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 bonding: A bond formed between a metal cation and a nonmetal anion due to the electrostatic force that binds ions of opposite charge together
  • Covalent bonding: An electron sharing between two nonmetals to allow the atoms to reach an octet in their valence shell
  • Metallic bonding: An electrostatic force of attraction between conduction electrons and positively charged metal ions

Ionic Compounds

Ionic compound:

Cations and anions held together by ionic bonds and forming extended crystal lattices 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 with electrons 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. It decreases according to the multiplicity of the bond: single bond > double bond > triple bond
  • Bond strength: energy required to break a bond. It increases according to the multiplicity of the bond: single bond < double bond < triple bond (when comparing similar bonds: bond length / atomic radius becomes a factor)

Empirical vs. Molecular Formula

  • Empirical formula: the chemical formula that conveys with the smallest possible whole numbers the ratioof combination of atoms in a compound
  • Molecular formula: the chemical formula that gives the number of each type of atom in a molecule

 

Molecule of glucose : 

Empirical formula = CH2O
Molecular formula = C6H12O6

The Law of Constant Composition

Samples of a pure compound always contain the same elements in the same mass proportions. 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 percent of the total mass contributed by each element in a compound. The percent by mass of each element, 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 the 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 the non-polyatomic ions. The word ion is omitted. If the metal is a variable charge metal, the charge is indicated in bracket 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 binary molecular compound: name of the 1st element in 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 beginning 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