Ionic Compounds Unit 6

Writing Formulas Elements occur in constant whole number ratios in a compound (Law of Definite Proportions). In a chemical formula symbols and subscripts are used to describe this ratio. For molecules the chemical (molecular) formula shows the actual number of atoms of each element in the compound In an ionic compound, the formula shows the lowest whole number ratio of ions in the ionic crystal. This ratio is called a formula unit.

Identifying Ionic Compounds Any substance made of metal cation and nonmetal anion OR contains any polyatomic ions Ionic bond = transfer of electrons 1 atom loses, 1 atom gains (cation/anion formed) the + and – particles are attracted to one anther and form a very stable bond (electrostatic attract.) Properties of ionic bonds: high melting point low malleability – break and shatter easily can conduct electricity under certain conditions Coordination number – the number of ions of opposite charge that surround the ion in a crystal Coordination number

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Naming Ionic Compounds Simply name the two ions, positive ion first (the positive ion is always first). Positive monatomic (one type of element) ions have the same name as the element. Na +  sodium ion Al 3+  aluminum ion

Naming Ionic Compounds Negative monatomic ions end in –ide. Cl -  chloride ion O 2-  oxide ion Negative polyatomic ions end in –ate or –ite. There is not a hard and fast rule which will allow you to determine if a polyatomic ion is –ate or –ite (which is why you have to memorize them), but there is one helpful shortcut.

Patterns for Polyatomic Ions The most common form of a polyatomic ion formed from a nonmetal combined with oxygen ends in –ate. -ate ion chlorate = ClO 3 - -ate ion plus 1 O  same charge, per- prefix perchlorate = ClO 4 - -ate ion minus 1 O  same charge, -ite suffix chlorite = ClO 2 - -ate ion minus 2 O  same charge, hypo- prefix, - ite suffix hypochlorite = ClO -

Patterns for Polyatomic Ions ¬Elements in the same column on the Periodic Table form similar polyatomic ions same number of O’s and same charge ClO 3 - = chlorate  BrO 3 - = bromate ­If the polyatomic ion starts with H, add hydrogen- before the ions name and add +1 to the charge CO 3 2- = carbonate  HCO 3 - = hydrogen carbonate

Writing Ionic Formulas IMPORTANT – When writing formulas, use appropriate subscripts so the total number of positive and negative charges cancel out and the compound is neutral. Now, lets see how good we are at writing some formulas and naming some ionic compounds.

Practice KBr Since this has a metal (K) and a nonmetal (Br), we say it is an ionic compound. So we name the positive ion – potassium and the negative ion with the ending changed to – ide, bromide. Potassium bromide Binary compound

Practice Calcium Chloride Again a metal and a nonmetal so it is ionic. Calcium would form an ion with a 2+ charge and chloride would be 1-. Ca 2+ Cl -  in order for the compound to be neutral, how many Cl - would there need to be for every Ca 2+ ?? 2 Cl - for every 1 Ca 2+ So the formula would be CaCl 2 Binary compound

Practice Na 2 CO 3 In this compound there are two ways to identify is as ionic. First, it has a metal and a nonmetal. Second, it has a polyatomic ion. So we name the ions, positive ion first. Sodium carbonate Ternary compound

Practice Magnesium Phosphate magnesium – Mg 2+ ; phosphate – PO 4 3- In order for the compound to be neutral we have to find the least common multiple between our two charges, 2 and 3. The LCM is 6. 2 goes into 6 – 3 times so Mg 3 ; 3 goes into 6 – 2 times so (PO 4 ) 2. 3 x +2 = +6 AND 2 x -3 = -6 Our compound is neutral. Mg 3 (PO 4 ) 2 Ternary compound

Important Precautions If we need more than one polyatomic ion (like in the previous example), it must be surrounded by parentheses before you add the subscript. Also, notice that if the subscript is 1 we do not write it. If the positive ion has more than one possible oxidation number, then when we write the name we must indicate which oxidation state it is. This is done by writing a roman numeral in parentheses which is equal to the positive charge.

Practice Copper (I) Oxide (I) indicates that the copper has a charge of 1+. Copper - Cu 1+ ; Oxide – O 2- Cu 2 O Iron (III) Oxide (III) indicates iron has a 3+ charge. Iron - Fe 3+ ; Oxide – O 2- Fe 2 O 3 PracticePractice and More Practice!!!More Practice

Naming Hydrates Hydrates are crystalline compounds which attract and hold water molecules. The water is called the water of hydration and can be removed (evaporated) by heating. After water is removed the crystal is said to be anhydrous.

Naming Hydrates To name hydrates simply name the compound (usually ionic) and then indicate the number of water molecules by using the same prefixes as in molecular compounds. CuSO 4 5 H 2 O Copper (II) sulfate pentahydrate Sodium carbonate heptahydrate Na 2 CO 3 7 H 2 O

NumberPrefix 1mono- 2di- 3tri- 4tetra- 5penta- 6hexa- 7hepta- 8octa- 9nona- 10deca-

Bonding In Metals valence electrons of metal atoms can be modeled as a sea of electrons – they are mobile and can drift from one part of the metal to the other Metallic Bond – the attraction of these “free-floating” electrons for the metal ionsMetallic Bond these bonds hold metals together and explain many of their physical properties but can we actually bond 2 metals together?

Alloys an alloy is a MIXTURE that is composed of two or more elements, at least one of which is a metal you can also think of an alloy as a solid solution made by dissolving metals in other metals alloys are important because their properties are often superior to those of their component elements

Substitutional Alloy atoms of approximately the same size replace each other in the crystal sterling silver, coins, solder, brass, 18-carrat gold, bronze

Interstitial Alloy atoms of smaller size fit into the space (interstices) between the larger atoms cast iron, steel, stainless steel, surgical steel

Amalgam metal solution that contains mercury this type of alloy may be a solid or liquid used to be very useful in dental amalgams (mixture of Hg, Ag, and Zn)