THE NUCLEAR MODEL OF THE ATOM

Lesson Objectives Distinguish between the three main subatomic particles. Understand the contributions of J. J. Thomson, Robert Millikan, and Ernest Rutherford to atomic theory. Describe the structure of the nuclear atom.

INTRODUCTION Dalton’s atomic theory represented an improvement over the idea of Democritus because the theory was based on experimental findings and the scientific method. However, his theory did have its shortcomings. He believed that atoms were indivisible, meaning that the atom was the smallest possible component of matter. Further investigations in the late 1800s proved that atoms can indeed be broken down into smaller particles. It is the unique number and arrangement of these subatomic particles that makes atoms of one element different from those of every other element. The three fundamental particles are called the proton, the neutron, and the electron.

DISCOVERY OF THE ELECTRON The first discovery of a subatomic particle resulted from experiments into the nature of the relationship between electricity and matter.

CATHODE RAYS In 1897, English physicist J. J. Thomson ( ) experimented with a device called a cathode ray tube, in which an electric current was passed through gases at low pressure. A cathode ray tube consists of a sealed glass tube fitted at both ends with metal disks called electrodes. The electrodes are then connected to a source of electricity. One electrode, called the anode, becomes positively charged while the other electrode, called the cathode, becomes negatively charged. Once this happens, a beam called a cathode ray travels from the cathode to the anode.

Investigations were carried out to determine the nature of the cathode ray. The results of two further experiments supported the hypothesis that the cathode ray consisted of a stream of particles. 1.When an object was placed between the cathode and the opposite end of the tube, it cast a shadow on the glass. 2.A cathode ray tube was constructed with a small metal rail between the two electrodes. Attached to the rail was a paddle wheel capable of rotating along the rail. Upon connecting the cathode ray tube to a power source, the wheel rotated from the cathode toward the anode. This suggested that the cathode ray was made of particles that must have mass.

In order to determine if the cathode ray consisted of charged particles, Thomson used magnets and charged plates to deflect the ray. His findings are summarized below. Cathode rays were deflected by a magnetic field in the same manner as a wire carrying an electric current, which was known to be negatively charged. Cathode rays were deflected away from a negatively charged metal plate and toward a positively charged plate. Thomson knew that opposite charges attract one another, while like charges repel one another. Together, the results of the cathode ray tube experiments showed that cathode rays are actually streams of tiny negatively charged particles moving at very high speeds. While Thomson originally called these particles corpuscles, they were later named electrons.

Thomson conducted further experiments which allowed him to calculate the charge-to-mass ratio (e/m e ) of the electron. In units of coulombs to grams, e/m e = 1.8 × 10 8 C/g. He found that this value was a constant and did not depend on the gas used in the cathode ray tube or on the metal used as the electrodes. He concluded that electrons were negatively charged subatomic particles present in atoms of all elements.

CHARGE AND MASS OF THE ELECTRON American physicist, Robert Millikan ( ), carried out a series of experiments between 1908 and 1917 that allowed him to determine the charge of a single electron. Millikan’s experiment was called the oil drop experiment. When tiny drops of oil were sprayed into a chamber, the oil drops picked up a static charge and were suspended between two charged plates.

Millikan was able to observe the motion of the oil drops with a microscope and found that the drops lined up in a specific way between the plates, based on the number of electric charges that they had acquired. From the data gathered in this experiment, he was able to accurately determine the charge of an individual electron. Then, using Thomson’s previous measurement of an electron’s charge-to-mass ratio, he was also able to calculate the mass of a single electron. Charge of one electron = −1.602×10 −19 C Mass of one electron = 9.11×10 −28 g

The incredibly small mass of the electron was found to be approximately 1/1840 the mass of a hydrogen atom, so scientists realized that atoms must also contain other, far more massive particles. Additionally, at least one of these particles must carry a positive charge because complete atoms are electrically neutral.

PROTONS AND NEUTRONS If cathode rays are electrons that are given off by the metal atoms of the cathode, then what remains of the atoms that have lost those electrons? We know several basic things about electrical charges. They are carried by particles of matter. Millikan’s experiment showed that they exist as whole-number multiples of a single basic unit. Atoms have no overall electrical charge, meaning that each and every atom contains an exactly equal number of positively and negatively charged particles. A hydrogen atom, the simplest kind of atom, contains only one electron. When that electron is removed, a positively charged particle should remain.

In 1886, Eugene Goldstein ( ) discovered evidence for the existence of this positively charged particle. Using a cathode ray tube with holes in the cathode, he noticed that there were rays traveling in the opposite direction from the cathode rays. He called these canal rays and showed that they were composed of positively charged particles. The proton is a positively charged subatomic particle that is present in all atoms. The mass of the proton is about 1840 times the mass of the electron.

In 1932, English physicist, James Chadwick ( ), discovered a third subatomic particle. The neutron is a subatomic particle with no electrical charge and a mass that is approximately the same as the mass of a proton. Below is a summary of the properties of the three fundamental subatomic particles.

DISCOVERY OF THE ATOMIC NUCLEUS The next step after the discovery of subatomic particles was to figure out how these particles were arranged in the atom. This is a difficult task because of the incredibly small size of the atom. Therefore, scientists set out to design a model of what they believed the atom could look like. The goal of each atomic model was to accurately represent.

Rutherford needed to come up with an entirely new model of the atom in order to explain his results. Because the vast majority of the alpha particles had passed through the gold, he reasoned that most of the atom was empty space. In contrast, the particles that were highly deflected must have experienced a tremendously powerful force within the atom. He concluded that all of the positive charge and the majority of the mass of the atom must be concentrated in a very small space in the atom’s interior, which he called the nucleus. The nucleus is the tiny, dense, central core of the atom and is composed of protons and neutrons.

Rutherford’s atomic model became known as the nuclear model. In this model, the protons and neutrons, which comprise nearly all of the mass of the atom, are located in a nucleus at the center of the atom. The electrons are distributed around the nucleus and occupy most of the volume of the atom. It is worth emphasizing just how small the nucleus is compared to the rest of the atom. If we could blow up an atom to be the size of a large professional football stadium, the nucleus would be about the size of a marble.

Rutherford’s model proved to be an important step toward a full understanding of the atom. However, it did not completely address the nature of the electrons and the way in which they occupied the vast space around the nucleus. It was not until some years later that a more complete understanding of the electron was achieved. This proved to be the key to understanding the chemical properties of elements.

Lesson Summary The three fundamental subatomic particles are the electron, the proton, and the neutron. Thomson used the cathode ray tube to discover the electron and determine its negative charge. Millikan determined the charge and mass of the electron with the oil-drop experiment. Rutherford’s gold foil experiment provided evidence for the atomic nucleus, a small dense core of the atom which contains the positive charge and most of the mass. The nuclear model of the atom is one in which the nucleus is composed of protons and neutrons, while electrons are distributed throughout the rest of the space.