Chemistry · Chapter 5
Study notes aligned to the official NEB syllabus.
A chemical bond is the attractive force that holds two or more atoms together in a molecule or a crystal. Atoms combine because a bonded arrangement has lower energy (greater stability) than the separate atoms.
Why atoms combine (octet theory / electronic theory of valency). The octet rule (Kossel and Lewis) states that atoms tend to gain, lose, or share electrons so as to acquire the stable eight-electron outer configuration of the nearest noble gas (or two electrons, the duplet, for hydrogen, lithium, etc.). The valence shell is the outermost shell, and the electrons in it are the valence electrons that take part in bonding.
The three principal types of strong bonds are the ionic (electrovalent) bond, the covalent bond, and the metallic bond.
An ionic bond is formed by the complete transfer of one or more electrons from a metal atom (low ionization energy) to a non-metal atom (high electron affinity). The resulting oppositely charged ions are held together by strong electrostatic force of attraction.
Example - sodium chloride. Sodium ($2,8,1$) loses one electron; chlorine ($2,8,7$) gains it:
$$ \begin{aligned} \ce{Na -> Na+ + e-} \qquad \ce{Cl + e- -> Cl-} \ \ce{Na+ + Cl- -> NaCl} \end{aligned} $$
Both ions now have a noble-gas octet ($\ce{Na+}$ like neon, $\ce{Cl-}$ like argon).
Conditions favouring ionic bonding: low ionization energy of the metal, high electron affinity of the non-metal, large electronegativity difference, and high lattice energy.
Properties of ionic compounds: high melting and boiling points (strong lattice forces); crystalline solids; conduct electricity when molten or dissolved (ions become free) but not when solid; generally soluble in polar solvents like water; and they are hard but brittle.
A covalent bond is formed by the mutual sharing of one or more pairs of electrons between two atoms, each atom contributing one electron to the shared pair. It usually forms between two non-metal atoms of similar electronegativity.
Properties of covalent compounds: usually gases, liquids, or soft solids with low melting/boiling points (weak intermolecular forces); poor conductors of electricity (no free ions); generally soluble in non-polar (organic) solvents; and they show directional bonding, giving definite molecular shapes.
A chemical bond is the attractive force that holds two or more atoms together in a molecule or a crystal. Atoms combine because a bonded arrangement has lower energy (greater stability) than the separate atoms.
Why atoms combine (octet theory / electronic theory of valency). The octet rule (Kossel and Lewis) states that atoms tend to gain, lose, or share electrons so as to acquire the stable eight-electron outer configuration of the nearest noble gas (or two electrons, the duplet, for hydrogen, lithium, etc.). The valence shell is the outermost shell, and the electrons in it are the valence electrons that take part in bonding.
The three principal types of strong bonds are the ionic (electrovalent) bond, the covalent bond, and the metallic bond.
An ionic bond is formed by the complete transfer of one or more electrons from a metal atom (low ionization energy) to a non-metal atom (high electron affinity). The resulting oppositely charged ions are held together by strong electrostatic force of attraction.
Example - sodium chloride. Sodium () loses one electron; chlorine () gains it:
Both ions now have a noble-gas octet ( like neon, like argon).
Conditions favouring ionic bonding: low ionization energy of the metal, high electron affinity of the non-metal, large electronegativity difference, and high lattice energy.
Properties of ionic compounds: high melting and boiling points (strong lattice forces); crystalline solids; conduct electricity when molten or dissolved (ions become free) but not when solid; generally soluble in polar solvents like water; and they are hard but brittle.
A covalent bond is formed by the mutual sharing of one or more pairs of electrons between two atoms, each atom contributing one electron to the shared pair. It usually forms between two non-metal atoms of similar electronegativity.
Properties of covalent compounds: usually gases, liquids, or soft solids with low melting/boiling points (weak intermolecular forces); poor conductors of electricity (no free ions); generally soluble in non-polar (organic) solvents; and they show directional bonding, giving definite molecular shapes.