Chemistry 3A Practice Exam 1 Key
Understanding fundamental chemistry concepts including periodic trends, Lewis structures, molecular geometry, bonding, and stoichiometry is crucial for success in Chem 3A.
Core Principles
- Periodic Trends: Ion size decreases with increasing nuclear charge and increases with more electron shells. Electronegativity generally increases across a period and decreases down a group.
- Lewis Structures & Formal Charge: The preferred Lewis structure minimizes formal charges on atoms and places negative formal charges on more electronegative atoms.
- Molecular Geometry: The arrangement of atoms around a central atom (molecular geometry) is determined by the electron group geometry and influences polarity.
- Bonding: Covalent bonds are strong intramolecular forces due to shared electrons, while hydrogen bonds are weaker intermolecular forces arising from partial charges.
Action Steps
- Calculate Formal Charge: Use the formula: # valence e - (# non-bonding e) - ½(# bonding e) to determine formal charges on atoms in Lewis structures.
- Determine Molecular Geometry: Based on electron group geometry, predict shapes like linear, trigonal planar, trigonal pyramidal, tetrahedral, or bent.
- Identify Polar/Nonpolar Bonds: Compare electronegativity differences between bonded atoms (e.g., C-O, N-C, C-H) to classify bonds.
- Convert Units: Utilize appropriate conversion factors to change between units like grams and nanograms, or moles to particles using Avogadro's number.
Key Terms
- Ionic Compound: A compound formed by electrostatic attraction between oppositely charged ions, typically between a metal and a nonmetal.
- Covalent Compound: A compound formed by the sharing of electrons between atoms, typically between nonmetals.
- Electronegativity: A measure of the tendency of an atom to attract a bonding pair of electrons.
- Electron Affinity: The energy change that occurs when an electron is added to a neutral atom in the gaseous state.
- Ionization Energy: The energy required to remove an electron from a neutral atom or ion in its gaseous state.
- Lewis Structure: A diagram showing the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule.
- Molecular Geometry: The three-dimensional arrangement of atoms that constitute a molecule.
- Polar Bond: A covalent bond between two atoms with different electronegativities, resulting in an uneven distribution of electron density.
- Nonpolar Bond: A covalent bond where the electrons are shared equally between two atoms with similar electronegativities.
- Hydrogen Bond (IMF): An intermolecular force between a hydrogen atom bonded to a highly electronegative atom (N, O, or F) and another nearby electronegative atom.
- Avogadro's Number: The number of constituent particles (usually molecules, atoms or ions) that are contained in the amount of substance given by one mole of that substance, approximately 6.022 x 10^23 particles/mol.
Real World Examples
- Potassium isocyanate (KOCN) was used as an herbicide and weedkiller.: This example illustrates the practical application of chemical compounds and the need for unit conversions (e.g., grams to nanograms) in handling them.
- Caffeine is a main compound in coffee beans.: This highlights how understanding molecular structure, polarity, and bonding (polar vs. nonpolar bonds) is relevant to common substances.
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