General Chemistry I: Hybridisation, Molecular Geometries, and Bonding Forces

Flashcards covering VSEPR theory, steric numbers, hybridisation, molecular shapes, crystal solid types, unit cells, and intermolecular versus intramolecular forces from General Chemistry I (CHM 101).

What is the primary function of the Valence Shell Electron Pair Repulsion (VSEPR) theory?

VSEPR theory is a chemical model used to predict the 3D geometry and shape of molecules based on the mutual electrostatic repulsion of valence electron groups surrounding a central atom.

Who originally proposed the VSEPR theory, and who further developed it in 1957?

It was originally proposed by Nevil Sidgwick and Herbert Powell in 1940, and further developed by Ronald Gillespie and Ronald Nyholm in 1957.

What is the formula used to calculate the Steric Number ($\text{SN}$) of a central atom?

$$\text{Steric Number (SN)} = (\text{Number of atoms bonded to the central atom}) + (\text{Number of lone pairs on the central atom})$$

How are multiple bonds (double or triple bonds) treated when determining electron domains and molecular geometry?

Double or triple bonds count as a single electron "group" or domain.

What are the four sequential steps to solve any molecular geometry problem using steric number?

1. Draw the correct Lewis structure.\n2. Count the bonded atoms (treating multiple bonds as one) and add the lone pairs on the central atom to find the $\text{SN}$.\n3. Match the $\text{SN}$ to the hybridisation and base geometry.\n4. Adjust for lone pairs to state the final molecular shape.

What is the Steric Number ($\text{SN}$), hybridisation, and molecular geometry of methane ($CH_4$)?

Methane has a Steric Number of $4$ ($4$ bonded atoms + $0$ lone pairs), an $sp^3$ hybridisation, and a tetrahedral molecular geometry.

What is the Steric Number ($\text{SN}$), electron-domain geometry, and molecular shape of water ($H_2O$)?

Water has a Steric Number of $4$ ($2$ bonded atoms + $2$ lone pairs), a tetrahedral electron-domain geometry ($sp^3$ hybridisation), and a bent (V-shape) molecular shape with a bond angle of $104.5^\circ$.

What is the Steric Number ($\text{SN}$), hybridisation, and molecular geometry of carbon dioxide ($CO_2$)?

Carbon dioxide has a Steric Number of $2$ ($2$ bonded atoms + $0$ lone pairs), an $sp$ hybridisation, and a linear molecular geometry with a $180^\circ$ bond angle.

What is the Steric Number ($\text{SN}$), hybridisation, and molecular geometry of sulphur trioxide ($SO_3$)?

Sulphur trioxide has a Steric Number of $3$ ($3$ bonded atoms + $0$ lone pairs), an $sp^2$ hybridisation, and a trigonal planar molecular geometry with a $120^\circ$ bond angle.

According to VSEPR theory, what are the formula classification, molecular shape, and bond angle of ammonia ($NH_3$)?

Ammonia has the formula $(AX_3E_1)$, a trigonal pyramidal molecular shape, and a bond angle of $107^\circ$.

What molecular shape and bond angle characterize sulphur dioxide ($SO_2$)?

Sulphur dioxide has a bent (V-shape) molecular shape ($AX_2E_1$ formula) and a bond angle of $< 120^\circ$.

What are the molecular shape and bond angles of phosphorus pentachloride ($PCl_5$)?

Phosphorus pentachloride has a trigonal bipyramidal molecular shape with bond angles of $90^\circ$ and $120^\circ$.

What is hybridisation according to Valence Bond Theory?

Hybridisation is the mathematical mixing of atomic orbitals ($s$, $p$, $d$) of similar energy to form new, identical hybrid orbitals suitable for bonding.

How are $sp^3$ hybrid orbitals formed, and what geometry and bond angle do they produce?

Mixing one $s$ orbital and three $p$ orbitals yields four equivalent $sp^3$ orbitals arranged in a tetrahedral geometry at $109.5^\circ$.

In ethene ($C_2H_4$), how do hybridised and unhybridised orbitals interact to form the double bond?

The three $sp^2$ orbitals form the planar $\sigma$ framework, while the leftover unhybridised $p$ orbitals overlap sideways to form a $\pi$ bond, creating the $C=C$ double bond.

What orbitals mix in $sp^3d^2$ hybridisation, and what are the resulting geometry and bond angles?

One $s$ orbital, three $p$ orbitals, and two $d$ orbitals mix from the same valence shell to form six equivalent orbitals with octahedral (square bipyramidal) geometry, featuring $90^\circ$ angles between adjacent bonds and $180^\circ$ across opposite bonds.

Why do ionic solids conduct electricity when molten but act as poor conductors when solid?

In the solid state, ions are locked in fixed positions in the lattice, whereas when molten, the ions are free to move and carry an electric current.

Why does graphite conduct electricity while diamond acts as an electrical insulator?

Both are covalent network solids, but diamond has all valence electrons fixed in covalent bonds, whereas graphite conducts due to delocalised electrons between its sheets or layers.

What are the coordination number and number of atoms per unit cell for Simple Cubic (SC), Body-Centred Cubic (BCC), and Face-Centred Cubic (FCC) lattices?

• Simple Cubic (SC): Coordination number $6$ ($1\text{ atom/unit cell}$)\n- Body-Centred Cubic (BCC): Coordination number $8$ ($2\text{ atoms/unit cell}$)\n- Face-Centred Cubic (FCC): Coordination number $12$ ($4\text{ atoms/unit cell}$)

What is the primary difference in role and strength between intramolecular forces and intermolecular forces?

Intramolecular forces are strong chemical bonds holding atoms together within a single molecule (energies $\sim 100\text{--}1000\,kJ\,mol^{-1}$), while intermolecular forces are weaker attractions between separate molecules that dictate physical properties such as boiling points, viscosity, and solubility.

What causes London dispersion forces, and what increases their strength?

London dispersion forces arise from instantaneous dipoles created by electron movement; their strength increases with an increasing number of electrons (larger molar mass).

Under what structural condition does hydrogen bonding occur?

Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a highly electronegative, small atom ($N$, $O$, or $F$), allowing the hydrogen to act as a bridge between two electronegative atoms.

What are the electronegativity difference thresholds that classify nonpolar covalent, polar covalent, and ionic bonds?

• Nonpolar covalent: $0.0\text{ to }0.4$\n- Polar covalent: $0.5\text{ to }1.9$\n- Ionic: $> 1.9\text{ to }2.0$

How does the "sea of electrons" model explain the electrical and thermal conductivity of metallic solids?

Valence electrons are delocalised and move freely around a fixed lattice of positive metal cations, facilitating the rapid transport of heat and electric current.