Ionic and Covalent Bonding
Lattice enthalpy, Lewis structures, bond parameters and Fajans' rules. (Chemistry › Chemical Bonding and Molecular Structure, NEET UG syllabus.)
What is Ionic and Covalent Bonding?
A chemical bond formed by the complete transfer of one or more electrons from one atom to another, resulting in the formation of oppositely charged ions that are held together by electrostatic forces.
Key formula / rule: Formal Charge
Key points
- Differentiate between ionic and covalent bonds based on electron transfer vs. sharing.
- Explain the factors influencing the formation of ionic and covalent bonds.
- Define and explain lattice enthalpy and its influencing factors.
- Draw correct Lewis structures for simple molecules and polyatomic ions, including octet rule exceptions.
Common exam trap
Confusing lattice enthalpy (for ionic compounds) with bond enthalpy (for covalent bonds).
Definitions
- Term
Ionic Bond
- Meaning
A chemical bond formed by the complete transfer of one or more electrons from one atom to another, resulting in the formation of oppositely charged ions that are held together by electrostatic forces.
- Term
Covalent Bond
- Meaning
A chemical bond formed by the mutual sharing of one or more pairs of electrons between two atoms.
- Term
Lattice Enthalpy
- Meaning
The energy released when one mole of an ionic compound is formed from its constituent gaseous ions under standard conditions. It is a measure of the strength of the ionic bond.
- Term
Lewis Structure
- Meaning
A diagram that shows the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule, representing valence electrons as dots.
- Term
Bond Length
- Meaning
The equilibrium distance between the nuclei of two atoms bonded together in a molecule.
- Term
Bond Angle
- Meaning
The angle formed between the orbitals containing bonding electron pairs around the central atom in a molecule.
- Term
Bond Enthalpy (Bond Energy)
- Meaning
The amount of energy required to break one mole of a particular type of bond in a gaseous molecule into its constituent gaseous atoms.
- Term
Bond Order
- Meaning
The number of chemical bonds between a pair of atoms. It indicates the stability of a bond.
- Term
Resonance
- Meaning
A phenomenon where the bonding in a molecule or ion cannot be represented by a single Lewis structure, but rather by a combination of two or more contributing structures (resonance structures) to form a resonance hybrid.
- Term
Fajans' Rules
- Meaning
A set of empirical rules that predict the ° of covalent character in an ionic bond based on the size and charge of the ions involved.
- Term
Polarizing Power
- Meaning
The ability of a cation to distort the electron cloud of an anion.
- Term
Polarizability
- Meaning
The ease with which the electron cloud of an anion can be distorted by a cation.
Learning objectives
Differentiate between ionic and covalent bonds based on electron transfer vs. sharing.
Explain the factors influencing the formation of ionic and covalent bonds.
Define and explain lattice enthalpy and its influencing factors.
Draw correct Lewis structures for simple molecules and polyatomic ions, including octet rule exceptions.
Calculate formal charges for atoms in Lewis structures.
Define and explain various bond parameters (length, angle, enthalpy, order).
Describe the concept of resonance and draw resonance structures.
State and apply Fajans' rules to predict the extent of covalent character in ionic compounds.
Formulae
- Name
Formal Charge
- Note
Used to determine the most stable Lewis structure by minimizing formal charges.
- Expression
Formal Charge = (Total number of valence electrons in the free atom) - (Total number of non-bonding (lone pair) electrons) - (1/2 * Total number of bonding (shared) electrons)
- Name
Born-Haber Cycle (Components for Lattice Enthalpy)
- Note
Where ΔHf is enthalpy of formation, ΔHsub is enthalpy of sublimation, IE is ionization enthalpy, ΔHdiss is bond dissociation enthalpy, EA is electron gain enthalpy, and UL is lattice enthalpy. This is an energy cycle, not a single formula.
- Expression
ΔHf = ΔHsub + IE + ΔHdiss + EA + UL
Prerequisites
Basic understanding of atomic structure (protons, neutrons, electrons).
Knowledge of electron configuration and valence electrons.
Familiarity with periodic trends (ionization enthalpy, electron gain enthalpy, electronegativity, atomic size).
Concept of stability and noble gas configuration.
Common mistakes
Confusing lattice enthalpy (for ionic compounds) with bond enthalpy (for covalent bonds).
Incorrectly applying the octet rule to elements that can have incomplete or expanded octets.
Failing to calculate formal charges correctly when drawing Lewis structures, leading to incorrect preferred structures.
Misinterpreting the factors in Fajans' rules, especially the effect of cation/anion size on polarizing power/polarizability.
Not recognizing resonance structures when applicable, leading to an incomplete understanding of molecular stability and properties.
Keywords
Ionic bond
Covalent bond
Lattice enthalpy
Lewis structure
Octet rule
Formal charge
Bond length
Bond angle
Bond enthalpy
Bond order
Resonance
Fajans' rules
Polarizing power
Polarizability
Born-Haber cycle
Practice preview
Lattice enthalpy is defined as the energy required to:…
easy
Which of the following compounds has the highest ionic character?…
medium
Among the alkali metal halides, the melting point of LiF is significantly higher than that of LiCl, LiBr, and LiI. This can be primarily attributed to:…
hard