1. Isotopes and Mass Number How Dalton’s atomic theory was wrong: –Atoms ARE divisible. (Who is credited with first splitting an atom?) –All atoms of a particular element are NOT identical. All atoms of an element do have the same number of protons and electrons, so what could differ?

2. Isotopes and Mass Number (cont.) Isotopes: atoms with the same number of protons (same element), but with different numbers of neutrons. Examples: –Potassium (K): K-39, K-40, K-41 –Chlorine (Cl): Cl-35 and Cl-37 –Uranium: U-235 and U-238

3. Isotopes and Mass Number (cont.) If atoms have the same number of protons and electrons, but different numbers of neutrons, what else will be different? Answer: MASS Mass number = number of protons + number of neutrons

4. Isotopes and Mass Number (cont.) Examples: –K-39 = 19 protons + 20 neutrons –K-40 = ___ protons + ___ neutrons –K-41 = ___ protons + ___ neutrons Isotope symbolic notation: mass number Element Symbol = 39 K atomic number 19 Isotope name is potassium-39

5. Isotopes and Mass Number (cont.) Review: 1.Number of protons is the same for all atoms of a particular element 2.Number of neutrons can vary ( = isotopes) What is the mass number? What is the atomic number? Number of neutrons = ?

6. Isotopes and Mass Number (cont.) Number of neutrons = mass number – atomic number Example: If an atom of neon has an atomic number of 10, and a mass number of 22: 1.How many protons? Neutrons? Electrons? 2.What is the NAME of the isotope? 3.What is the SYMBOL of the isotope?

7. Practice & Assignment Practice: p.101 (14) Study Guide Workbook: pp. 21-22 (13-23)