Lecture notes: CHEM103 Fall 2008 – September 18

Outline for the day:

ATOMIC STRUCTURE

·        review:

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?

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

 mass number

 charge

 atomic number

 number of atoms

X              ß “X” = any atomic symbol

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:

carbon-12, carbon-13

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?

…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

AND

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