Lecture Notes: Atoms, Molecules, and Compounds

Key Definitions

• Atom: The smallest unit of an element, composed of protons, neutrons, and electrons.
• Molecule: A group of two or more atoms bonded together. Molecules can be composed of the same or different types of atoms.
• Pure Element: Substances composed of only one type of atom.
• Compound: A substance composed of different types of atoms bonded together.

Examples of Atoms and Molecules

• Helium (He):
  • Composed of atoms.
  • Pure element (one type of atom).
• Hydrogen Gas (H2):
  • Composed of molecules (two hydrogen atoms).
  • Pure element (one type of atom).
• Water (H2O):
  • Composed of molecules (two hydrogen and one oxygen atom).
  • Not a pure element; it's a compound.

Additional Examples

• Oxygen (O2):
  • A molecule (two oxygen atoms).
  • Pure element (one type of atom).
• Carbon Dioxide (CO2):
  • A molecule (one carbon and two oxygen atoms).
  • Compound (different atoms).
• Neon (Ne):
  • Composed of atoms.
  • Pure element (one type of atom).
• Fluorine (F2):
  • A molecule (two fluorine atoms).
  • Pure element (one type of atom).

Atoms vs. Ions

• Atom: Electrically neutral, equal number of protons and electrons.
• Ion: Unequal number of protons and electrons, resulting in a charge.
  • Cation: Positively charged ion (more protons than electrons).
  • Anion: Negatively charged ion (more electrons than protons).

Atomic Examples

• Aluminum Atom and Ion:
  • Atomic number: 13 (13 protons).
  • Aluminum Ion (3+ charge): 10 electrons.
• Phosphorus Atom and Anion:
  • Atomic number: 15 (15 protons).
  • Phosphorus Anion (3- charge): 18 electrons.

Compounds

• Ionic Compounds: Composed of metals and nonmetals. Involves transfer of electrons creating cations and anions.
  • Example: Sodium Chloride (NaCl).
• Covalent (Molecular) Compounds: Composed of nonmetals. Electrons are shared between atoms.
  • Example: Water (H2O).

Compound Examples and Classification

• Carbon Monoxide (CO): Molecular compound (two nonmetals).
• Magnesium Chloride (MgCl2): Ionic compound (metal and nonmetal).
• Sulfur Hexafluoride (SF6): Molecular compound (two nonmetals).
• Calcium Oxide (CaO): Ionic compound (metal and nonmetal).

Exceptions

• Ammonium Chloride (NH4Cl): Despite consisting of only nonmetals, it's ionic due to ionic characteristics (positive and negative ions).

Summary

• Atoms vs. Molecules: Atoms are single, molecules are multiple atoms bonded.
• Pure Elements vs. Compounds: Pure elements are one type of atom, compounds consist of different types.
• Ionic vs. Covalent Compounds: Ionic involves metals and nonmetals, covalent involves nonmetals sharing electrons.