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4.2: Structure of the Atom

Created by: CK-12

Lesson Objectives

  • Describe the properties of cathode rays that led to the discovery of the electron.
  • Describe the experiment carried out by Rutherford and his co-workers that led to the discovery of the nucleus.
  • List the properties of protons, neutrons, and electrons.
  • Describe the properties of X-rays and how they were discovered.
  • Describe the currently accepted model of the atom.

Lesson Vocabulary

  • electron: A negatively charged particle that has a very small mass compared to the mass of other subatomic particles and surrounds the atom.
  • plum pudding model: An experiment, led by J.J. Thomson, which proposed that the atom was comprised of negatively charged particles in a disperse field of positively charged particles.
  • radioactivity: When certain substances emit charged particles.
  • gold foil experiment: An experiment, led by Ernest Rutherford, which proposed that atoms consist of a small positively charged nucleus surrounded by negatively charged electrons.
  • proton: A positively charged particle that has a relatively large mass compared to electrons. Protons can be found in the nucleus of the atom.
  • neutron: An uncharged particle with a mass nearly equal to that of the proton. Neutrons can be found in the nucleus of the atom.

Check Your Understanding

  • Describe Dalton’s atomic theory.
  • List two changes that were made to Dalton's theory based on more recent evidence.

Introduction

After the development of Dalton’s atomic theory, several important discoveries were made that led to a new understanding of the atom. A number of experiments revealed that the atom is comprised of subatomic particles called electrons, neutrons, and protons. Most of the atom's mass is concentrated in a central nucleus, which contains protons and neutrons. The nucleus is surrounded by much less massive electrons, which account for most of the volume occupied by the atom. The electron was the first subatomic particle to be discovered, followed by discoveries of the nucleus, the proton, and the neutron. In this lesson, we are going to study these important discoveries and how they led to our current understanding of the atom.

Discovery of Cathode Rays

In 1877, William Crookes (1832-1919) was studying how electrical current behaves in a vacuum tube. In one experiment, he passed an electric current through an evacuated phosphorous-coated glass cylinder with an object in the center, as shown in Figure below.

Diagram of Crookes tube.

Upon passing a current through the tube, Crookes noticed that a "shadow" was cast by the object in the tube. The phosphorus on the terminal end of the tube became brightly fluorescent, except for the region directly behind the central object. He interpreted this to mean that the electrical current was blocked by the object. He reasoned that the electrical current, which he later called cathode rays, was composed of streams of particles. Crookes’s work was later expanded upon by several other scientists. One scientist in particular, J. J. Thomson, was able to show that cathode rays could be deflected by a magnetic field, as shown in the following video: http://www.youtube.com/watch?v=M1REuKMeI34 (0:58).

Thomson’s interpretation of this effect was that cathode rays must consist of charged particles that have mass. Thomson presented his work in 1897, where he referred to these negatively charged particles as corpuscles. Later on, this name was changed and negatively charged particles became known as electrons. Thomson revised the model of the atom into what became known as the plum pudding model. He hypothesized that the atom was comprised of negatively charged particles in a field of positive charge (positively charged particles had not yet been discovered). This proposed arrangement was compared to the arrangement of plums in plum pudding, as illustrated in Figure below.

Plum pudding model of the atom.

The Charge of the Electron

In 1909, Robert Millikan and Harvey Fletcher devised what is known as the oil drop experiment to determine the charge of a single electron. The oil drop experiment consisted of an apparatus in which small, charged oil drops were passed through an electric field. The electric field was created by two oppositely charged parallel metal plates. The rate at which the oil drops fell through the field was used to determine the magnitude of the charge of an electron.

Millikan's oil drop experiment.

The following video illustrates this experiment and explains how the charge of an electron was determined: http://www.youtube.com/watch?v=XMfYHag7Liw (1:14).

Using this information, Millikan calculated the charge of an electron to be 1.5924 × 10−19 coulombs. A coulomb (C) is the SI unit for electric charge, where 1 coulomb = 1 ampere × 1 second. (Remember, an ampere is an SI base unit for electric current.) Today, the accepted value for the charge of an electron is 1.602176487 × 10−19 C. Despite the relatively simple apparatus with which it was determined, Millikan’s value was within 1% of the currently accepted value. Combining this value with information from J. J. Thomson's experiments, Millikan was also able to calculate the mass of an electron. The currently accepted value is 9.10938215 × 10-31 kg.

