Putting Atoms Together (7.1) and How Atoms Combine (7.3)

Key Concepts

  • Atoms: Smallest unit of an element with its properties.

  • Molecules: Particles made of two or more atoms joined by chemical bonds (e.g., N2N2​ - molecular element, not a compound).

  • Compounds: Made of two or more different elements chemically combined.

7.1: Key Topics

  • Diatomic Elements:

    • Elements that exist as two atoms bonded for stability (e.g., H2,O2,N2H2​,O2​,N2​).

    • Acronym: HOFBrINCl (Hydrogen, Oxygen, Fluorine, Bromine, Iodine, Nitrogen, Chlorine).

  • Chemical Formulas:

    • Notation indicating the type and number of atoms in a substance (e.g., H2O,NH3H2​O,NH3​).

  • Why Atoms Combine:

    • To achieve stability by gaining, losing, or sharing electrons, aiming for a full valence shell (like noble gases).

  • Molecular Compounds:

    • Consist of non-metals bonded covalently (e.g., CH4,C2H5OHCH4​,C2​H5​OH).

    • Properties vary based on composition and structure.

  • Counting Atoms:

    • Example: C6H12O6C6​H12​O6​: Carbon = 6, Hydrogen = 12, Oxygen = 6.

7.3: How Atoms Combine

  • Covalent Bonds:

    • Formed by non-metals sharing electrons (e.g., molecular compounds).

  • Ionic Bonds:

    • Metals lose electrons (form cations), non-metals gain electrons (form anions).

    • Example: Sodium (Na+Na+) reacts with Chlorine (Cl−Cl−) to form ionic compounds.

  • Lewis Structures:

    • Simplified representation showing only valence electrons around element symbols for easier visualization.

Periodic Table Trends

  • Valence electrons increase from left to right (Group 1: 1 e-, Group 18: 8 e-).

  • Stability achieved with a full valence shell.

Stability and Reactivity

  • Stability:

    • Atoms are stable when protons = electrons or the valence shell is full.

    • Example: NeNe is stable, LiLi is not.

  • Reactivity:

    • Influenced by:

      1. Number of electrons to gain/lose for stability.

      2. Distance of valence electrons from the nucleus.

Ions

  • Formation:

    • Atoms gain or lose electrons to become charged (e.g., Li+,O2−Li+,O2−).

  • Types:

    • Cations: Lose electrons (positive charge).

    • Anions: Gain electrons (negative charge).

  • Examples:

    • Mg2+Mg2+, N3−N3−, Cl−Cl−.

Bonding Examples

  • Using Diagrams:

    • Bohr-Rutherford and Lewis dot diagrams visualize atomic bonding.

  • Practice Examples:

    • Bonding between MgMg and SS, CaCa and NN, F2F2​, and CC with BrBr.