Atoms, Molecules, and Chemical Quantities: Study Notes for GOB Chemistry

Atoms, Molecules, and Chemical Quantities

Vocabulary and Key Concepts

This section introduces foundational terms and concepts essential for understanding atomic structure, molecular geometry, and chemical quantities in GOB Chemistry.

• Valence Electrons: Outer electrons responsible for bonding between atoms.

• Core Electrons: Inner electrons not involved in bonding.

• Lewis Dot Structures: 2D representations of compounds showing valence electrons as dots around element symbols.

• Octet Rule: Atoms tend to have 8 electrons in their valence shell (except for hydrogen and helium).

• VSEPR Theory: 3D representation of compounds based on electron pair repulsion.

• Electron Geometry (EG): Indicates how many things (atoms or lone pairs) are attached to a central atom.

• Molecular Geometry (MG): Differentiates between electron lone pairs and atoms bonded to the central atom.

• Bond Polarity: Indicates difference in electronegativity between atoms in a bond.

• Molecular Polarity: Describes how equally electrons are shared in a molecule.

• Intermolecular Forces: Forces between molecules, including dipole-dipole, hydrogen bonding, and London dispersion.

• Dipole-Dipole: Unequal sharing of electrons between polar molecules.

• Hydrogen Bonding: Strong intermolecular force occurring only in N-H, O-H, and F-H bonds.

• London Dispersion Forces: Weak forces present in all compounds due to temporary dipoles.

• Mole: Specific unit representing $6.022 \times 10^{23}$ particles.

• Avogadro's Number: $6.022 \times 10^{23}$, the number of particles in one mole.

• Molar Mass: Sum of atomic masses from the periodic table for a compound, expressed in g/mol.

• Dimensional Analysis: Tool used to convert units in chemical calculations.

• Scientific Notation: Allows for small/large numbers to be written efficiently (e.g., $6.022 \times 10^{23}$).

• Stoichiometry: Conversion between element/compound quantities utilizing subscripts and molar ratios.

Lewis Dot Structures and Molecular Geometry

Drawing Lewis Dot Structures

Lewis dot structures are used to represent the arrangement of valence electrons in molecules. They help predict molecular shape and polarity.

• NH3 (Ammonia): N has 5 valence electrons, each H has 1. Total: 8 electrons. EG: Tetrahedral, MG: Trigonal pyramidal. Polar.

• CO2 (Carbon Dioxide): C has 4, each O has 6. Total: 16 electrons. EG: Linear, MG: Linear. Non-polar.

• PCl3 (Phosphorus Trichloride): P has 5, each Cl has 7. Total: 26 electrons.

• H2S (Hydrogen Sulfide): H has 1, S has 6. Total: 8 electrons. EG: Tetrahedral, MG: Bent. Polar.

• CCl4 (Carbon Tetrachloride): C has 4, each Cl has 7. Total: 32 electrons. EG: Tetrahedral, MG: Tetrahedral.

• BF3 (Boron Trifluoride): B has 3, each F has 7. Total: 24 electrons. Non-polar, all electrons are shared equally.

• PBr3 (Phosphorus Tribromide): P has 5, each Br has 7. Total: 26 electrons. Polar, lone pair on P makes electrons unequally shared.

• CF3Br (Bromotrifluoromethane): C has 4, each F has 7, Br has 7. Total: 32 electrons. EG: Tetrahedral, MG: Tetrahedral.

Electron Geometry vs. Molecular Geometry

Electron geometry considers all electron groups (bonds and lone pairs) around the central atom, while molecular geometry considers only the arrangement of atoms.

• Electron Geometry (EG): Tells how many things (atoms or lone pairs) are attached to the central atom. Example: Tetrahedral (4 things).

• Molecular Geometry (MG): Differentiates between electron lone pairs and atoms bonded to the central atom. Example: Trigonal pyramidal (3 atoms, 1 lone pair).

Intermolecular Forces

Types of Intermolecular Forces

Intermolecular forces determine physical properties such as boiling and melting points.

• Hydrogen Bonding: Occurs in molecules with N-H, O-H, or F-H bonds (e.g., H2O, NH3).

• Dipole-Dipole: Occurs in polar molecules (e.g., HBr, NaCl).

• London Dispersion: Present in all molecules, especially non-polar ones (e.g., CH4).

Examples:

• H2O → Hydrogen bonding

• HBr → Dipole-dipole

• CH4 → London dispersion

• NH3 → Hydrogen bonding

• NaCl → Dipole-dipole

Chemical Quantities and Calculations

Mole Calculations and Avogadro's Number

The mole is a fundamental unit in chemistry for counting particles. Avogadro's number ($6.022 \times 10^{23}$) is used to convert between moles and number of particles.

• Number of particles in moles: $\text{Particles} = \text{moles} \times 6.022 \times 10^{23}$

• Example: 2.5 moles RbCl × $6.022 \times 10^{23}$ = $1.6 \times 10^{24}$ particles

Dimensional Analysis and Unit Conversion

Dimensional analysis is used to convert between units such as grams, moles, and number of particles.

• Converting Celsius to Kelvin: $T_K = T_C + 273$

• Example: 45°C + 273 = 318 K

Molar Mass Calculations

Molar mass is calculated by summing the atomic masses of all atoms in a compound.

• Example: Cr(NO3)3: Cr = 51.996, N = 14.007 × 3, O = 15.999 × 9. Total = 238.012 g/mol

Stoichiometry: Mass, Mole, and Particle Conversions

Stoichiometry involves converting between mass, moles, and number of particles using molar mass and Avogadro's number.

• Example: How many molecules in 10.0 g H2O?

  • 10.0 g H2O × (1 mole / 18.015 g) × ($6.022 \times 10^{23}$ molecules / 1 mole) = $3.34 \times 10^{23}$ molecules

• Example: How many grams in $5.00 \times 10^{24}$ molecules of C2H6?

  • $5.00 \times 10^{24}$ molecules × (1 mole / $6.022 \times 10^{23}$) × (30.07 g / 1 mole) = 250 g C2H6

Sample HTML Table: Molar Mass Calculation

The following table summarizes the calculation of molar mass for selected compounds:

Compound	Element	Atomic Mass (g/mol)	Number of Atoms	Total Mass (g/mol)
H2O	H	1.008	2	2.016
H2O	O	15.999	1	15.999
Total Molar Mass				18.015
NaCl	Na	22.990	1	22.990
NaCl	Cl	35.453	1	35.453
Total Molar Mass				58.443

Summary Table: Intermolecular Forces in Selected Compounds

Compound	Type of Intermolecular Force
H2O	Hydrogen Bonding
HBr	Dipole-Dipole
CH4	London Dispersion
NH3	Hydrogen Bonding
NaCl	Dipole-Dipole

Additional info:

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

• All calculations use standard dimensional analysis and conversion factors.

• Electron geometry and molecular geometry are crucial for predicting molecular shape and polarity.