1—10.
X-RAY METHOD
13
determinations in this manner.
For example, Bearden has found
'
for the copper K,,1 line, 3. wave-length of 1.5405 angstroms. .
The angle 0 through which an x—ray beam is diffracted from a
crystal may be measured and used in the well-known Bragg law
=
2d sin-9 *
(1—24)
to compute the grating constant d, provided the Wave-length )\
of the ray is known.
The order of \ the diffracted beam is
n(=1, 2,3, etc.).
.
Consider a cubic crystal of rock salt as made up of many
small elementary cubes with alternate sodium and chlorine atoms
at the corners.
Let the crystal be one which weighs 58.454 grams
(one gram—rholecule, M), which is the sum of 22.997 grams of
sodium and 35.457” grams of chlorine.
denition of
'
Avogadro’s number, it will then contain N atoms of each or. 2N
.
.
3
atoms in all.
Along each edge there W1ll be V 2N atoms, each at
a distance d centimeters from the next.
The length of each edge
'
.
3
-
.
.
W1ll then be d'V 2N and the volume of the crystal W1ll be d3-2N
cubic centimeters.
By denition, the density p of the crystal is
its mass divided by its volume, so that
’
,
”
-
…
,N =
1—25
_
2pd3
<
.
>
A similar equation may
obtained for calcite (CaCO;;), where
,
_
theelementary space lattice is a rhômbohedron or distorted cube.
;
_
To allow for
included in the deno_mi…
,
_nator
1—25 For ÇaÏÇîÎÇ6, M _=_.100-07, P = 2-7102,
spacer! is known,
N=6022><1023
î
'
-
e=4804><10"1068u
;
lg,
mulüplymgthenghthandSldeby(ï—551112®»Where518theumtdecrement0f ‘
'