More on Ionic and Covalent Compounds

What is a compound? A chemical made of two or more atoms bonded together in a set ratio

Intramolecular forces Forces that hold the atoms in a compound together (aka chemical bonds) Can be: Ionic Covalent Metallic

Ionic Compounds (salts) Formed when valence electrons are transferred amongst atoms. This creates cations and anions which then attract each other.

Naming Ionic Compounds State the name of the cation first If it is a transition metal with more than one oxidation state, include the charge with roman numerals. Examples: Na = sodium Cs = cesium Fe = iron (II) or iron (III) *depending on what it is bonded to

If the anion is an element, you write the beginning of its name, but change the ending to “–ide” Examples: Na2O = sodium oxide (note that the “2” does not affect the name) KCl = potassium chloride

If the anion is more than one element, you simply write the name of the polyatomic ion. Examples: NaNO3 = sodium nitrate CsClO3 = cesium chlorate Ba3(PO4)2 = barium phosphate

Writing Formulas for Ionic Compounds For a compound to be stable, the overall charge must equal zero. So, figure out the charge of the cation (always positive) and the charge of the anion (always negative) and determine how many of each you need to balance the compound.

Examples Magnesium oxide Mg = 2+ O = 2- MgO Beryllium cyanide Be=2+ CN = 1- Be(CN)2 Lead (II) nitride Pb = 2+ N = 3- Pb3N2

Covalent Compounds (molecules) Formed when atoms share one or more pairs of valence electrons.

Types of covalent bonds Nonpolar Electrons are being shared evenly. Polar Electrons are spending more time with one type of atom than the other. **Which type forms is determined by the electronegativity of the involved atoms (how strongly an atom pulls electrons toward itself)

Polar Covalent If two atoms have a larger difference in electronegativity, then they will share electrons unevenly. For example, oxygen has an electronegativity of 3.0, while hydrogen has an electronegativity of 2.1. Since oxygen is more electronegative, it will get to spend more time with the shared electrons.

Nonpolar covalent If two atoms have a very similar electronegativity, then they will share electrons evenly. For example, carbon has an electronegativity of 2.5, while selenium has an electronegativity of 2.4. These are pretty close, so the atoms share evenly.

Electronegativity and Bond Type Electronegativity Difference Type of Bond Formed 0.0 to 0.2 nonpolar covalent 0.3 to 1.4 polar covalent > 1.5 ionic **I have checked several resources, and each lists a slightly different set of numbers. As best I can tell, these are arbitrarily chosen and should be used only as a guideline.

Naming Covalent Compounds As before, you use the full name of the first element and change the ending of the second element to “- ide.” We use prefixes to indicate the number of each type of atom involved.

Memorize for quiz Prefix number indicated mono- 1 di- 2 tri- 3 tetra- 4 penta- 5 hexa- 6 hepta- 7 octa- 8 nona- 9 deca- 10

Examples C3H8 = tricarbon octahydride N2O = dinitrogen monoxide H2O = dihydrogen monoxide H2O2 = dihydrogen dioxide P4O6 = tetraphosphorus hexaoxide

The exception If there is only one atom of the first chemical, we leave out the “mono-”. Examples: CO = Carbon monoxide CO2 = Carbon dioxide

Diatomic Molecules There are certain elements that are only found in nature as a covalent molecule made of two identical atoms. All of them are gases at room temperature. BrINClHOF Bromine – Br2 Hydrogen –H2 Iodine - I2 Oxygen – O2 Nitrogen - N2 Fluorine – F2 Chlorine- Cl2