Transcript CHAPTER 4
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.
ISOTOPES OF HYDROGEN
name
hydrogen-1
protium
abundance n0
99.98%
0
hydrogen-2 deuterium
0.0156%
1
hydrogen-3
trace
amounts
2
tritium
all have 1 p+ and 1 e–
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
TRITIUM (T)
formed continuously in upper
atmosphere in nuclear rxns
by cosmic rays
radioactive – decays
spontaneously
ATOMIC MASS
atomic mass of an element =
weighted average of the atoms
in a naturally-occurring sample
of the element
CALCULATION OF
ATOMIC 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
CALCULATION OF
ATOMIC 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
CALCULATION OF
ATOMIC MASS
Calculate the atomic mass of boron.
Two isotopes:
boron-10 (10.01) boron-11(11.01)
19.9%
80.1%
atomic mass:
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.
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