Chemistry, Third edition

30 3 · INSIDE THE ATOM

model has been compared with currants in a bun, with the electrons as the currants

and the dough as the positive charge.

In 1909 a group led by Lord Rutherford (1871–1937) carried out an experiment in

which alpha particles from a radioactive material were fired at a very thin layer of gold

metal in an evacuated bulb. (Alpha particles are much smaller than gold atoms.)

Rutherford’s team repeated the experiment many times. They found that

although most of the alpha particles passed through the foil undisturbed, very

occasionally an alpha particle underwent a massive deflection – like a table tennis

ball bouncing off a wall. Only about 1 in every 20 000 alpha particles bounced back

in this way, and the deflections were so rare that many experimenters might have

simply ignored them. But Rutherford and his workers realized that such deflections

were very significant indeed. Rutherford proposed that the mass of an atom was

concentrated in its centre (the nucleus). The chances of an alpha particle striking

the nucleus were very small but, when such a collision did occur, the considerable

mass of the nucleus ensured that the alpha particle was greatly deflected (Fig. 3.1).

The ‘insides’ of atoms: a summary

The Rutherford model may be summarized as follows:

1.Atoms consist of electrons circling the nucleus of the atom.Electrons are nega-

tively charged. By agreement, electrons are assigned a charge of 1. The circular

paths of the electrons are called shellsororbits. The radius of an atom is equal to

the radius of the outermost electron shell.

2.The nucleus consists of two kinds of particles, protons (which possess a relative

charge of 1) and neutrons(with a mass approximately equal to that of the proton but

with no electrical charge). Protons and neutrons are collectively known as nucleons.

Nucleus

Fig. 3.1Explanation of the

Rutherford experiment. The

path of the alpha particles is

shown by arrows. Only if an

alpha particle makes a direct

hit on a nucleus does it suffer

a large deflection.

Joseph John (‘JJ’) Thomson
(left) and Ernest Rutherford.
Both were awarded Nobel
Prizes, Thomson in 1906 (for
his work on atomic structure)
and Rutherford in 1908 (for his
work on radioactivity). Much to
Thomson’s delight, his son,
George, also won the Nobel
Prize for Physics (in 1937).