Ionic Bonds and Compounds | General Chemistry 1

Ionic bonds and compounds are studied in this chapter: the difference between ionic and covalent bonds, the ionic charges and chemical formulas, the transition metal ions, the sizes of ions, the ionic bond energies.

Ionic and Covalent Bonds

Octet rule: Atoms tend to form molecules in such a way as to reach an octet in the valence shell and attain a noble-gas configuration.

 

Carbon: Z=6 ⇒ 6 electrons ⇒ 1s22s22p2 ⇒ 4 electrons in its valence shell
C needs 4 additional electrons to get Neon’s electronic configuration.
C will form 4 covalent bonds with other atoms in order to get 4 additional electrons.

 


The properties of substances are determined by the type of bonding within the substance.

There are 3 different types:
 

Ionic bonding = bond form between a metal and a nonmetal
The electrostatic force that binds oppositely charged ions together (formed by the loss or gain of electrons by an element in order to follow the octet rule)
Ionic compounds tends to form 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 Cl readily gains one e-:
Na+ + Cl- → NaCl. There is an ionic bond between Na and Cl.

 

Covalent bonding = electron sharing between two nonmetals
Covalent compounds tend to exist as individual molecules with shared electrons
In solution, covalent compounds do not form ions and are therefore poor conductors of electricity


Metallic bonding = electrostatic attractive force between conduction electrons and positively charged metal ions.

Ionic Charges and Chemical Formulas

Chemical formula of 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

Transition Metal Ions

Ground state configuration of transition metal ions ≠ order as neutral atoms:
- energy of 3d < energy of 4s
- energy of 4d < energy of 5s
Order of orbital energies: 1s < 2s < 2p < 3s < 3p < 3d < 4s < 4p < 4d < 5s


18-electron rule: one relatively stable electron configuration for transition metals is a configuration with 18 electrons in the outer shell.
Transition metals do not follow the octet rule.
 

Ag = 47 ⇒ 1s22s22p63s23p63d104s24p64d105s1
Ag tends to lose the e- in the 5s orbital ⇒ Ag+ ⇒ 18 electrons in the outer shell

 

Caution: this rule is not always true (16 electrons or half-filled orbitals are also observed for example)
 

Ni: Z = 28 ⇒ 1s22s22p63s23p63d84s2
Ni tends to lose the 2 e- in the 4s orbital ⇒ Ni2+ ⇒ 16 electrons in the outer shell

Sizes of Ions

Cations: no more (or less) electrons in the outer shell of the corresponding neutral parent atom + excess positive charge toward the nucleus
⇒ cations are smaller than neutral parent atom
 

Mg: Z = 12 ⇒ 1s22s22p63s2; Mg2+ ⇒ 1s22s22p6 ⇒ no more electrons in the shell n = 3.
Mg2+ is smaller than Mg

 

Anions: increasing electron-electron repulsion
⇒ anions are larger than neutral parent atom

Ionic Bond Energies

Formation of an ionic compound from its corresponding elements is calculated by imagining a 3 steps process:


1) An electron is removed from one atom in the gas phase to form a cation
⇒ Energy of this step = ionization energy of the corresponding element (endothermic)

2) An electron is added to another atom in the gas phase to form an anion
⇒ Energy of this step = electron affinity of the corresponding element (exothermic)

3) The gas phase ions come together to form an ionic bond
⇒ Energy of this step = Coulomb energy (exothermic)

 

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

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

3) Step 3: Na+ (g) + Cl- (g) → Na+Cl- (g)
Energy = Coulomb’s energy = Ecoulomb = 231 x +1-1283 = -0.816 aJ
(dNa-Cl = 283 ppm)


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