Ionic Bonding and Properties of Ionic Compounds | General Chemistry 1

Ionic bonding and ionic compounds are studied in this chapter: types of bonding, ionic compounds and their names, lattice energies, energies of formation of ionic solids, electronegativity, dipole moment, and percent ionic character.

Types of Bonding

The properties of substances are determined by the type of bonding within the substance. There are 3 main types of bonding:
 

  • Ionic bonds

Ionic bonds are formed between a metal cation and a non-metal anion due to the electrostatic force that binds ions of opposite charge. This type of bond results from the complete transfer of one or more electrons from one atom to another, leading to the formation of 2 charged particles, called ions.
 

  • Covalent bonds

Covalent bonds involve the sharing of a pair of electrons between two non-metals to allow the atoms to reach an octet in their valence shell. This is the most common kind of chemical bond.
 

Water (H2O) is a covalent compound where each hydrogen atom shares an electron with the oxygen atom, resulting in a molecule where the oxygen has an octet and the hydrogens have a complete outer shell.

 

  • Metallic bonds

Metallic bonds are formed by an electrostatic attractive force between conduction electrons (free electrons) and positively charged metal ions. This type of bonding is characterized by the sharing of free electrons among a structure of positively charged metal ions, creating a "sea of electrons" that allows metals to conduct electricity and heat.

Ionic Compounds

Ionic compound:

Ionic compound is a chemical compound composed of cations and anions held together by ionic bonds. Ionic compounds form when atoms transfer electrons to achieve a stable electron configuration, typically forming an extended crystal lattice of alternating ions.
 

Na readily loses one e- to become a cation Na+
Cl readily gains one e- to become an anion Cl-
NaCl is an ionic compound held by the ionic bond between Na+ and Cl-: Na+ + Cl- → NaCl

 

Properties of ionic compounds:

  • Crystal lattice structure: Ionic compounds form a regular, repeating pattern of ions in a crystal lattice, which maximizes the electrostatic attractions and minimizes the repulsions between ions.
  • Conductivity: In the solid state, ionic compounds do not conduct electricity. However, when dissolved in water or melted, they dissociate into ions and become good conductors of electricity.
  • High melting and boiling points: The strong electrostatic forces between ions in an ionic compound result in high melting and boiling points.

 

How to determine the chemical formula of an ionic compound:

  1. Determine the charge of the ions.
  2. Balance the positive and negative charges (ensure that the total positive charge and total negative charge in the compound are equal, resulting in a neutral compound).
  3. Write the resulting chemical formula.
     

Chemical formula of the ionic compound formed between aluminum and fluorine:

  1. Aluminum ion: Al loses three electrons to become Al3+.
    Fluorine ion: F gains one electron to become F-.

  2. Balancing charges: To balance the charges, you need three F- ions to balance the charge of one Al3+ ion.

  3. Chemical Formula: The resulting formula is AlF3.

Naming Ionic Compounds

Naming ions:

  • Monatomic cations:

Monatomic cations are single atoms that have lost one or more electrons, resulting in a positive charge. They are named by adding the word "ion" to the name of the element. For elements that can have more than one possible charge (especially transition metals), the charge is indicated with Roman numerals in parentheses.
 

  • Monatomic anions:

Monatomic anions are single atoms that have gained one or more electrons, resulting in a negative charge. They are named by changing the ending of the element name to "-ide" and adding the word "ion".
 

Monatomic cations:

Mg2+ = magnesium ion
Fe2+ = iron(II) ion
Fe3+ = iron(III) ion

Monatomic anions:

Cl- = chloride ion
O2- = oxide ion
N3- = nitride ion

 

  • Polyatomic ions:

Polyatomic ions are groups of atoms that behave as a single ion with a specific charge. The names of common polyatomic ions often do not follow the same rules as monatomic ions and must be memorized.
 

NH4+ = ammonium ion
NO3- = nitrate ion
SO42- = sulfate ion

 

Naming ionic compound

  • Binary ionic compounds:

Most ionic compounds are binary ionic compounds, which consist of 2 ions derived from 2 different elements: one metal element (a cation) and 1 nonmetal element or polyatomic ion (an anion). 

Naming convention:

  1. The name of the cation is given first, followed by the name of the anion.
  2. For monatomic anions, the anion name ends in "-ide".
  3. The word "ion" is omitted in the compound name.
  4. If the metal is a variable charge metal, the charge is indicated in parentheses with Roman numerals.

