Ions and Chemical Bonding: Transfer of Electrons and Compound Formation

Ions: Transfer of Electrons

Types of Particles and Bonds in Compounds

Atoms combine to form compounds through the transfer or sharing of electrons, resulting in different types of chemical bonds. Understanding how ions form and interact is fundamental to the study of chemistry.

• Ions are atoms or groups of atoms with an overall charge, formed by the loss or gain of electrons.

• Chemical bonds hold atoms together in compounds and are classified as ionic (transfer of electrons) or covalent (sharing of electrons).

• Most elements (except noble gases) are found in nature as compounds.

Compounds and the Octet Rule

Stability Through Electron Configuration

The octet rule states that atoms are stabilized by having eight valence electrons, corresponding to a fully filled s and p sublevel in their outermost energy level. This rule explains the tendency of atoms to form compounds.

• Noble gases naturally have eight valence electrons and are chemically stable.

• Other atoms achieve stability by gaining, losing, or sharing electrons to fulfill the octet rule.

• Hydrogen and helium follow the duet rule (two electrons in the first energy level).

• The octet rule does not apply to transition elements.

Formation of Ions

Cations and Anions

Ions are formed when atoms lose or gain electrons to achieve a stable electron configuration.

• Cations are positive ions formed when metals lose electrons.

• Anions are negative ions formed when nonmetals gain electrons.

• Metals tend to lose electrons, while nonmetals tend to gain electrons.

Example: Sodium Ion Formation

• A sodium atom (Na) loses one electron from its 3s1 orbital to form a sodium ion (Na+).

• Electron configuration changes from $1s^2 2s^2 2p^6 3s^1$ to $1s^2 2s^2 2p^6$.

• The sodium ion has the same electron configuration as neon, a noble gas.

Example: Magnesium Ion Formation

• Magnesium (Mg) loses two electrons to form Mg2+.

• Electron configuration changes from $1s^2 2s^2 2p^6 3s^2$ to $1s^2 2s^2 2p^6$.

Example: Chloride Ion Formation

• Chlorine (Cl) gains one electron to form Cl-.

• Electron configuration changes from $1s^2 2s^2 2p^6 3s^2 3p^5$ to $1s^2 2s^2 2p^6 3s^2 3p^6$.

Formulas and Names of Common Ions

Main Group Ions and Naming Conventions

The charge of ions formed by main group elements can often be predicted from their group number in the periodic table.

• Cations (positive ions) are named after the element (e.g., sodium ion, magnesium ion).

• Anions (negative ions) are named by replacing the ending of the element with -ide (e.g., chloride, oxide, nitride).

Element	Ion Symbol	Ion Name
Sodium	Na+	Sodium ion
Magnesium	Mg2+	Magnesium ion
Chlorine	Cl-	Chloride ion
Nitrogen	N3-	Nitride ion
Oxygen	O2-	Oxide ion

Writing Ionic Formulas

Charge Balance in Ionic Compounds

Ionic compounds are formed by combining cations and anions in ratios that result in a neutral overall charge. The empirical formula represents the smallest whole-number ratio of ions.

• Write the symbol and charge for each ion.

• Balance the charges by adjusting the number of each ion.

• The sum of positive and negative charges must equal zero.

• The metal (cation) is written first, followed by the nonmetal (anion).

Example: Lithium and Nitrogen

• Lithium forms Li+; nitrogen forms N3-.

• Three lithium ions are needed to balance one nitride ion: $3(+1) + 1(-3) = 0$.

• Formula: Li3N

Example: Aluminum and Oxygen

• Aluminum forms Al3+; oxygen forms O2-.

• Two aluminum ions and three oxide ions are needed: $2(+3) + 3(-2) = 0$.

• Formula: Al2O3

Covalent (Molecular) Compounds

Sharing of Electrons

Covalent bonds are formed when two or more nonmetals share electrons to achieve a stable electron configuration. The resulting compounds are called molecular compounds.

• Molecules are discrete units with specific numbers of atoms.

• The molecular formula gives the actual number of atoms of each element in a molecule.

• Examples: $H_2O$ (water), $CH_4$ (methane), $C_2H_6$ (ethane).

Empirical vs Molecular Formulas

Comparison of Formula Types

Formula Type	Used For	Description
Empirical Formula	Ionic Compounds	Smallest whole-number ratio of ions
Molecular Formula	Molecular Compounds	Actual number of atoms in a molecule

Polyatomic Ions

Compounds Containing Polyatomic Ions

A polyatomic ion is a group of covalently bonded atoms with an overall ionic charge. These ions participate in ionic bonding similarly to monatomic ions.

Ion Name	Formula
Ammonium	NH4+
Nitrate	NO3-
Hydroxide	OH-
Carbonate	CO32-
Phosphate	PO43-
Sulfate	SO42-

• Formulas for compounds containing polyatomic ions are written using the same charge-balancing principles as for simple ions.

Summary Table: Ionic vs Molecular Compounds

Property	Ionic Compounds	Molecular Compounds
Bond Type	Transfer of electrons	Sharing of electrons
Constituent Elements	Metal + Nonmetal	Nonmetals only
Formula Type	Empirical formula	Molecular formula
Structure	3D lattice of ions	Discrete molecules

Practice Problems

Sample Questions

• Write the symbol and name for the ion with 7 protons and 10 electrons. Answer: N3-, nitride ion.

• Write the symbol and name for the ion with 20 protons and 18 electrons. Answer: Ca2+, calcium ion.

• Write the correct formula for the ionic compound formed from lithium and oxygen. Answer: Li2O.

• Write the correct formula for the ionic compound formed from aluminum and chlorine. Answer: AlCl3.

• Write the correct formula for the ionic compound formed from magnesium and nitrogen. Answer: Mg3N2.

Additional info: Some context and examples have been expanded for clarity and completeness, including the summary tables and stepwise explanations for writing ionic formulas.