General Chemistry: Molecular Structure, Bonding, Intermolecular Forces, and Stoichiometry Study Guide

Molecular Geometry and Hybridization

Electron Geometry and Molecular Geometry

Understanding the three-dimensional arrangement of atoms in a molecule is essential for predicting chemical behavior. Electron geometry considers all electron groups (bonding and lone pairs), while molecular geometry considers only the positions of atoms.

• Electron Geometry (eg): The spatial arrangement of all electron groups around a central atom.

• Molecular Geometry (mg): The arrangement of only the atoms (ignoring lone pairs).

• Common Geometries:

  • Tetrahedral: 4 electron groups

  • Trigonal bipyramidal: 5 electron groups

  • Octahedral: 6 electron groups

• Example: For XeF4, the electron geometry is octahedral, but the molecular geometry is square planar due to two lone pairs.

Polarity of Molecules

Polarity depends on both the molecular geometry and the difference in electronegativity between atoms. A molecule is polar if it has a net dipole moment.

• Nonpolar: Symmetrical molecules where dipoles cancel.

• Polar: Asymmetrical molecules or those with lone pairs that do not cancel dipoles.

• Example: SF4 is polar due to its seesaw shape, while SF6 is nonpolar (octahedral, symmetrical).

Hybridization

Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals suitable for bonding.

• sp: Linear geometry, 2 electron groups

• sp2: Trigonal planar, 3 electron groups

• sp3: Tetrahedral, 4 electron groups

• sp3d: Trigonal bipyramidal, 5 electron groups

• sp3d2: Octahedral, 6 electron groups

• Example: In HCN, the carbon is sp hybridized.

Determining Hybridization and Geometry in Complex Molecules

To determine the hybridization and geometry of specific atoms in a molecule, count the number of electron groups (bonds and lone pairs) around each atom.

• Example: In a molecule with two labeled carbons, one with three groups (sp2, trigonal planar) and one with four groups (sp3, tetrahedral).

Chemical Bonding

Types of Bonds

Chemical bonds can be classified as nonpolar covalent, polar covalent, or ionic based on the difference in electronegativity between the bonded atoms.

• Nonpolar Covalent: Electrons shared equally (e.g., Cl2).

• Polar Covalent: Electrons shared unequally (e.g., HCl).

• Ionic: Electrons transferred (e.g., NaCl).

• Example: CF4 contains polar bonds but is nonpolar overall due to symmetry.

Pi Bonds

A pi (π) bond is formed by the side-by-side overlap of unhybridized p orbitals. Pi bonds are present in double and triple bonds, in addition to sigma (σ) bonds.

• Single bond: 1 sigma bond

• Double bond: 1 sigma + 1 pi bond

• Triple bond: 1 sigma + 2 pi bonds

• Example: The C=C bond in ethene (C2H4) contains a pi bond.

Intermolecular Forces

Types of Intermolecular Forces

Intermolecular forces (IMFs) are forces of attraction between molecules, affecting physical properties like boiling and melting points.

• Dispersion (London) Forces: Present in all molecules, especially nonpolar ones.

• Dipole-Dipole Forces: Occur between polar molecules.

• Hydrogen Bonding: Strong dipole-dipole interaction when H is bonded to N, O, or F.

• Example: In CH3CH2CH2OH (propanol), all three types are present.

Boiling Point Trends

Boiling point increases with stronger intermolecular forces and higher molecular mass.

• Order of strength: Hydrogen bonding > Dipole-dipole > Dispersion

• Example: H2O has a higher boiling point than N2 or CO due to hydrogen bonding.

Stoichiometry and Chemical Calculations

Molar Mass and Formula Units

Molar mass is the mass of one mole of a substance, expressed in g/mol. It is calculated by summing the atomic masses of all atoms in a formula unit.

• Example: The molar mass of Pb3(PO4)2 is calculated as follows:

$3 \times M_{Pb} + 2 \times (M_P + 4 \times M_O)$

Calculating Moles, Atoms, and Molecules

Use Avogadro's number ($6.022 \times 10^{23}$) to convert between moles and number of particles.

• Moles = $\frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$

• Number of molecules = moles $\times$ Avogadro's number

• Number of atoms = number of molecules $\times$ number of atoms per molecule

• Example: To find the number of Fe atoms in 354 g of Fe:

$\text{Moles Fe} = \frac{354}{55.85}$ $\text{Atoms Fe} = \text{Moles Fe} \times 6.022 \times 10^{23}$

Isotopic Abundance and Average Atomic Mass

The average atomic mass of an element is calculated using the masses and natural abundances of its isotopes.

• Formula:

$\text{Average atomic mass} = \sum (\text{isotope mass} \times \text{fractional abundance})$

• Example: For Ga with two isotopes:

$\text{Average mass} = (68.9256 \times 0.6011) + (70.9247 \times 0.3989)$

Chemical Equations and Balancing

Balancing Chemical Equations

Balancing ensures the same number of each atom on both sides of the equation, in accordance with the Law of Conservation of Mass.

• Adjust coefficients to balance atoms.

• Example: For the reaction $NH_3(g) + O_2(g) \rightarrow N_2(g) + H_2O(l)$, the balanced equation is:

$4 NH_3(g) + 3 O_2(g) \rightarrow 2 N_2(g) + 6 H_2O(l)$

Writing Net Ionic and Molecular Equations

When mixing solutions, write the balanced molecular equation, then the net ionic equation if required.

• Example: Reaction of aluminum acetate with ammonium phosphate to form aluminum phosphate and ammonium acetate:

$Al(C_2H_3O_2)_3(aq) + (NH_4)_3PO_4(aq) \rightarrow AlPO_4(s) + 3 NH_4C_2H_3O_2(aq)$

Reference Data

Periodic Table and Constants

The periodic table provides atomic numbers, symbols, and atomic masses. Constants such as Avogadro's number and the gas constant are essential for calculations.

• Avogadro's number: $6.022 \times 10^{23}$ mol-1

• Gas constant (R): $0.0821$ L·atm·mol-1·K-1

• Standard temperature: $0^\circ$C = 273.15 K

Sample Table: Types of Intermolecular Forces

Type of Force	Occurs Between	Relative Strength	Example
Dispersion (London)	All molecules	Weakest	CH4
Dipole-Dipole	Polar molecules	Intermediate	HCl
Hydrogen Bonding	H with N, O, or F	Strongest	H2O

Sample Table: Hybridization and Geometry

Hybridization	Electron Groups	Geometry	Bond Angle
sp	2	Linear	180°
sp2	3	Trigonal planar	120°
sp3	4	Tetrahedral	109.5°
sp3d	5	Trigonal bipyramidal	90°, 120°
sp3d2	6	Octahedral	90°

Additional info:

• Some context and explanations have been expanded for clarity and completeness.

• Tables have been inferred and constructed to summarize key concepts.