Chapter 6

Ions are charged molecules or atoms. Ions form ionic compounds in which oppositely charged ions are held together by electrostatic attraction. When ions form compounds the total charge must sum to zero.

Oxidation numbers are represent the charge of an atom in a molecule when the electrons in bonds are assumed to go with the element with the higher electronegativity.

See rules for assigning oxidation numbers on page 117.

How do we use the periodic table to predict the charge of ions.

Assign oxidation numbers to the atoms in the following molecules.

MnO₂

P₂O₅

Na₂CO₃

Cr₂O₇^2-

CO₃^2-

H₂SO₄

Oxidation numbers give us a way of keeping track of the electrons lost or gained in an oxidation-reduction reaction.

For ionic compounds the chemical formulas are easy to determine. One need only exchange the charge numbers for the ions and reduce if possible.

e.g. Na⁺¹ + Cl^-1 ==> NaCl Sodium cholride

Na⁺¹ + O^2- ==> Na₂O Sodium oxide

Al³⁺ + Cl^- ==> AlCl₃ Aluminum oxide

Cr³⁺ + S^2- ==> Cr₂S₃ Chromium suldife

Fe³⁺ + O^2- ==> Fe₂O₃ Iron(III) oxide

Some ionic compounds consist of polyatomic anions; however, if you treat the whole polyatomic anion as a single entity you will encounter little problem.

e.g. Na⁺¹ + SO₄^2- ==> Na₂SO₄ Sodium sulfate

Al³⁺ + SO₄^2- ==> Al₂(SO₄)₃ Aluminum sulfate

The names of the monatomic negative ions (anions) use the root of the element and end in ide.

Oxygen oxide O^2-

Chlorine chloride Cl^-1

Bromine bromide Br^-1

Iodine iodide I^-1

Fluorine fluoride F^-1

Nitrogen nitride N^-3

Crobon carbide C^-4

Phosphorous Phosphide P^3-

Sulfur Sulfide S^-2

Polyatomic negative ions (Polyatomic anions) are named using the root + ate ending. Most other polyatomic anions can be named if you memorize this list and follow the rules below.

Bromate BrO₃^1-

Chlorate ClO₃^1-

Nitrate NO₃^1-

Sulfate SO₄^2-

Carbonate CO₃^2-

Chromate CrO₄^2-

Manganate MnO₄^2-

Arsenate AsO₄^3-

Phosphate PO₄^3-

Elements with one less oxygen than the "ate" ions use the root and end in ite. Root-ite.

Bromite BrO₂^1-

Chlorite ClO₂^1-

Arsenite AsO₃^3-

Sulfite SO₃^2-

Elements with two less oxygens than the "ate" ions use the prefix hypo then the root and end in ite. Hypo-root-ite.

Hyporomite BrO^1-

Hypohlorite ClO^1-

Hyposulfite SO₂2^-

Elements with one more oxygen than the "ate" ions, use the prefix per then the root and end in ate. Per-root-ate.

Perromate BrO₄^1-

Chlorite ClO₄^1-

Permanganate MnO₄^1-

Some times the above ions have hydrogen associated with them in which case you use the prefix hydrogen (or bi)

Hydrogen sulfate HSO₄^1- Bisulfite

Hydrogen carbonate HCO₃^1- Bicarbonate

Monohydrogen phosphate HPO₄^2-

Dihydrogen phosphate H₂PO₄^1-

Positive ions (cation) simply give the name of the metal. If the metal exhibits more than one charge, you must give the charge in roman numerals following the ion.

Fe²⁺ Iron (II) Ferrous

Fe³⁺ Iron (III) Ferric

Pb²⁺ Lead (II) Plumbus

Pb⁴⁺ Lead (IV) Plumbic

Al³⁺ Aluminum

Na¹⁺ Sodium

When naming ionic compounds name the cation first followed by the anion.

When naming acids you must pay particular attension to the ion forming the acid.

First acids formed from monatomic anions. Hydro-root-ic-acid

Cl^1- HCl Hydrochloric acid

S^2- H₂S Hydrosulfuric acid

F^1- HF Hydrofluoric acid

Acids that are formed from ions that end in ite are named by replacing the ite with ous acid.

Hypo-root-ite ==> Hypo-root-ous acid

Root-ite ==> Root-ous acid

Acids that are formed from ions that end in ate are named by replacing the ate with ic acid.

Per-root-ate ==> Per-root-ic acid

Root-ate ==> Root-ic acid

ClO^1- HClO Hypochlorous acid

ClO₂^1- HClO₂ Chlorous acid

ClO₃^1- HClO₃ Chloric acid

ClO₄^1- HClO₄ Perchloric acid

Naming molecular compounds compounds.

You must use prefixes to indicate the number of each element present except when there is only one of the first element.

Prefixes Meaning Prefixes Meaning

Mono 1 Hexa 6

Di 2 Hepta 7

Tri 3 Octa 8

Tetra 4 Nona 9

Penta 5 Deca 10

When the element begins in a vowel, you omit the a at the end of the prefix.

NO₂ Nitrogen dioxide CO Carbon monoxide

N₂O Dinitrogen monoxide CO₂ Carbon dioxide

N₂O₄ Dinitrogen tetroxide P₃O₄ Triphosphorous tetroxide

SO₃ Sulfur trioxide