Ionic Compounds Unit 6. Writing Formulas Elements occur in constant whole number ratios in a...
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Transcript of Ionic Compounds Unit 6. Writing Formulas Elements occur in constant whole number ratios in a...
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 easilycan conduct electricity under certain conditionsCoordination number – the number of ions of opposite
charge that surround the ion in a crystal
Accessed 11-30-09 http://images.google.com/imgres?imgurl=http://www.everyscience.com/Chemistry/Inorganic/Ionic_Solids/.images/ni6as.gif&imgrefurl=http://www.everyscience.com/Chemistry/Inorganic/Ionic_Solids/b.1297.php&usg=__agLG82xXC9Yj5oqyhykNAtqKDKc=&h=480&w=640&sz=8&hl=en&start=14&um=1&itbs=1&tbnid=R3WSQd3Jddh8JM:&tbnh=103&tbnw=137&prev=/images%3Fq%3Dcoordination%2Bnumber%2Bof%2Bionic%2Bcompounds%26hl%3Den%26safe%3Dactive%26rlz%3D1T4EGLC_enUS321US342%26um%3D1
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 ionAl3+ aluminum ion
Naming Ionic Compounds
• Negative monatomic ions end in –ide.Cl- chloride ionO2- 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 ionchlorate = ClO3
-
• -ate ion plus 1 O same charge, per- prefixperchlorate = ClO4
-
• -ate ion minus 1 O same charge, -ite suffixchlorite = ClO2
-
• -ate ion minus 2 O same charge, hypo- prefix, -ite suffixhypochlorite = ClO-
Patterns for Polyatomic Ions
Elements in the same column on the Periodic Table form similar polyatomic ionssame number of O’s and same charge
ClO3- = chlorate BrO3
- = bromate If the polyatomic ion starts with H, add hydrogen- before the ions name and add +1 to the charge
CO32- = carbonate HCO3
- = 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
• KBrSince 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 ChlorideAgain a metal and a nonmetal so it is ionic.Calcium would form an ion with a 2+ charge and chloride would be 1-.Ca2+Cl- in order for the compound to be neutral, how
many Cl- would there need to be for every Ca2+??2 Cl- for every 1 Ca2+
• So the formula would be CaCl2
• Binary compound
Practice
• Na2CO3
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 – Mg2+ ; phosphate – PO43-
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 Mg3; 3 goes into 6 – 2 times so (PO4)2.
3 x +2 = +6 AND 2 x -3 = -6 Our compound is neutral.Mg3(PO4)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 - Cu1+ ; Oxide – O2-
• Cu2O• Iron (III) Oxide
(III) indicates iron has a 3+ charge.Iron - Fe3+ ; Oxide – O2-
• Fe2O3
Practice and 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.
• CuSO4 • 5 H2OCopper (II) sulfate pentahydrate
• Sodium carbonate heptahydrateNa2CO3 • 7 H2O
Number Prefix
1 mono-
2 di-
3 tri-
4 tetra-
5 penta-
6 hexa-
7 hepta-
8 octa-
9 nona-
10 deca-
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 ions
• 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)