Lecture notes: CHEM103 Fall 2008 – September 18
Outline for the day:
a. mass spectrometry
· elemental isotopes & isotopic abundance
· atomic symbols (atomic number & mass number)
· conversions between amu, gram, atom, mole…
applied to calculations using atomic mass
MASS SPECTROMETRY REVISITED…
WHAT DOES THIS LOOK LIKE?
WHY ARE THERE MULTIPLE PEAKS?
THEN HOW IS THIS POSSIBLE – WHY DO THESE ATOMS DIFFER FROM EACH OTHER?
RELATIONSHIPS BETWEEN ISOTOPES:
SAME number of PROTONS, but DIFFERENT number of NEUTRONS
CHEMICALLY the SAME: they react to form the same compounds, BUT…
different mass (obvious!)
so they react at different rates (and there are other different mass-related behaviors…)
PLUS, the issue of stability – may be radioactive!
HOW DO WE DESCRIBE THE CONTENTS OF DIFFERENT ISOTOPES OF AN ATOM?
(SIMPLY & COMPACTLY, BUT GETTING ALL THE DETAILS…)
ATOMIC SYMBOLS, INCLUDING MASS NUMBER AND ATOMIC NUMBER
number of atoms
Mass #: the mass of a single atom, measured in amu (note: round to an integer)
Atomic #: the number of protons in an atom of this type
- from this, you can get the # of protons – duhh…
- IF a neutral atom, you also know the # of electrons
- by subtracting the Atomic # from the Mass #, you can calculate the # of neutrons
Charge: the total charge of the atom accounting for protons AND electrons
calculated as: [number of protons] – [number of electrons]
- from this, you can get the # of electrons in a charged atom (an ION)
Examples using atomic number & mass number:
hydrogen, deuterium, tritium
palladium isotopes: how many protons, neutrons, electrons in 106Pd, 108Pd, 105Pd?
WHY ARE THE ATOMIC WEIGHTS LISTED ON THE PERIODIC TABLE ALWAYS NON-INTEGER,
… WHILE THE MASS NUMBER IS ALWAYS AN INTEGER???
RELATIONSHIP OF MASS NUMBER TO ATOMIC MASS (ATOMIC WEIGHT)
REMEMBER: MASS NUMBER ≠ ATOMIC MASS !!!
Examples of isotopic abundance and the calculation of average atomic mass:
What is the global average atomic mass of oxygen?
16O (mass = 15.994 914 amu) ; abundance = 99.757%
17O (mass = 16.999 131 amu) ; abundance = 0.038%
18O (mass = 17.999 160 amu); abundance = 0. 205%
What is the global abundance (in %) of carbon-13 (13C)?
GIVEN (only) TWO ISOTOPES:
12C (mass = 12.000 000 amu)
13C (mass = 13.003 354 838 amu)
AND their (average) atomic mass = 12.010 78 amu
NOTE: (1 amu is DEFINED as the mass of 1/12 of an atom of carbon-12)
MOLES, ATOMS & MASS (IN AMU & GRAMS)
Question: HOW MANY ATOMS ARE THERE PER GRAM OF C-12?
12C (mass = 12.000 000 amu)
1 amu = 1.6606 x 10-24 g
Question: HOW MANY ATOMS ARE THERE IN 12 GRAMS OF C-12?
NOTES: NA named in honor of Amadeo Avogadro, not by him…
…a mole, like a dozen, doesn’t specify WHAT it counts so the units are:
6.022×1023 mol-1 (per mole)
1 mole of “.” stacked end to end ≈ the radius of the GALAXY! (~50,000 light years) or 5×1020 m
1 mole of water molecules ≈ a small-ish gulp…
SO – atomic mass (average) given in units of g/mol OR amu/atom – these are equivalent!
sometimes called the atomic weight, or better: molar mass