 

CaS = calcium sulfide
CaBr2 = calcium bromide

Fe2(SO4)3 = iron(III) sulfate

 

  • Compounds with polyatomic ions: 

The name of a compound with polyatomic ion includes the cation followed by the name of the polyatomic ion.
 

Fe2(SO4)3 = iron(III) sulfate
(NH4)2SO4 = ammonium sulfate

Lattice Energies

Lattice of ionic compounds:

An ionic compound consists of a vast, repeating array of positively charged cations and negatively charged anions. This arrangement is known as a lattice. The lattice structure is crucial for understanding the stability and properties of ionic compounds.
 

Lattice for NaCl:

NaCl forms a cubic lattice where each sodium ion (Na+) is surrounded by 6 chloride ions (Cl-), and each chloride ion is surrounded by 6 sodium ions.
This arrangement maximizes the electrostatic attraction between the oppositely charged ions, contributing to the stability of the crystal.

 

Lattice energy:

Lattice energy (U) is the amount of energy required to convert one mole of an ionic solid into its constituent ions in the gas phase. It measures the strength of the ionic bonds within the solid. The greater the lattice energy, the more stable the compound. 

Factors influencing lattice energy:

  • Magnitudes of charges: Higher charges on the ions result in greater lattice energies.
  • Distance between ions: Smaller distances between the ions result in greater lattice energies.

 

 

Calculate the lattice energy:

The interaction between electric charges can be described using Coulomb's law:
 

F = k z1z2d2

F = interaction force between the charges (in Newtons)
k = Coulomb constant = 231 aJ.pm (1 aJ = 10-18 J)
z1 = charge of ion 1 (in C)
z2 = charge of ion 2 (in C)
d = distance between the nuclei of the ions (in m)

 

Energy in physics is defined as force multiplied by distance. Therefore, the lattice energy U can be expressed as:
 

 U = - F x d = - k z1z2d

U = lattice energy (in J)
k = Coulomb constant = 231 aJ.pm (1 aJ = 10-18 J)
z1 = charge of ion 1 (in C)
z2 = charge of ion 2 (in C)
d = distance between the nuclei of the ions (in m)

Formation of Ionic Solids

Hypothetical steps for formation of ionic compounds:

The formation of an ionic compound from its corresponding elements can be conceptualized through a 3-step hypothetical process:

  • Step 1 - Formation of cation: 
    An electron is removed from an atom in the gas phase to form a cation. This step requires the ionization energy I of the corresponding element.

  • Step 2 - Formation of an anion:
    An electron is added to another atom in the gas phase to form an anion. This step releases energy equivalent to the electron affinity EA of the corresponding element.

  • Step 3 - Formation of ionic bond:
    T
    he ions in the gas phase come together to form an ionic bond. This step involves the lattice energy U.


The total formation energy of the ionic compound is the sum of the 3 energies: I + EA + U.
 

Formation energy of NaCl (Na+Cl-):

Na (g) + Cl (g) → Na+Cl- (g)

  1. Na (g) → Na+ (g) + e-
    Energy: first ionization energy of Na = INa = 0.824 aJ

  2. Cl (g) + e- → Cl- (g)
    Energy: first electron affinity of Cl = EACl = -0.580 aJ

  3. Na+ (g) + Cl- (g) → Na+Cl- (g)     (dNa-Cl = 283 ppm)
    Energy: lattice energy = U = -231 x +1-1283 = -0.816 aJ

Total energy of this reaction: Ereaction = INa + EACl + Ecoulomb = -0.572 aJ

Electronegativity

Electronegativity:

Electronegativity is a measure of an atom's ability to attract electrons to itself. In a chemical bond, electrons are shared between atoms, but they are more attracted to the atom with higher electronegativity. This tendency determines how the electron density is distributed in a molecule or a polyatomic ion.
 

Trends in electronegativity:

  • Across a period: Electronegativity increases from left to right across a period in the periodic table. This is because atoms have more protons and thus a stronger pull on the shared electrons.
  • Down a group: Electronegativity decreases from top to bottom down a group. As the atomic radius increases, the added electron shells shield the outer electrons from the nucleus' pull, reducing electronegativity.

These trends are similar to those observed for electron affinity.

