Chemical Bonding and Molecular Structure Flashcards

Comprehensive vocabulary flashcards covering ionic bonding, covalent bonding, VSEPR theory, valence bond theory, and molecular orbital theory based on the BCHCT-131 lecture notes.

Effective Nuclear Charge ($Z^*$)

The net positive nuclear charge felt by valence shell electrons in an atom, calculated as $Z^* = Z - S$, where $Z$ is the atomic number and $S$ is the screening constant.

Screening Constant ($S$)

A value indicating how much other electrons in the same and lower energy levels decrease the nuclear charge felt by a specific electron due to the $Z$ protons in the nucleus.

Valence Shell

The outermost electron shell of an atom containing the valence electrons involved in chemical bonding.

Ionisation Energy ($I$)

The minimum energy required to remove the least tightly bound electron from an isolated gaseous atom in its ground state, expressed in $kJ\,mol^{-1}$.

Ionisation Potential

The ionisation energy of an atom expressed in units of electron volt ($eV$), where $1\,eV = 96.5\,kJ\,mol^{-1}$.

Electron Affinity ($E_a$)

The energy change associated with the gain of an electron by a gaseous atom in its ground state, expressed in $kJ\,mol^{-1}$.

Electronegativity ($\chi$)

A measure of the relative tendency or power of an atom in a chemical bond to attract shared bonding electrons toward itself.

Ionic Bond

A chemical bond formed by the complete transfer of one or more valence electrons from one atom to another, producing positively and negatively charged ions held together by electrostatic attraction.

Covalent Bond

A chemical bond resulting from the mutual sharing of one or more pairs of valence electrons between atoms of the same or different elements.

Coordinate Bond

A type of covalent bond formed when both shared electrons of the bonding pair are contributed by a single atom known as the donor.

Coordination Number

The maximum number of oppositely charged ions surrounding a central ion in an ionic crystal lattice.

Lattice Energy ($\Delta H_L$)

The energy released when one mole of a solid ionic compound is formed from its constituent gaseous ions.

Enthalpy of Formation ($\Delta H_f$)

The overall enthalpy change involved in the formation of one mole of a compound from its constituent elements in their standard states.

Born-Haber Cycle

A thermochemical cycle based on Hess's law of heat summation used to calculate experimental lattice energy or electron affinity values from measurable thermodynamic quantities.

Born-Landé Equation

An equation based on the electrostatic model used to calculate theoretical lattice energy: $U = -\frac{1.389 \times 10^4\,A Z_+ Z_-}{a} \left(1 - \frac{1}{n}\right)\,kJ\,mol^{-1}$.

Hydration Energy

The amount of energy released when one mole of gaseous ions is dissolved in a large volume of water to form an infinitely dilute solution.

Enthalpy of Solution ($\Delta H_{\text{sol}}$)

The net enthalpy change associated with dissolving one mole of an ionic solid in a solvent to form a solution of infinite dilution, given by $\Delta H_{\text{sol}} = \Delta H_{\text{hyd}} - \Delta H_L$.

Polarisation

The distortion of the spherical electron cloud of an anion caused by the electrostatic attraction exerted by an adjacent cation.

Polarising Power

The ability of a cation to attract and distort the electron cloud of an adjacent anion.

Polarisability

The ease with which the electron cloud of an anion or molecule can be distorted by an adjacent cation or an applied electric field.

Fajan's Rules

A set of empirical rules stating that covalent character in an ionic bond is favoured by small, highly charged cations, large, highly charged anions, and cations with non-noble gas electronic configurations.

Bond Polarity

A measure of how unequally the bonding electron pair is shared between two atoms in a chemical bond.

Dipole Moment ($\mu$)

The quantitative measure of molecular polarity, defined as the product of the magnitude of electric charge ($q$) and the separation distance ($r$): $\mu = q \times r$.

Molar Polarisation ($P_M$)

The total dielectric polarisation produced in one mole of a substance subjected to an electric field, expressed as $P_M = \frac{N_A}{3\varepsilon_0}\left(\alpha + \frac{\mu^2}{3kT}\right)$.

