ATOMIC MASS

1. ATOMIC MASS • amu = atomic mass unit • If atomic mass is determined by the number of p+ and e–, why isn’t it a whole number? • Most elements occur as a mixture of two or more isotopes.

2. ISOTOPES OF HYDROGEN name abundance n0 hydrogen-1 protium 99.98% 0 hydrogen-2 deuterium 0.0156% 1 hydrogen-3 tritium trace amounts 2 all have 1 p+ and 1 e–

3. DEUTERIUM (D) • D2O is called heavy water • concentrated with n0 during slow electrolysis of water • has greater density and higher boiling point than H2O • not radioactive

4. TRITIUM (T) • formed continuously in upper atmosphere in nuclear rxns by cosmic rays • radioactive – decays spontaneously

5. ATOMIC MASS atomic mass of an element = weighted average of the atoms in a naturally-occurring sample of the element

6. CALCULATION OFATOMIC MASS must know three values: • the number of stable isotopes of the element • the mass of each isotope • the natural percent abundance of each isotope

7. CALCULATION OFATOMIC MASS Calculate the atomic mass of chlorine. Two isotopes: • Cl-35 (34.97) Cl-37(36.97) 75.77% 24.23% atomic mass = 0.7577(34.97) + 0.2423(36.97) = 35.45 amu

8. CALCULATION OFATOMIC MASS Calculate the atomic mass of boron. Two isotopes: • boron-10 (10.01) boron-11(11.01) 19.9% 80.1% atomic mass:

9. MASS DEFECT – MISSING MASS? • Add the masses of particles that make up an atom; the sum is always larger than actual atomic mass. • The missing mass is the matter converted into energy when the nucleus was formed from its component protons and neutrons.

10. MASS DEFECT Calculate mass defect of chlorine-35: • actual mass is 5.81 x 10-23 grams • e– = 9.11 x 10-28 g • p+ = 1.67 x 10-24 g • n0 = 1.67 x 10-24 g