 

Covalent, polar covalent, and ionic bond:

The difference in electronegativity between two bonded atoms determines the type of bond formed and the polarity of the bond. This difference can categorize bonds into three types:

  • Purely covalent (nonpolar) bond
    Electronegativity difference less than 0.5
    Electrons are shared equally between the two atoms because their electronegativities are very similar.
  • Polar covalent bond
    Electronegativity difference between 0.5 and 2.0
    Electrons are shared unequally. The atom with higher electronegativity attracts the shared electrons more, creating a dipole moment.
  • Ionic bond
    Electronegativity difference more than 2.0
    Electrons are transferred from the less electronegative atom to the more electronegative atom, resulting in the formation of ions.

 

H2 ⇒ no difference of electronegativity ⇒ nonpolar bond

HCl (electronegativity: H = 2.1, Cl = 3.0) ⇒ difference of electronegativity = 0.9 ⇒ polar covalent bond

NaCl (electronegativity: H = 0.9, Cl = 3.0) ⇒ difference of electronegativity = 2.1 ⇒ ionic bond

Dipole Moment & Percent Ionic Character

Dipole moment (μ):

A dipole moment is a quantitative measure of the polarity of a bond. It occurs when there is a separation of positive and negative charges within a molecule, indicating a region with a slight positive charge (δ+) and a region with a slight negative charge (δ-). It is represented by a crossed arrow above the Lewis structure, pointing from δ+ to δ-.
 

 

 

Dipole moments are typically expressed in debye units (D), where 1D = 3.336 x 10-30 C.m. The dipole moment μ can be calculated using the formula:
 

μ = Q x r

μ = dipole moment (in C.m)
Q = charge (in C)
r = distance between the charges (in m)


 

Percent ionic character:

Percent ionic character is a quantitative measure of how ionic a bond is, compared to a purely covalent bond. It compares the observed dipole moment of a bond with the dipole moment calculated assuming complete transfer of electrons:
 

% ionic character = μ(observed)μ(calculated) x 100

 

Check your knowledge about this Chapter

The 3 primary types of bonding are:

  • Ionic bonds
  • Covalent bonds
  • Metallic bonds

An ionic bond is a bond formed between a metal cation and a non-metal anion due to the electrostatic force that binds ions of opposite charge, whereas a covalent bond involves the sharing of a pair of electrons between two non-metals to allow the atoms to reach an octet in their valence shell

  • A covalent compound is an individual molecule whose electrons are shared between its atoms forming covalent bonds. In solution, covalent compounds do not form ions and are therefore poor conductors of electricity
  • An ionic compound is a chemical compound composed of cations and anions held together by ionic bonds and forming extended crystal lattice of alternating ions. In solution, ionic compounds form ions and are therefore good conductors of electricity

Metallic bonds are formed by an electrostatic attractive force between conduction electrons and positively charged metal ions. This type of chemical bonding may be described as the sharing of free electrons among a structure of positively charged ions

  1. Determine the charge of the elements
  2. Balance the positive and negative charges to be neutral
  3. Write the resulting chemical formula
     

Chemical formula of the ionic compound formed between aluminum and fluorine:

  1. Al ⇒ Al3+ ; F ⇒ F-
  2. To be neutral: 1 cation Al3+ for 3 anions F-
  3. Chemical formula: AlF3

Name of the cation + name of the anion with the suffix – ide for non-polyatomic ions. The word ion is omitted. If the metal is a variable charge metal, the charge is indicated in parentheses with Roman numerals
 

CaS = calcium sulfide
CaBr2 = calcium bromide
Fe2(SO4)3 = iron(III) sulfate

The formation of an ionic compound from its corresponding elements is calculated by imagining a 3-step process:

  1. An electron is removed from an atom in the gas phase to form a cation
    Energy of this step: ionization energy of the corresponding element
  2. An electron is added to another atom in the gas phase to form an anion
    Energy of this step: electron affinity EA of the corresponding element
  3. The ions in the gas phase come together to form an ionic bond
    Energy of this step: Coulomb energy Ecoulomb

Coulomb energy Ecoulomb (in J):
 

Ecoulomb = kQ1Q2d

k = Coulomb constant = 231 aJ.pm (1 aJ = 10-18 J)
Q1 = charge of the ion 1
Q2 = charge of the ion 2
d = distance between the centers of the two ions (in m)