Discovery of the X-ray

Right around the time that Thomson was formulating his model of the atom, a scientist named Wilhelm Conrad Roentgen was studying the behavior of electricity in discharge tubes. These were partially evacuated gas-filled tubes which would conduct an electric current, similar to the Crookes tube used in the cathode ray experiments. He devised an experiment in which he covered a discharge tube with black cardboard, and several feet away, he placed a sheet of paper that had been chemically treated with a compound called barium-platinum cyanide. When he applied voltage to the discharge tube, he noticed the screen several feet away became fluorescent, emitting light. This was peculiar, because the tube had been completely covered by material that would block the escape of any cathode rays. Roentgen continued to explore this phenomenon. He moved the screen further away, he turned the screen around, and he placed objects between the screen and the discharge tube. In all cases, the screen still fluoresced when the discharge tube was turned on. Then Roentgen had his wife place her hand atop a photographic plate, and the rays were shone towards the plate. After developing the plate, he observed an image of his wife's hand that “showed the shadows thrown by the bones of her hand and that of a ring she was wearing” (Wilhelm Conrad Roentgen - Biography). Figure below is an image of what he saw.

The skeleton of Roentgen's wife's hand, as captured on the photographic plate. What do you think the dark spot in the picture might be caused by?

This was the first "roentgenogram" ever taken. He interpreted this to mean that another ray, other than the cathode rays, was being produced that could penetrate and travel through objects at a distance. He called these X-rays, and he received the Nobel Prize in Physics in 1901 for his brilliant work. Modern day X-rays that one might receive for a medical exam operate on the same principles that Roentgen discovered. Several major discoveries followed shortly after Roentgen’s discovery of X-rays. Just two months later, in 1896, radioactivity was discovered by a Frenchman named Henri Becquerel. Becquerel discovered that certain substances, like uranium salts, emit charged particles. Following this work, Marie and Pierre Curie began to study the behavior of various radioactive substances in 1897. In fact, Madame Curie coined the word “radioactivity.” Their work resulted in the discovery of mass changes in radioactive elements, which later became known as radioactive decay. They also identified two new radioactive elements, which later became known as polonium and radium. The Curies were awarded the Nobel Prize in Physics in 1903 for their work. Marie Curie later won a Nobel Prize in Chemistry (1911) for her contributions to our understanding of radioactivity. We will study radioactivity further in the chapter Nuclear Chemistry.

Discovery of the Nucleus

Ernest Rutherford (1871-1937).

By 1900, it was known that the electron carried a negative charge. It was also known that the electron makes up an extremely small fraction of the mass of an atom. Ernest Rutherford set out to determine how the remainder of the mass and charge was distributed in the atom. Rutherford was a physicist from New Zealand who was working under the direction of J. J. Thomson. He conducted several experiments on the radioactive properties of uranium. He discovered that uranium released two different types of particles, which he referred to as alpha (α) particles, which were positively charged, and beta (β) particles, which were negatively charged. It was later shown that beta particles were simply free electrons.

Gold Foil Experiment

One of Rutherford’s famous experiments was called the gold foil experiment (illustrated in the Figure below). In this experiment, Rutherford used a radioactive source to direct alpha particles toward a very thin sheet of gold foil. Surrounding the foil was a screen that fluoresced when struck by the alpha particles.

Rutherford's gold foil experiment.

Here is a short video of his experiment: http://www.youtube.com/watch?v=5pZj0u_XMbc (0:47).

As shown in the video, most alpha particles easily passed through the gold foil and struck the fluorescent screen behind the foil. However, there were some instances in which the alpha particles were deflected very strongly, often back toward the emission source. If the plum pudding model were correct, all of the alpha particles would be expected to pass through the gold foil with little or no deflection. The strong deflection experienced by a small portion of the alpha particles could be better explained by an atom that contained a very small, dense nucleus. Because some of the alpha particles emitted from the source were repelled by the nucleus, Rutherford concluded that the nucleus must be made up of these positively charged alpha particles, which he named protons. He proposed that atoms consist of a small, positively charged nucleus surrounded by negatively charged electrons, as shown in the Figure below.

