Ionic and Covalent Compounds | General Chemistry 1

Ionic and covalent compounds are studied in this chapter: difference types of bonding, ionic compounds and their chemical formula, empirical vs. molecular formula, the law of constant composition, naming binary compounds, ionic bond energies

Types of Bonding

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

  • Ionic bonds: bonds formed between a metal cation and a non-metal anion due to the electrostatic force that binds ions of opposite charge
  • Covalent bonds: bonds that involve the sharing of a pair of electrons between two non-metals to allow the atoms to reach an octet in their valence shell
  • Metallic bonds: bonds 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

Ionic Compounds

Ionic compound:

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
 

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

 

How to determine the chemical formula of an ionic compound:

  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

Covalent Compounds

Covalent compound:

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
 

Cl: Z = 17 ⇒ 1s2s2p3s3p5 ⇒ 7 valence electrons

If 2 chlorine atoms each share an electron:

  •  they have 8 electrons in their outer shell
  • they reach the electron configuration of argon 

⇒ formation of Cl2

 

Bond length vs. bond strength:

  • Bond length: distance between 2 covalently bonded nuclei.
    Bond length decreases with increasing bond multiplicity: single bond > double bond > triple bond. It is also directly proportional to the size of the atoms: the bond length increases with increasing atomic size
  • Bond strength: energy required to break a bond.
    Bond strength increases as the multiplicity of the bond increases: single bond < double bond < triple bond. When comparing bonds of the same multiplicity, the strength of the bond generally increases as the bond length decreases

Empirical vs. Molecular Formula

  • Empirical formula: the chemical formula with the simplest whole number ratio of atoms present in a compound
  • Molecular formula: the chemical formula that gives the number of each element in a compound

 

Molecule of glucose : 

Empirical formula = CH2O
Molecular formula = C6H12O6

The Law of Constant Composition

The law of constant composition states that samples of a pure compound are always made up of the same elements in the same proportion or mass ratio. From a molecular or empirical formula, we can calculate what percentage of the total mass is contributed by each element in a compound
 

Percent composition by mass:

The percentage of the total mass contributed by each element in a compound. The mass percent of an element in a compound, for example A in AB, is given by:
 

%A = n x atomic mass of Amolecular (or formula) mass of AB x 100%

n = number of atoms A in a molecule (or formula unit) of compound AB

 

Percent composition of H2O:

%H = 2 x 1.00818.016 x 100 = 11.19%

%O = 16.0018.016 x 100 = 88.81%

MH = 1.008 amu
MO = 16.00 amu
MH2O = 18.016 amu

Naming Binary Compounds

Ionic compound

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

Name of an ionic compound: 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

 

Molecular compound

Binary molecular compounds are composed of 2 nonmetals

Name of a binary molecular compound: name of the 1st element of the formula + name of the 2nd element with the suffix ide. Prefixes are used to indicate the number of atoms of a given element in a molecule (mono- = 1, di- = 2, tri- = 3, tetra- = 4 …). The prefix mono- is generally omitted for the first element. The final a or o of the prefix is combined with a name starting with a vowel
 

HCl = hydrogen chloride
CO2 = carbon dioxide
PCl5 = phosphorus pentachloride

Ionic Bond Energies

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. 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)

 

Formation energy of the ionic compound 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: Coulomb’s energy = Ecoulomb = 231 x +1-1283 = -0.816 aJ

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

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

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

  • 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
  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
  • Bond length is the distance between 2 covalently bonded nuclei
  • Bond strength is the energy required to break a bond

Bond length decreases with increasing bond multiplicity: single bond > double bond > triple bond. It is also directly proportional to the size of the atoms: the bond length increases with increasing atomic size

The strength of a bond between 2 atoms increases as the multiplicity of the bond increases: single bond < double bond < triple bond. When comparing bonds of the same multiplicity, the strength of the bond generally increases as the bond length decreases

The molecular formula is the chemical formula that gives the number of each element in a compound, while the empirical formula is the simplest whole number ratio of atoms present in a compound
 

Molecule of glucose : 

Molecular formula = C6H12O6
Empirical formula = CH2O

The law of constant composition states that samples of a pure compound are always made up of the same elements in the same proportion or mass ratio

Percent composition by mass is the percentage of the total mass contributed by each element in a compound

The mass percent of an element in a compound, for example A in AB, is given by:
 

%A = n x atomic mass of Amolecular (or formula) mass of AB x 100%

n = number of atoms A in a molecule (or formula unit) of compound AB

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

Name of the 1st element of the formula + name of the 2nd element with the suffix – ide. Prefixes are used to indicate the number of atoms of a given element in a molecule (mono- = 1, di- = 2, tri- = 3, tetra- = 4 …). The prefix mono- is generally omitted for the first element. The final a or o of the prefix is combined with a name starting with a vowel
 

HCl = hydrogen chloride
CO= carbon dioxide
PCl5 = phosphorus pentachloride