Chemical bond: A mutual electrical attraction between the nuclei and valence electrons of different atoms.
Ionic bonding: Type of chemical bonding that results from the electrical attraction between cations and anions.
Covalent bonding: Type of chemical bonding resulting from the sharing of electron pairs between two atoms.
Nonpolar-covalent bond: A covalent bond in which the bonding electrons are shared equally by the bonded atoms.
Polar-covalent bond: A covalent bond in which the bonded atoms have an unequal attraction for the shared electrons.
Introduction to Chemical Bonding
Atoms seldom exist as independent particles in nature. They are usually bonded to form stable arrangements.
Main Idea: Atoms form compounds by gaining, losing, or sharing electrons, which minimizes their potential energy.
Types of Chemical Bonds
Ionic and Covalent Bonds
Ionic Bonding: Occurs when atoms completely give up electrons to other atoms, resulting in the formation of cations (positive ions) and anions (negative ions).
Covalent Bonding: Involves the sharing of electron pairs between atoms. In purely covalent bonding, the shared electrons are owned equally by the two bonded atoms.
Character of Bonds
Bonding is not purely ionic or covalent but falls somewhere in between, determined by the electronegativity of the atoms involved.
Electronegativity: A measure of an atom's ability to attract electrons. The greater the difference in electronegativity, the more ionic character a bond possesses.
Ionic character classification:
1.7 or less: More covalent
0.3 to 1.7: Polar covalent
d 0.3 or less: Nonpolar covalent
Hydrogen (H): Forms nonpolar covalent bonds with itself (H₂).
Covalent Bonding and Molecular Compounds
Molecules and Molecular Compounds
A molecule: Neutral group of atoms held together by covalent bonds.
A molecular compound: Composition is given by its chemical formula, indicating the relative numbers of atoms involved.
Molecular formula: Shows the types and counts of atoms in a molecule. Example: Water (H₂O).
A diatomic molecule: Contains only two atoms (e.g., O₂).
Bond Energy and Structure
Bond Energy: The energy required to break a chemical bond.
Bond lengths vary and are indicative of the strength and energy of the covalent bond.
The bond length of a hydrogen-hydrogen bond is 75 pm.
Energy Relation in Covalent Bond Formation
As atoms approach, potential energy decreases until a stable bond forms (minimum potential energy).
Atoms typically maintain a bond length at which their potential energy is minimized.
The Octet Rule and Exceptions
The Octet Rule
Atoms tend to form bonds to achieve a noble gas electron configuration (8 electrons in the valence shell).
Example with Fluorine (F): F₂ molecule shares electrons to fill its valence shell creating an octet.
Exceptions to the Octet Rule
Hydrogen: Forms covalent bonds with only 2 electrons.
Boron: May be surrounded by 6 electrons.
Certain elements can participate in expanded octets when they form bonds with highly electronegative elements (e.g., SF₆).
Lewis Structures for Covalent Bonds
Electron-dot notation: Used to represent valence electrons and visualize bond formations.
A Lewis structure represents atomic symbols for nuclei and inner-shell electrons and uses dot pairs or dashes for shared electron pairs.
Examples
Hydrogen bonds: H-H (single bond)
Fluorine molecule representation: :F:F: for two bonded fluorine atoms.
Types of Covalent Bonds
Single bond: One pair of electrons shared.
Double bond: Two pairs of electrons shared, represented as C=C.
Triple bond: Three pairs of electrons shared, represented as N≡N.
Bond Energies Comparison
Generally, double bonds are shorter and stronger than single bonds, while triple bonds are the strongest.
Resonance Structures
Some molecules cannot be represented by a single Lewis structure and require resonance structures that average multiple configurations. Example: O₃ (ozone).
Ionic Bonding and Ionic Compounds
Characteristics
Ionic Compounds: Composed of positive and negative ions held together by ionic bonds.
Formula Unit: The simplest collection representing the ionic compound's formula (e.g., NaCl).
Ionic compounds typically form crystalline solids with orderly arrangements of ions minimizing potential energy.
Characteristics of Ionic Bonding
Ionic bonds result from the attraction between cations and anions.
Strong attractive forces between oppositely charged ions give ionic compounds high melting and boiling points making them hard and brittle.
Properties of Ionic Compounds
Ionic compounds conduct electricity only when molten or dissolved in water as their ions can then move freely.
Metallic Bonding
Properties and Structure
Metallic Bonding: Involves the sharing of free electrons among a lattice of metal atoms. This results in high conductivity and malleability.
Metals can be hammered into shapes due to the flexibility of metallic bonds under applied force without breaking.
Conclusion
Understanding chemical bonding, along with its types and characteristics, provides a basis for predicting the behavior and properties of various compounds in chemical reactions.