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Ionic Compounds: Bonding and Naming

Molecular Compounds: Bonding and Naming

Ionic bonding: This type of bonding involves the complete giving up of 1 or more electrons from one atom to another, resulting in a positively charged ion called a “cation” and a negatively charged ion called an “anion”. This giving and receiving of electrons allows each atom to obtain a “noble gas” electron configuration, making each ion more stable than the original atoms. These oppositely charged ions then stick together like magnets in a repeating pattern.

Naming Ionic Compounds from Chemical Formulas: When naming ionic compounds, follow the following guidelines:

1. Name the cation first. Then, name the anion.

2. For a metal cation, it is named the same as the element. If the metal can have multiple charges (i.e. a transition metal), you must indicate its charge with a Roman numeral. (See above chart. The shaded area indicates where transition elements are.)

Example: Al³⁺ is simply called Aluminum, but Pb⁴⁺ would be Lead (IV) and Cu⁺ would be Copper (I)

3. For non-metal monatomic ions (one-atom ions), the name of the anion is simply the first syllable of the non-metal followed by an “-ide” ending.

Example: O^2- is oxide, Cl^- is chloride, P^3- is phosphide.

4. For polyatomic ions, there is no change in the name of the ion.

Molecular (or Covalent) bonding: This type of bonding involves the sharing of electrons between atoms so that each atom can have 8 electrons in its outermost (or valence) shell. These electrons are shared in pairs, with the two nuclei of the “bonded” atoms pulling on them in a sort of tug-of-war fashion. Atoms that are more electronegative have a stronger “pull”. (See more about electronegativity on the next page.) Each molecule is a separate entity, unlike in ionic compounds where the ions are continuous.

Naming Covalent Compounds from Chemical Formulas: When naming covalent compounds, follow the following guidelines:

For covalent compounds, the same two elements can have numerous combinations due to the fact that there are many ways to share electrons. As such, covalent naming uses the following prefixes:

Prefix	Meaning	Prefix	Meaning
Mono-	1	Hexa-	6
Di-	2	Hepta-	7
Tri-	3	Octa-	8
Tetra-	4	Nona-	9
Penta-	5	Deca-	10

Name the first element with its appropriate prefix (unless the prefix is mono. No “mono-“ prefix for the first element.) Then, name the second element with its appropriate prefix AND change the ending to an “-ide”.

Example: N₂O is Dinitrogen monoxide, CO₂ is Carbon dioxide.

Empirical Formula: A chemical formula indicating the lowest whole number ratio of atoms within a compound. All ionic formulas are empirical formulas since ions don’t form molecules.

Molecular Formula: A chemical formula indicating the exact number of atoms of each type necessary to make up one molecule of a substance.

Writing Ionic Formulas

1. Identify the ions present along with their charges.

Example: Magnesium Phosphate would have the ions Mg²⁺ and PO₄^3-.

2. Find the lowest common multiple of the two different charges.

Example: The lowest common multiple is 6.

3. Determine how many cations would be needed to give you a total positive charge equal to the LCM (lowest common multiple) and how many anions would be needed to give you a total negative charge equal to the LCM.

Example: 3 Mg²⁺ =+6 and 2 PO₄^3- = -6

4. Write the formula for the compound using chemical symbols and indicate how many of each ion with subscripts. If there is more than 1 polyatomic ion needed, use parentheses.

Example: Mg₃(PO₄)₂

Determining the Charge of a Transition Metal

1. Calculate the total negative charge from all the anions. Then, recognize that there will be an equal amount of positive charge.

2. Divide the total positive charge by the number of transition metal ions to get the charge per transition metal.

Example: Name the compound with the formula Fe₂(SO₄)₃.

CHEM 1411 Ch 3 img 2.png $LaTeX: \frac{+6}{2\:Fe}=+3\:charge\:per\:Fe$

The name of the above compound is therefore “Iron(III) sulfate”.

Ion and Acid Naming Chart

*Ion Type*	*Change in # of Oxygens*	*Ion Naming Scheme*	*Example Ion*	*Acid Naming Scheme*	*Example Acid*
Polyatomic	None	(root)-ate	Chlorate: ClO₃^-	(root)-ic acid	Chloric acid: HClO₃
Polyatomic	+1	Per-(root)-ate	Perchlorate: ClO₄^-	Per-(root)-ic acid	Perchloric acid: HClO₄
Polyatomic	-1	(root)-ite	Chlorite: ClO₂^-	(root)-ous acid	Chlorous acid: HClO₂
Polyatomic	-2	Hypo-(root)-ite	Hypochlorite: ClO^-	Hypo-(root)-ous acid	Hypochlorous acid: HClO
Monatomic	N/A	(root)-ide	Chloride: Cl^-	Hydro-(root)-ic acid	Hydrochloric acid: HCl

Important Note: The “root” for sulfate (SO₄^2-) and sulfide (S^2-) changes from “sulf-“ to “sulfur” and the “root” for phosphate (PO₄^3-) changes from “phosph-“ to “phosphor-“ in acid naming.

Molar Mass of a Compound: The molar mass of a compound is the mass of 1 mole of that compound. Molar mass is calculated by taking the sum of all the molar masses of each atom in the compound. See the following example on how to calculate molar mass:

Example: Find the molar mass of Al₂(SO₄)₃:

Al: 2 x 26.98 = 53.96
S: 3 x 32.07 = 96.21
O: 12 x 16.00 = 192.00
342.17 g/mol

$LaTeX: Mass\:Percent\:of\:Element\:X=\frac{mass\:of\:element\:X\:in\:1\:mole\:of\:a\:compound}{molar\:mass\:of\:the\:compound}\times100\%$

Example: What is the mass percent of oxygen in Al₂(SO₄)₃?

$LaTeX: Mass\:\%\:of\:O\:in\:Al_2\left(SO_4\right)_3=\frac{192.00\:g\:O}{342.17\:g\:Al_2\left(SO_4\right)_3}\times100\%=56.112\%$

Mole Conversion Relationships