0.2 The structure of an atom  (Page 2/6)

The atoms we choose to shoot at are gold, in the form of a very thin gold foil of thickness about $10^{-4}\mathrm{cm}$ . The objects we "throw" are actually α particles,which are positively charged and fairly massive, emitted by radioactive polonium nuclei. The α particles aredirected in a very precise narrow line perpendicular to and in the direction of the gold foil. We then look forα particles at various angles about the gold foil, looking both for particles which have been deflected as they passthrough the foil or which have been reflected as they bounce off of the foil. The scattering experiment is illustrated here .

The result of the experiment is initially counter-intuitive. Most of the α particles passthrough the gold foil undeflected, as if there had been nothing in their path! A smaller number of the particles are deflected sharplyas they pass through the foil, and a very small fraction of the α particles are reflected backwards off of the goldfoil. How can we simultaneously account for the lack of any deflection for most of the α particles and for thedeflection through large angles of a very small number of particles?

First, since the majority of the positively charged α particles pass through the gold foilundeflected, we can conclude that most of the volume of each gold atom is empty space, containing nothing which might deflect anα particle. Second, since a few of the positively charged α particles are deflected very sharply, thenthey must encounter a positively charged massive particle inside the atom. We therefore conclude that all of the positive charge andmost of the mass of an atom is contained in a nucleus . The nucleus must be very small, very massive, and positively charged if it is to account for the sharpdeflections. A detailed calculation based assuming this model reveals that the nucleus must be about 100,000 times smaller thanthe size of the atom itself. The electrons, already known to be contained in the atom, must be outside of the nucleus, since thenucleus is positively charged. They must move in the remaining space of the much larger volume of the atom. Moreover, in total,the electrons comprise less than 0.05% of the total mass of an atom.

This model accounts for observation of both undeflected passage most of α particles and sharpdeflection of a few. Most α particles pass through the vast empty space of the atom, which is occupied only byelectron. Even the occasional encounter with one of the electrons has no effect on an α particle’s path, sinceeach α particle is much more massive than an electron. However, the nucleus is both massive and positivelycharged, but it is also small. The rare encounter of anα particle with the nucleus will result in very large deflections; a head-on collision with a gold atom nucleus will sendan α particle directly back to its source.

Observation 2: x-ray emission

Although we can now conclude that an atom has a nuclear structure, with positive charge concentrated in a verysmall nucleus and a number of electrons moving about the nucleus in a much larger volume, we do not have any information on how manyelectrons there are in an atom of any given element or whether this number depends on the type of atom. Does a gold atom have the samenumber of electrons as a silver atom? All we can conclude from the data given is that the number of positive charges in the nucleusmust exactly equal the number of electrons moving outside the nucleus, since each atom is neutral. Our next difficulty is that wedo not know what these numbers are.