Lewis Electron-Dot Symbols

Representations of atoms or ions where the chemical symbol of the element is surrounded by dots corresponding to the number of its valence electrons.

Formal Charge

The theoretical charge assigned to an atom in a molecule, calculated as: $\text{Formal Charge} = \text{Valence Electrons} - \text{Unshared Electrons} - \text{Bonding Pairs}$.

Bond Length

The equilibrium interatomic distance between the centers of two covalently bonded atomic nuclei.

Covalent Radius

Half of the interatomic distance of a single covalent bond formed between two identical atoms of the same element.

Bond-Dissociation Energy

The average enthalpy change required to break one mole of a specific covalent bond in a gaseous substance.

Valence Shell Electron Pair Repulsion (VSEPR) Theory

A model predicting molecular geometry based on the principle that electron pairs around a central atom arrange themselves in space to minimize mutual electrostatic repulsion.

Valence Bond (VB) Theory

A quantum mechanical model describing a covalent bond as the pairing of valence electron spins resulting from the overlap of atomic orbitals of individual atoms that retain their identity.

Born-Oppenheimer Approximation

An approximation in molecular quantum mechanics that treats atomic nuclei as stationary relative to electron motion due to their substantially larger mass.

Resonance

A concept describing a molecule whose true electronic structure is an intermediate hybrid of two or more contributing canonical Lewis structures differing only in electron placement.

Resonance Energy

The energy difference between the actual experimental enthalpy of a resonance-stabilised molecule and the calculated enthalpy of its most stable contributing canonical structure.

Hybridisation

The mathematical mixing of atomic orbitals of different energies on the same atom to generate an equal number of new, equivalent hybrid orbitals of lower total energy.

$\sigma$ (Sigma) Bond

A covalent bond formed by the head-on overlap of atomic orbitals along the internuclear axis, giving an electron density that is axially symmetrical.

$\pi$ (Pi) Bond

A covalent bond formed by the side-by-side (lateral) overlap of atomic orbitals, creating electron density concentrated above and below the internuclear axis.

Molecular Orbital (MO) Theory

A quantum mechanical approach where atomic orbitals combine to form polycentric molecular orbitals that encompass all nuclei in a molecule.

Linear Combination of Atomic Orbitals (LCAO)

A mathematical method constructing molecular orbital wave functions through the additive or subtractive superposition of individual atomic orbital wave functions.

Constructive and Destructive Interference

Superposition of wave functions where in-phase waves combine additively to increase amplitude (constructive), while out-of-phase waves combine subtractively to cancel amplitude (destructive).

s-s Combination of Atomic Orbitals

The linear combination of two $1s$ atomic orbitals producing a lower-energy $\sigma 1s$ bonding molecular orbital and a higher-energy $\sigma^* 1s$ antibonding molecular orbital.

Bonding Molecular Orbital

A molecular orbital formed by constructive interference of atomic orbital wave functions, characterised by increased electron density between nuclei and lower energy than isolated atomic orbitals.

Antibonding Molecular Orbital

A molecular orbital formed by destructive interference of atomic orbital wave functions, characterised by a nodal plane between nuclei and higher energy than isolated atomic orbitals.

Nonbonding Molecular Orbital

A molecular orbital formed when atomic orbital combination results in zero net overlap, leaving orbital energy unchanged relative to isolated atomic orbitals.

Bond Order

A measure of net bonding strength in a molecule, defined as half the difference between the number of bonding electrons ($N_b$) and antibonding electrons ($N_a$): $\text{Bond Order} = \frac{1}{2}(N_b - N_a)$.

Gerade ($g$)

A symmetry designation for a molecular orbital whose wave function sign remains unchanged upon inversion through its center of symmetry.

Ungerade ($u$)

A symmetry designation for a molecular orbital whose wave function sign changes upon inversion through its center of symmetry.

Paramagnetic Substance

A substance containing one or more unpaired electron spins, causing it to be weakly attracted into an external magnetic field.