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