Chapter 3: Covalent and Ionic Compounds – Structure, Naming, and Counting Atoms

Covalent and Ionic Compounds

Introduction

This chapter introduces the fundamental concepts of covalent and ionic compounds, focusing on how atoms combine, how to write and name chemical formulas, and how to count atoms and molecules using the mole concept. Understanding these principles is essential for studying chemical reactions and properties of substances in GOB Chemistry.

Ionic Compounds—Electron Give and Take

Formation of Ionic Compounds

• Ionic compounds are formed when a metal reacts with a nonmetal, resulting in the transfer of electrons from the metal to the nonmetal.

• This transfer creates cations (positively charged ions) and anions (negatively charged ions).

• The electrostatic attraction between cations and anions is called an ionic bond.

• The resulting substance is an ionic compound.

Example: Sodium (Na) transfers an electron to chlorine (Cl), forming Na+ and Cl-, which combine to make NaCl (sodium chloride).

Writing Formulas of Ionic Compounds

• The ratio of ions in an ionic compound is determined by the charges of the cation and anion.

• Ions combine so that the total charge of the compound is zero (electrical neutrality).

• Subscripts in chemical formulas indicate the number of each ion present.

• Charges are not shown in the final formula.

Example: For Ca2+ and Cl-, the formula is CaCl2.

Steps to Check a Chemical Formula

1. Determine the charge of each ion (use the periodic table for main-group elements).

2. Combine ions in a ratio that results in a net charge of zero.

3. Check the formula by ensuring the charges and subscripts balance.

Example: Al3+ and O2- combine to form Al2O3:

$\text{Al}^{3+} + \text{O}^{2-} \rightarrow \text{Al}_2\text{O}_3$

Predicting Number and Charge in a Chemical Formula

• The formula must have no net charge (sum of positive and negative charges is zero).

• The charge of the anion is typically known from the periodic table or polyatomic ion list.

Practice: Writing Formulas for Ionic Compounds

• Na+ and SO42- → Na2SO4

• Al3+ and NO3- → Al(NO3)3

• Sn4+ and O2- → SnO2

• Ca2+ and SO42- → CaSO4

• Mg2+ and PO43- → Mg3(PO4)2

• Ba2+ and Br- → BaBr2

Naming Ionic Compounds

• The name of an ionic compound combines the names of the cation and anion, with the cation first.

• The word "ion" is omitted in the compound name (e.g., NaCl is "sodium chloride").

• For transition metals, a Roman numeral indicates the cation's charge (e.g., CuO is "copper(II) oxide").

• If a polyatomic ion is present, its name remains unchanged (e.g., Ca3(PO4)2 is "calcium phosphate").

Example Table: Naming Ionic Compounds

Formula	Name
NaCl	Sodium chloride
CaCl2	Calcium chloride
CuO	Copper(II) oxide
Ca3(PO4)2	Calcium phosphate

Covalent Compounds—Electron Sharing

Formation of Covalent Bonds

• Covalent compounds are formed when two nonmetals share valence electrons to achieve an octet.

• The shared electrons belong to both atoms, resulting in a covalent bond.

• The smallest unit of a covalent compound is a molecule.

Example: Two hydrogen atoms share electrons with one oxygen atom to form H2O (water).

Lewis Structures and Electron-Dot Symbols

• Lewis structures use electron-dot symbols to show how atoms share electrons in covalent bonds.

• Each dot represents a valence electron; a line (–) represents a shared pair (bonding pair).

• Atoms share unpaired electrons to complete their octets.

Example: The Lewis structure for methane (CH4):

$\mathrm{H} - \mathrm{C} - \mathrm{H}$ $\quad |$ $\mathrm{H}$ $\quad |$ $\mathrm{H}$

Types of Covalent Bonds

• Single bond: One pair of electrons shared (e.g., H–H).

• Double bond: Two pairs of electrons shared (e.g., O=O).

• Triple bond: Three pairs of electrons shared (e.g., N≡N).

Example: Carbon dioxide (CO2) has two double bonds: O=C=O.

Drawing Lewis Structures for Covalent Compounds

1. Count the total number of valence electrons from all atoms (add for anions, subtract for cations).

2. Arrange the atoms, connecting them with single bonds.

3. Complete the octets of outer atoms, then place remaining electrons on the central atom.

4. If the central atom lacks an octet, form double or triple bonds as needed.

Example: For CO2:

$\mathrm{O} = \mathrm{C} = \mathrm{O}$

Naming Binary Covalent Compounds

• Binary covalent compounds contain only two elements.

• Name the first element as it appears.

• Name the second element with the suffix "-ide."

• Use Greek prefixes to indicate the number of each atom (except "mono-" is often omitted for the first element).

Greek Prefixes Table

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

Example: CO2 is "carbon dioxide"; N2O4 is "dinitrogen tetroxide."

Determining Compound Type

• If the compound contains a metal and a nonmetal, it is ionic.

• If the compound contains only nonmetals, it is covalent.

Counting Atoms and Compounds: The Mole Concept

The Mole and Avogadro's Number

• Chemists use the mole as a counting unit, similar to a dozen.

• One mole contains $6.022 \times 10^{23}$ particles (Avogadro's number).

• The mass of one mole of an element (in grams) equals its atomic mass in amu.

Example: 1 mole of carbon-12 atoms has a mass of 12 grams and contains $6.022 \times 10^{23}$ atoms.

Converting Between Atoms and Moles

• Use Avogadro's number as a conversion factor:

$1\ \text{mole} = 6.022 \times 10^{23}\ \text{particles}$

• To convert from moles to atoms: multiply by Avogadro's number.

• To convert from atoms to moles: divide by Avogadro's number.

Molar Mass and Formula Weight

• The formula weight of a compound is the sum of the atomic masses of all atoms in its formula (in amu).

• The molar mass is the mass of one mole of a compound (in grams per mole, g/mol).

• For large molecules (e.g., proteins), the dalton (Da) is used, equivalent to the amu.

Example: The molar mass of NaCl is calculated as:

$\text{Molar mass of NaCl} = 22.99\ \text{g/mol (Na)} + 35.45\ \text{g/mol (Cl)} = 58.44\ \text{g/mol}$

Steps for Converting Between Units

1. Determine the desired unit for your answer.

2. Establish the given information.

3. Set up the problem using appropriate conversion factors.

4. Solve the problem.

5. Check your answer for reasonableness.

Summary Table: Key Differences Between Ionic and Covalent Compounds

Property	Ionic Compounds	Covalent Compounds
Bond Type	Electron transfer	Electron sharing
Constituent Elements	Metal + Nonmetal	Nonmetal + Nonmetal
Smallest Unit	Formula unit	Molecule
Physical State	Usually solid	Solid, liquid, or gas
Electrical Conductivity	Conducts when molten or dissolved	Does not conduct

Additional info: Some context and examples were inferred and expanded for clarity and completeness, including the summary tables and stepwise procedures.