Chapter 3: Naming Covalent Compounds and Counting Atoms in Chemistry

Naming Covalent Compounds

Introduction to Covalent Compounds

Covalent compounds are chemical substances formed by the sharing of electrons between nonmetal atoms. Understanding how to name these compounds is essential for clear communication in chemistry.

• Binary compounds are composed of only two elements.

• They are named using a systematic three-step procedure.

Three-Step Procedure for Naming Binary Covalent Compounds

1. Name the first element in the formula.

2. Name the second element in the formula and change its ending to -ide.

3. Designate the number of each element present using the appropriate Greek prefix.

• Indicating the number of each element is important because nonmetals can combine in multiple ratios.

• Some compounds, such as water (H2O), ammonia (NH3), and nitrous oxide (N2O), are known by their traditional names.

Greek Prefixes for Covalent Compound Naming

The following table lists the prefixes used to indicate the number of atoms of each element in a covalent compound:

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

Determining Compound Type: Ionic vs. Covalent

Before naming a compound, it is necessary to determine whether it is ionic or covalent:

• Ionic compounds consist of metals and nonmetals.

• Covalent compounds consist of only nonmetals.

The following decision process is used:

• If the compound contains a metal from Group 1A, 2A, or 3A, use Roman numerals and no Greek prefixes.

• If the compound is covalent (all nonmetals), use Greek prefixes.

Examples of Covalent Compound Names

• SCl2: sulfur dichloride

• NF3: nitrogen trifluoride

• CO2: carbon dioxide

• PBr3: phosphorus tribromide

• N2O4: dinitrogen tetroxide

• P2O5: diphosphorus pentoxide

Counting Atoms and Compounds: The Mole Concept

Introduction to the Mole

Chemists use the mole as a counting unit to relate the mass of an element in grams to the number of atoms it contains. The mole is analogous to other counting units, such as a dozen.

• 1 dozen = 12 items

• 1 mole = $6.022 \times 10^{23}$ items (atoms, molecules, etc.)

Molar Mass

The molar mass of an element (or compound) is the mass in grams of one mole of that substance. It is numerically equal to the atomic (or molecular) mass in atomic mass units (amu).

• Molar mass is expressed in units of grams per mole (g/mol).

• For example, 12 grams of carbon-12 contains 1 mole of atoms.

Avogadro's Number

The number of atoms in one mole is known as Avogadro's number ($N$), named after Amedeo Avogadro:

• $6.022 \times 10^{23}$ atoms = 1 mole of atoms

• This number is used as a conversion factor between atoms and moles.

Converting Between Atoms and Moles

To convert between atoms and moles, use Avogadro's number as a conversion factor:

• Atoms to moles: divide by $6.022 \times 10^{23}$

• Moles to atoms: multiply by $6.022 \times 10^{23}$

Steps for Unit Conversion in Chemistry

1. Determine the unit for your final answer.

2. Establish the given information.

3. Decide how to set up the problem.

4. Solve the problem.

5. Check your answer.

Molar Mass and Formula Weight

The formula weight of a compound is the sum of the atomic masses of all atoms in its formula. The molar mass is numerically equal to the formula weight, but expressed in grams per mole.

• The dalton (Da) is another unit equivalent to the amu, used mainly in biochemistry for large molecules.

Example: Calculating Molar Mass

• For Ca3(PO4)2:

  • Ca: 3 × 40.08 g/mol = 120.24 g/mol

  • P: 2 × 30.97 g/mol = 61.94 g/mol

  • O: 8 × 16.00 g/mol = 128.00 g/mol

  • Total molar mass = 310.18 g/mol

• For NaClO3:

  • Na: 22.99 g/mol

  • Cl: 35.45 g/mol

  • O: 3 × 16.00 g/mol = 48.00 g/mol

  • Total molar mass = 106.44 g/mol

Additional info: The notes also mention the importance of distinguishing between ionic and covalent compounds for proper naming, and the use of traditional names for some common compounds.