Previous Background: Thomson’s Atomic Model
Since the early 1900s, two characteristics of atoms were already known: they contain electrons and they are electrically neutral. For an atom to be neutral it must contain an equal number of positive and negative charges. Thomson proposed that an atom could be visualized as a uniform positively charged sphere, inside which the electrons were contained like raisins in a cake. This model, called the “raisin pudding model,” was accepted as a theory for some years.
Thomson’s atomic model, sometimes called the “raisin pudding” model because of its resemblance to the traditional English dessert containing raisins, the electrons are embedded in a uniform positively charged sphere.
Ernest Rutherford’s experiment
In 1910 a New Zealand physicist. Ernest Rutherford, who studied with Thomson at Cambridge University, used alpha particles to demonstrate the structure of atoms. Together with his colleague Hans Geiger and an undergraduate student named Ernest Marsden. Rutherford performed a series of experiments using very thin films of gold and other metals as a target for particles from a radioactive source. They observed that most of the particles passed through the foil without deflection, or with a slight deflection. Occasionally, some alpha particles were scattered (or deflected) from their trajectory at a large angle. In some cases, the alpha particles returned along the same path to the radioactive source. This was the most surprising discovery because. According to Thomson’s model, the positive charge of the atom was so diffuse that the a particles would be expected to pass through the sheets without deflection or with minimal deflection, Rutherford’s comment on this discovery was as follows:
Some time later, Rutherford was able to explain the results of the a-particle scattering experiment using a new model of the atom. According to Rutherford, most atoms must be empty space. This explains why most of the a particles passed through the gold foil undergoing little or no deflection. Rutherford proposed that the positive charges of atoms were concentrated in a dense central cluster within the atom, which he called the nucleus. When an “A” particle passed near the nucleus in the experiment, a large repulsive force acted on it, resulting in a large deflection. Moreover, when an “A” particle impinged directly on the nucleus, it experienced such a great repulsion that its trajectory was completely reversed.
The positively charged particles in the nucleus are called protons. In further experiments it was found that protons have the same amount of charge as electrons and that their mass is 1.67262 × 10-24 g. Approximately 1840 times the mass of negatively charged particles, electrons. Up to this point, scientists visualized the atom as follows: the mass of the nucleus makes up most of the total mass of the atom, but the nucleus occupies only 1/1013 of the total volume of the atom.
The atomic (and molecular) dimensions will be expressed here, according to the SI (International System), with a unit called the picometer (pm), where: 1 pm = 1 × 10-12 m.
The radius of an atom is approximately 100 pm, while the radius of the atomic nucleus is only 0.005 pm. One can appreciate the relative difference between the size of an atom and its nucleus by imagining that if an atom were the size of the Houston Astrodome stadium, the volume of its nucleus would be comparable to that of a small marble. While the protons are confined in the nucleus of the atom, the electrons are considered to be scattered around the nucleus and at some distance from it.