Chemical Bonding and Ionic Bonds

Study Notes

Chemical Bonding

A chemical bond is an electrostatic force that holds atoms together in a compound.
When atoms are closer together, the system has lower energy, indicating bond formation.
There are three types of chemical bonds: ionic, metallic, and covalent.

Ionic Bonding

Ionic compounds form a 3D lattice structure, influenced by ion radii and stoichiometry.
Electrons are transferred to form cations (positively charged) and anions (negatively charged).
Ionic bonds arise from electrostatic attraction between oppositely charged ions.
The strength of attraction depends on ion size and charge (charge density).
Ionic interactions are isotropic meaning they are non-directional.
Electronegativity differences greater than 2 generally lead to ionic bonds.
Compounds formed between Group 1 and 2 with Group 16 and 17 elements are typically ionic.
Lattice energy measures the strength of ionic attraction and influences properties such as melting point, hardness, and solubility.
Lattice energy is the energy required to separate one mole of ionic solid into gaseous ions.
- For example, NaCl(s) → Na+(g) + Cl–(g) LE = +788 kJ mol-1
- For example, Na2O(s) → 2Na+(g) + O2–(g) LE = +2488 kJ mol-1
- For example, MgO(s) → Mg2+(g) + O2–(g) LE = +3800 kJ mol-1
Ionic compounds typically exhibit the following properties:
- They exist as hard, crystalline, and brittle solids.
- They have high melting points.
- They do not conduct electricity in solid form.
- They conduct electricity when molten or in solution (for soluble compounds) as ions are released to carry the current.

Metallic Bonding

Elements with low ionization energies often have mobile valence electrons, creating a "sea of electrons" and metallic bonding.
For example: Fe(s)
Metals typically show the following properties:
- Good electrical conductivity in both solid and molten states due to mobile electrons.
- Malleable and ductile (atoms can slide across each other in the electron sea).
- Variable, but often low melting points due to the relatively weak attraction between nuclei and mobile electrons.
- High boiling points as the attraction between nuclei and mobile electrons must be overcome.

Covalent Bonding

In covalent bonds, electrons are shared between two atoms.
The bond length is the specific distance between the bonding nuclei where the molecule's energy is minimized.
A single covalent bond involves a pair of shared electrons.
Some atoms can share more than one electron pair, leading to double bonds (two shared pairs) or triple bonds (three shared pairs).
Lone pairs or non-bonding pairs of electrons refer to pairs of electrons in the valence shell that are not involved in bonding.
Polar covalent bonds form when electrons are shared unequally between two different atoms due to electronegativity differences.
The atom with higher electronegativity (EN) attracts the electron pair, resulting in a partial negative charge, while the other atom has a partial positive charge.

An indirect correlation exists between bond strength and bond length: stronger bonds are generally shorter.

                                 Bond     Energy Bond length
                                         (kJ mol-1) (pm)
                                  H-H       432           74
                                  C-H       413           109
                                 Cl-Cl      239           199
                                 Br-Br      193           228
                                  C-C       347           154
                                  C=C       614           134
                                  C≡C       839           121