Overview
This lecture explains how to determine the number of protons, neutrons, and electrons in atoms and ions using sodium and other examples, along with key formulas and sample problems.
Atomic Structure Basics
- The atomic number (smaller number) equals the number of protons in an atom.
- The mass number (larger, whole number) equals the sum of protons and neutrons for a specific isotope.
- An element’s identity is determined by its atomic number, not its mass number.
- Isotopes have the same number of protons but different numbers of neutrons.
Key Formulas
- Number of protons = Atomic number.
- Number of neutrons = Mass number − Atomic number.
- Number of electrons = Atomic number − Charge (for ions).
Atoms vs. Ions
- For neutral atoms, protons = electrons.
- For cations (positively charged ions), electrons = atomic number − positive charge (fewer electrons than protons).
- For anions (negatively charged ions), electrons = atomic number − negative charge (more electrons than protons).
Worked Examples
- Sodium-23: 11 protons (atomic number), 12 neutrons (23-11), 11 electrons (neutral, 11-0).
- Aluminum-27 3+: 13 protons, 14 neutrons (27-13), 10 electrons (13-3).
- Sulfide ion (S²⁻): 16 protons, 17 neutrons (33-16), 18 electrons (16-(-2) = 16+2).
- Silicon-29: 14 protons, 15 neutrons (29-14), 14 electrons (neutral atom).
Identifying Unknown Elements
- Given an ion’s charge, number of neutrons, and electrons, use: electrons = atomic number − charge to find the atomic number.
- Find the mass number by adding neutrons and protons.
- Use the atomic number to identify the element from the periodic table (e.g., atomic number 22 = titanium).
Key Terms & Definitions
- Atomic Number — Number of protons in an atom; determines element identity.
- Mass Number — Total protons plus neutrons in a specific isotope.
- Isotope — Atoms of the same element (same protons) with different neutrons.
- Cation — Positively charged ion (fewer electrons than protons).
- Anion — Negatively charged ion (more electrons than protons).
Action Items / Next Steps
- Practice calculating protons, neutrons, and electrons for atoms and ions.
- Use the periodic table to look up atomic numbers for unfamiliar elements.