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?
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
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