Essential Chemistry Concepts: Atoms, Bonds, and Reactions

Atomic Structure

• Protons — positive charge, found in nucleus
• Neutrons — no charge, found in nucleus
• Electrons — negative charge, orbit nucleus in energy levels

Key Terms

• Atomic Number (Z): number of protons
• Mass Number (A): protons + neutrons
• Isotopes: atoms of the same element with different numbers of neutrons
• Ions: atoms that gain or lose electrons
  • Gain electrons → negative ion (anion)
  • Lose electrons → positive ion (cation)

Important Formulas

• Mass Number: A = Z + N
• Average Atomic Mass: weighted average of all isotopes

Electron Configuration

Electrons fill energy levels from lowest to highest energy. Example: Oxygen (8 electrons) → 1s² 2s² 2p⁴

Types of Elements & Periodic Trends

Types of Elements

• Metals: shiny, conduct heat/electricity, form positive ions
• Nonmetals: dull, brittle, poor conductors, form negative ions
• Metalloids: have mixed properties (Si, B, Ge)

Periodic Trends

• Atomic Radius:
  • Gets smaller left → right
  • Gets larger top → bottom
• Ionization Energy: energy needed to remove an electron
  • Gets higher left → right
  • Gets lower top → bottom
• Electronegativity: how strongly an atom attracts electrons (Highest = Fluorine)

Bond Types & Molecular Shapes

Bond Types

• Ionic Bonds: electrons transferred (metal + nonmetal)
• Covalent Bonds: electrons shared (nonmetal + nonmetal)
• Metallic Bonds: “sea of electrons” around metal atoms

Polarity Explained

• Nonpolar: electrons shared equally
• Polar: electrons shared unequally (creates partial charges)

Common Molecular Shapes

Law of Conservation of Mass & Balancing

What the Law Means

Matter cannot be created or destroyed. This is why equations must be balanced.

How to Balance Equations

1. Count atoms on both sides
2. Add coefficients to balance
3. Never change subscripts
4. Re-check totals

Example

Unbalanced: H₂ + O₂ → H₂O | Balanced: 2H₂ + O₂ → 2H₂O

Mole Concept & Avogadro’s Number

What a Mole Is

A mole is a counting unit: 1 mole = 6.022 × 10²³ particles

Conversions

• Mass → Moles: divide by molar mass
• Moles → Mass: multiply by molar mass
• Moles → Particles: multiply by Avogadro’s number
• Gas Volume at STP: 1 mole = 22.4 L

Reaction Rates

Factors That Increase Rate

• Higher temperature
• Higher concentration
• Greater surface area
• Catalyst (lowers activation energy)

Summary of Formulas

• Mass Number: A = Z + N
• Moles: n = mass ÷ molar mass
• Gas Volume at STP: V = n × 22.4 L
• Particles: particles = moles × 6.022×10²³
• Conservation of Mass: reactants = products