1. Relative Atomic Mass & Isotopes 5.1

2. Until recently it was impossible to weigh individual atoms

3. The relative scale English chemist John Dalton (1766-1844) simply assigned an arbitrary mass to the carbon * atom It was assigned an exact mass of 12 The mass of the elements were calculated “relative” to carbon * It was attempted using O and H but the relative masses were either too small or non-integral numbers

4. Relative Atomic Mass For example, –H is 12 times lighter than C –Mg is 2 times heavier than C

5. Why does carbon weigh 12.01? 6 C 12.01 No units because this is NOT the mass of an atom (relative mass only) Carbon is made up of several isotopes, not just carbon-12

6. Isotopes 6 C 12.01 Average Atomic Mass Carbon-12 Carbon-13 Carbon-14 Naturally occurring carbon contains 3 isotopes that are not present in equal amounts

7. Isotopes Atoms from the same element with different amounts of neutrons in the nucleus

8. Mass Spectrometer can find the relative abundance of each isotope in an element

9. How many isotopes does this element have? A typical mass spectrum for an element.

10. How would you find the average mass of a group of objects that have different masses and abundances? 80% 10% 10% 9g 12g 22g = 9 + 12 + 22 = 14.3 g 3 Doesn’t take into account that 80% of the spoons are 9g. Must calculate the WEIGHTED average. = (80% x 9) + (10% x 12) + (10% x 22) = 10.6 g

11. This applies to atoms and their isotopes 80% 10% 10% 9g 12g 22g Mg-24Mg-26Mg-25 80% 10% 10% VS Find the atomic mass for Mg (remember: no units!)

12. Relative Atomic Mass (also called Atomic Weight)

13. Relative Molecular Mass It is the sum of the relative weights of the atoms in the molecular formula Eg. H 2 O Mass of O: 15.9994 Mass of H: 1.00794 1.00794 Molecular mass: 18.0153

14. Find the molecular weight: Eg. H 2 SO 4

15. Molecular weight vs Formula weight H 2 SO 4 is a discrete covalent molecule vs NaCl and SiO 2 which are infinitely large ionic and covalent lattices, respectfully Because these represent empirical formulas, it’s proper term is “formula weight” for the ratio of atoms present eg. NaCl is 1:1 (23 + 35.5 = 58.5)