Chapter 6: Ionic and Molecular Compounds – Structure, Formation, and Properties

Chapter 6: Ionic and Molecular Compounds

6.1 Ions: Transfer of Electrons

Atoms form ions by losing or gaining electrons to achieve a stable electron configuration, typically an octet. This process is fundamental to the formation of ionic and covalent bonds in compounds.

• Ions are charged particles formed when atoms lose or gain electrons.

• Octet Rule: Atoms tend to gain, lose, or share electrons to achieve eight valence electrons, similar to noble gases.

• Ionic Bonds: Formed by the transfer of electrons from a metal to a nonmetal.

• Covalent Bonds: Formed by the sharing of electrons between nonmetals.

Table 6.1: Types of Particles and Bonds in Compounds

Type	Ionic Compounds	Molecular Compounds
Particles	Ions	Molecules
Bonds	Ionic	Covalent
Example	Na+, Cl- ions	H2O molecules, C6H12O6 molecules

Ionic and Covalent Bonds

Ionic and covalent bonds are the two primary types of chemical bonds that hold atoms together in compounds.

• Ionic bonds occur when valence electrons of a metal atom are transferred to a nonmetal atom, resulting in the formation of oppositely charged ions.

• Covalent bonds occur when nonmetal atoms share electrons to attain a noble gas arrangement.

• Example: Sodium (Na) transfers an electron to chlorine (Cl) to form NaCl (ionic bond); two hydrogen atoms share electrons with oxygen to form H2O (covalent bond).

Positive Ions: Loss of Electrons

Metals form positive ions, or cations, by losing electrons. This process is common for elements in Groups 1A, 2A, and 3A.

• Metals have low ionization energies and readily lose valence electrons.

• Loss of electrons results in a cation with a positive charge.

• Cations are named by their element name (e.g., sodium ion).

• Example: Sodium (Na) loses one electron to form Na+:

$\text{Na} \rightarrow \text{Na}^+ + e^-$

Negative Ions: Gain of Electrons

Nonmetals form negative ions, or anions, by gaining electrons. This is typical for elements in Groups 5A, 6A, and 7A.

• Nonmetals have high ionization energies and readily gain electrons.

• Gain of electrons results in an anion with a negative charge.

• Anions are named by using the first syllable of the element name followed by -ide (e.g., chloride ion).

• Example: Chlorine (Cl) gains one electron to form Cl-:

$\text{Cl} + e^- \rightarrow \text{Cl}^-$

Chemistry Link to Health: Some Important Ions in the Body

Ions play crucial roles in biological systems, regulating body functions and maintaining cellular processes.

Ion	Occurrence	Function	Source
Na+	Principal cation outside the cell	Regulates and controls body fluids	Salt, cheese, pickles
K+	Principal cation inside the cell	Regulates body fluids and cellular functions	Bananas, potatoes, orange juice, milk
Ca2+	Cation outside the cell; 90% of calcium in the body in bones	Major cation in bones, needed for muscle contractions	Milk, yogurt, cheese, greens, spinach
Mg2+	Cation outside the cell; 50% of magnesium in the body in bones	Essential for certain enzymes, muscles, nerve control	Chlorophyll, nuts, grains

6.2 Ionic Compounds

Ionic compounds are formed from the electrostatic attraction between positive and negative ions. Their formulas are determined by balancing the charges of the ions involved.

• Ionic compounds consist of positive and negative charges held together by strong electrical attractions.

• Example: Sodium chloride (NaCl) is formed from Na+ and Cl- ions.

Properties of Ionic Compounds

Ionic compounds exhibit distinct physical properties due to the nature of ionic bonding.

• Consist of positive and negative ions.

• Have attractions called ionic bonds between oppositely charged ions.

• Have high melting points.

• Are solids at room temperature.

Chemical Formulas of Ionic Compounds

The chemical formula of an ionic compound reflects the lowest whole-number ratio of ions that results in a net charge of zero.

• Symbols and subscripts are written in the lowest whole-number ratio.

• The sum of ionic charges in the formula equals zero.

• Key Principle: Total positive charge = total negative charge.

• Example: For NaCl:

$\text{Na}^+ + \text{Cl}^- \rightarrow \text{NaCl}$ $1(+1) + 1(-1) = 0$

Subscripts in Formulas

Subscripts in ionic formulas indicate the number of each ion needed to balance the charges.

• For magnesium chloride (MgCl2):

$\text{Mg}^{2+} + 2\text{Cl}^- \rightarrow \text{MgCl}_2$ $1(+2) + 2(-1) = 0$

Writing Ionic Formulas from Ionic Charges

To write the formula for an ionic compound, balance the total positive and negative charges.

• Example: Sodium sulfide (Na2S):

$2\text{Na}^+ + \text{S}^{2-} \rightarrow \text{Na}_2\text{S}$ $2(+1) + 1(-2) = 0$

Learning Check Examples

• Ion with 16 protons and 18 electrons: The element with 16 protons is sulfur (S). With 18 electrons, it has gained 2 electrons, forming S2-.

• Ionic formula for Ba2+ and Cl-: To balance charges, two chloride ions are needed for each barium ion: BaCl2.

Additional info: These principles are foundational for understanding chemical bonding, compound formation, and the role of ions in biological systems.