cation (H+) or an anion (H-‐). Generally, in combination with other non-‐metals, hydrogen does not form ions. Hydrogen in water (H 2 0), ammonia (NH 3 ), methane (CH 4 ) and millions of other compounds is neither a cation nor an anion. In a few compounds, called acids,
hydrogen forms hydrogen cations. In even fewer compounds is hydrogen present as the anion. Ionic Compounds and Their Nomenclature An ionic compound is a substance composed of oppositely charged ions held together by the attractive forces between opposite charges (Coulomb attractive forces). There are two general types of ionic compounds. The first type, and the simplest, contains simple cations and anions. If you look at the periodic table, you will notice a stair-‐step shaped line starting at boron (element 5) and continuing down and right to astatine (element 85). All elements on the left hand side of this line are metals (with the exception of hydrogen), and all elements on the right hand side of this line are nonmetals. We recognize NaCl as an ionic compound since it contains the metal Na and the nonmetal Cl. We know that CO 2 is not an ionic compound, since it is composed only of nonmetallic elements. The substance CoNi is not an ionic compound, since it only contains metals, but Na 2 O is an ionic compound, since it contains both a metal and a nonmetal. The second type of ionic compound is composed either partly or entirely of polyatomic ions such as ammonium ( NH 4 +^ ), sulfate ( SO 4!^2 ), carbonate ( CO 3!^2 ), etc. In order to recognize these types of ionic compounds, you must be able to recognize the polyatomic ions when you see them in chemical formulas. NOTE : in chemical formulas, the ion charges are NOT shown – you need to know the charge each polyatomic ion has in its ionic form. For example, when ammonium ions combine with chloride ions, the formula for the compound is NH 4 Cl, and NOT NH 4
Cl ! . Showing the charges of ions when they are combined into compounds is WRONG! In writing the chemical formulas of ionic compounds, it is customary to indicate the cation(s) first, and then the anion(s). If sodium ions combine with carbonate ions we represent this compound as Na 2 CO 3. If ammonium ions combine with sulfate ions, we represent this compound as (NH 4 ) 2 SO 4. The formula for our first compound clearly indicates two sodium ions (Na+) combined with a single carbonate ion ( CO 3!^2 ). The two +1 charges provided by the sodium ions exactly cancel the single -‐2 charge from the carbonate ion, and the resulting compound has a net charge of zero (it is electrically neutral). The formula for our second compound is slightly more complicated, but is not particularly difficult. The formula shows the ammonium ion, NH 4 +^ , enclosed within a set of parenthesis. These parenthesis are used whenever needed to prevent writing two or more subscripts consecutively. Using the representation ( NH 4
) 2 allows us to see that there are two separate ammonium ions present in this
represented as KLiSO 4 and would be named potassium lithium sulfate. These types of compounds are relatively rare, and not particularly helpful in learning the rules of nomenclature. For some compounds containing polyatomic anions, several different names can be assigned to the same material. Some of these names are customary or trivial names, while some are based upon more formalized rules of nomenclature. For example, the compound with the formula NaH 2 PO 4 has at least six different names still currently used: sodium phosphate monobasic, monosodium orthophosphate, monosodium phosphate, sodium acid phosphate, sodium biphosphate, and sodium dihydrogen phosphate. Perhaps someday common sense will break out and everyone will name this substance sodium dihydrogen phosphate, but I won’t hold my breath. As a practical matter, compounds containing one divalent or higher cation and several monovalent anions, such as AlClBrI, are so rarely encountered that the student need not worry about them. More commonly encountered are substances such as NaKO, or BaKN. Rules for naming these materials are the same as for their polyatomic anion analogs. These compounds are sodium potassium oxide and barium potassium nitride respectively. The last class of ionic compounds is those containing transition metal cations. Since these cations have a range of positive charges, we write the cation name with the charge indicated by Roman numerals in parenthesis. The formula for a compound containing Fe+3^ and Cl-‐^ ions is FeCl 3 , and its name is “iron (III) chloride”. This allows us to distinguish between this compound and a related material composed of Fe+2^ ions and Cl-‐^ ions. The formula for this compound is FeCl 2 , and the name is “iron (II) chloride”. Simply calling either of these substances “iron chloride” is not useful, because this name doesn’t distinguish between the two varieties of iron cations. Once again, a numerical prefix such as di, tri, tetra, and so on, is neither required nor correct when the compound is composed of one type of cation and anion. Naming iron (III) chloride “iron trichloride” is just wrong.
Vocabulary. The following terms are defined and explained in the text. Make sure that you are familiar with the meanings of the terms as used in chemistry. Understand that you may have been given incomplete or mistaken meanings for these terms in earlier courses. The meanings given in the text are correct and proper. Ion Simple ion Polyatomic ion Cation Anion Homework:
- For the following elements, write the formula including charge of the simple ion that is commonly formed, and the name of the ion formed. Ca Br Na S I O K N Al Mg Ba Sr Li Cl F
- From the following names of ions, write the chemical formula of the ion and the correct electrical charge. Be sure to include the proper sign (+ or -‐). Iron(III) Sulfide Manganese(VII) Acetate Hg(II) Chromium(III) Ammonium Tin(IV) Phosphate Vanadium(II) Hypochlorite Hydrogen carbonate Hydronium Nitrite Chloride
- For the following compounds, write all ions needed to form the compound (including the charge). Write the name of the compound. CaSO 4 NaOH AlBr 3 SrS LiI Na 2 O KClO Na 3 N Al 2 S 3 Mg(OH) 2 BaCO 3 Sr(NO 3 ) 2 LiF SnCl 4 FeF 3
5. For the following compounds, write the correct formula.
Iron (III) nitrate Barium sulfide Manganese (IV) chloride Copper (II) acetate Mercury (II) bromide Chromium (III) oxide Ammonium carbonate Tin (IV) sulfate Strontium hydrogen phosphate Calcium chloride. Magnesium perchlorate Zinc (II) nitrite
Answers:
Ca+ Calcium ion Br-‐ Bromide Na+ Sodium ion S -‐^2 Sulfide
I-‐
Iodide
O-‐^2
Oxide K+ Potassium ion
N-‐^3
Nitride Al+ Aluminum ion Mg+ Magnesium ion Ba+ Barium ion Sr+ Strontium ion Li+ Lithium ion Cl-‐ Chloride
F-‐
Fluoride
Fe+3^ S-‐^2 Mn+ C 2 H 3 O 2 -‐^ Hg+2^ Cr+ NH 4 +^ Sn+4^ PO 4 -‐^3 V+2^ ClO-‐^ HCO 3 -‐ H 3 O+^ NO 2 -‐^ Cl-‐
Iron (III) nitrate Fe(NO 3 ) 3 Barium sulfide BaS Manganese (IV) chloride MnCl 4 Copper (II) acetate Cu(C 2 H 3 O 2 ) 2 Mercury (II) bromide HgBr 2 Chromium (III) oxide Cr 2 O 3 Ammonium carbonate (NH 4 ) 2 CO 3 Tin (IV) sulfate Sn(SO 4 ) 2 Strontium hydrogen phosphate SrHPO 4 Calcium chloride CaCl 2 Magnesium chlorate Mg(ClO 3 ) 2 Zinc (II) nitrite Zn(NO 2 ) 2
