Overview
The transcript explains differences among atoms, molecules, ions, elements, and compounds, and how to identify ionic versus covalent compounds using examples.
Atoms vs. Molecules
- Atom: Single electrically neutral particle consisting of one nucleus with electrons.
- Molecule: Particle made of two or more atoms chemically bonded.
- Molecules can contain same element atoms (e.g., O2) or different elements (e.g., H2O).
Elements, Compounds, and Particle Types
- Pure element: Substance with only one type of atom in each particle.
- Compound: Substance with different types of atoms in each particle.
- Particle type identification:
- Single atom per particle → atom (e.g., He, Ne).
- Multiple atoms per particle → molecule (e.g., H2, O2, CO2, H2O).
Examples Summary
- Helium: Atoms; pure element; each particle one He atom.
- Hydrogen gas (H2): Molecules; pure element; each particle two H atoms.
- Water (H2O): Molecules; compound; H and O atoms bonded.
- Oxygen gas (O2): Molecules; pure element; two O atoms bonded.
- Carbon dioxide (CO2): Molecules; compound; C and O atoms.
- Neon: Atoms; pure element; single Ne atoms.
- Fluorine (F2): Molecules; pure element; two F atoms bonded.
Atoms vs. Ions
- Atoms: Electrically neutral; number of protons equals number of electrons.
- Ions: Charged particles; unequal numbers of protons and electrons.
- Cation: Positively charged ion (fewer electrons than protons).
- Anion: Negatively charged ion (more electrons than protons).
Electron Count Formula
- Electrons = atomic number − charge
- For negative charge, subtracting a negative increases electron count.
Worked Ion Examples
- Aluminum atom vs. Al3+:
- Atomic number = 13 (protons = 13).
- Mass number given as 27; neutrons = 27 − 13 = 14.
- Aluminum atom: electrons = 13 − 0 = 13 (neutral).
- Al3+: electrons = 13 − (+3) = 10; net charge +3.
- Phosphorus-31 atom vs. P3−:
- Atomic number = 15 (protons = 15).
- Neutrons = 31 − 15 = 16.
- P atom: electrons = 15 − 0 = 15.
- P3−: electrons = 15 − (−3) = 18; net charge −3.
Ionic vs. Covalent (Molecular) Compounds
- Ionic compounds: Typically metal + nonmetal; formed by electron transfer; composed of cations and anions.
- Covalent (molecular) compounds: Typically nonmetal + nonmetal; formed by electron sharing; no full charges on atoms.
- General rule:
- Metal + nonmetal → ionic (e.g., NaCl, CaO, MgCl2).
- Nonmetal + nonmetal → covalent (e.g., H2O, CO, SF6).
- Exception highlighted:
- Ammonium salts (e.g., NH4Cl): Contain polyatomic ion NH4+ and Cl−; overall ionic despite only nonmetals present.
Classification Table
| Substance | Particle Type | Element or Compound | Ionic or Covalent | Notes |
|---|
| He | Atoms | Pure element | N/A | Each particle is one He atom |
| H2 | Molecules | Pure element | Covalent | Two H atoms bonded |
| H2O | Molecules | Compound | Covalent | H and O share electrons |
| O2 | Molecules | Pure element | Covalent | Two O atoms bonded |
| CO2 | Molecules | Compound | Covalent | C and O atoms bonded |
| Ne | Atoms | Pure element | N/A | Single Ne atoms |
| F2 | Molecules | Pure element | Covalent | Two F atoms bonded |
| NaCl | Formula units | Compound | Ionic | Na+ and Cl−; electron transfer |
| CO | Molecules | Compound | Covalent | C and O nonmetals |
| MgCl2 | Formula units | Compound | Ionic | Mg2+ with Cl− |
| SF6 | Molecules | Compound | Covalent | S and F nonmetals |
| CaO | Formula units | Compound | Ionic | Ca2+ and O2− |
| NH4Cl | Formula units | Compound | Ionic | NH4+ and Cl−; exception case |
Key Terms & Definitions
- Atom: Smallest unit of an element retaining chemical identity; neutral.
- Molecule: Group of atoms bonded together; can be element or compound.
- Pure element: Substance of only one type of atom in particles.
- Compound: Substance with two or more different types of atoms.
- Ion: Charged atom or group of atoms due to electron gain or loss.
- Cation: Positively charged ion (loss of electrons).
- Anion: Negatively charged ion (gain of electrons).
- Ionic compound: Lattice of cations and anions formed by electron transfer.
- Covalent (molecular) compound: Atoms bonded by shared electrons.
Action Items / Next Steps
- Practice classifying substances as atoms or molecules, element or compound.
- Apply electron formula: electrons = atomic number − charge for ions.
- Use metal/nonmetal rule to predict ionic vs. covalent; remember ammonium exceptions.