Dictionary of Chemistry [6th Ed.]

time known as supramolecular syn-

thons.

Système International d’Unités

See si units.

Szilard–Chalmers effect An ef-

fect discovered by L. Szilard (1898–

1964) and T. A. Chalmers in 1934; it

has been used to separate radioactive

products in a nuclear reaction involv-

ing the absorption of a neutron and

the emission of gamma rays. If a ma-

terial absorbs a neutron and subse-

quently emits a gamma ray, the

emission of the gamma ray causes

the nucleus to recoil. Frequently, the

recoil energy is sufÜcient to break

the chemical bond between the atom

and the molecule of which it forms

part. Thus, although the atom that

has absorbed the neutron is an iso-

tope of the original atom it is in a dif-

ferent form chemically, enabling

separation to take place. For exam-

ple, if an aqueous solution of sodium

chlorate (NaClO 3 ) is subjected to

bombardment by slow neutrons, the

Cl^37 is converted to Cl^38 , with many

of the Cl^38 atoms breaking from the

chlorate and moving into the solu-

tion in the form of chloride ions.

This is an example of a ‘hot atom’ re-

action. The Cl^38 can be precipitated

out using silver nitrate.

Système International d’Unités 516

s