Rutherford's atomic model.

Bohr's Atomic Model

In 1913, shortly after Rutherford’s work on the nucleus, Neils Bohr proposed what became known as a planetary model of the atom. Bohr’s model was based upon the work done by Max Planck and Albert Einstein, who at the time were studying quantum theory which looks at the energy associated with matter. The planetary model was useful for relating atomic structure to the wavelengths of light that an element emits when heated. Bohr’s model, as well as the work of Planck and Einstein, will be discussed in the chapter Electrons in Atoms.

Discovery of the Neutron (1932)

In 1932, James Chadwick discovered the neutron. Chadwick was an English physicist who was mentored by Rutherford. His experiment consisted of bombarding beryllium atoms with alpha particles through a paraffin wax target and studying the effects. From his analysis, he concluded that the nucleus also contains a particle which has equal mass to the proton, but unlike the proton, is electrically neutral - hence the name neutron. Here is a short video clip describing Chadwick’s experiment: http://www.youtube.com/watch?v=HnmEI94URK8 (2:14).

Chadwick’s work resulted in a new understanding of the nucleus of the atom; it is comprised of both protons and neutrons. Because the masses of subatomic particles are so small, a new unit, called an atomic mass unit (amu), was defined. Protons and neutrons each have a mass of approximately one amu. The Table below describes the characteristics of the three subatomic particles we have discussed.

Particle Charge (C) Mass (kg) Mass (amu) Location
proton +1.6022 × 10-19 1.67262 × 10-27 1.0073 (~1) nucleus
electron (e-) -1.6022 × 10-19 9.10938 × 10-31 5.4858 × 10-4 (~0) outside nucleus
neutron 0 1.67493 × 10-27 1.0087 (~1) nucleus

Lesson Summary

  • Experiments conducted during the early twentieth century revealed that the atom is comprised of subatomic particles called electrons, neutrons, and protons.
  • In 1877, William Crookes discovered cathode rays, which later became known as electrons. Crookes discovered these rays using an apparatus he developed called the Crookes tube or cathode-ray tube.
  • Electrons, which were initially called cathode rays, are negatively charged and have a very small mass compared to the masses of other subatomic particles.
  • In 1895, Wilhelm Conrad Roentgen discovered X-rays.
  • In 1897, J. J. Thomson showed that cathode rays are deflected in a magnetic field and proposed that cathode rays are streams of negatively charged particles. Thomson proposed the plum-pudding model of the atom. This model described the atom as a disperse field of positive charge containing small negatively charged particles.
  • In 1909, the magnitude of the charge carried by an electron was determined by Robert Millikan in an experiment known as the oil drop experiment. Information from this experiment was later used to also calculate the mass of an electron.
  • In 1919, Rutherford discovered the presence of a positively charged nucleus with his famous gold foil experiment.
  • Rutherford proposed a new atomic model that described the atom as comprised of a positively charged nucleus surrounded by negatively charged electrons. In this model, most of the atom was thought to be empty space.*Protons are positively charged and have a relatively large mass compared to electrons. Protons can be found in the nucleus of the atom.
  • In 1932, Chadwick discovered the neutron, a particle with a mass similar to that of the proton but without any electrical charge.
  • Neutrons are particles with a mass similar to that of the proton, but they have no electrical charge. Neutrons also reside in the nucleus.

Review Questions

  1. What id Crookes discover in his cathode ray tube experiments?
  2. Describe the atom using Thomson's plum pudding model. Draw a picture of this model.
  3. How did Millikan set up his oil drop experiment?
  4. Describe how Roentgen took his first x-ray.
  5. How did Rutherford's gold foil experiment contradict the plum pudding model of the atom?
  6. What is the modern view of the nucleus and its composition?
  7. Sketch a modern view of the atom indicating the locations of protons, neutrons, and electrons.
  8. List the properties of electrons, neutrons, and protons.

Further Reading / Supplemental Links

Points to Consider

  • In this lesson we learned that neutrons and protons in the nucleus have similar mass. How might we measure the amount of mass contained in different elements?

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