Development of the Atom

4. Protons and Neutrons

In 1886 scientists discovered that a cathode-ray tube emitted rays not only from the cathode but also from the positively charged anode. These rays traveled in the opposite direction of the cathode rays. Like the cathode rays, they are deflected by electrical and magnetic fields, but in directions opposite to the way cathode rays are deflected. Thomson was able to show that there rays had a positive electrical charge. Years later, scientists determined that the rays were composed of positively charged subatomic particles called protons. The amount of charge on a proton and on an electron is equal but opposite, but the mass of a proton is much greater than the mass of an electron. The mass of a proton was found to be only slightly less than the mass of a hydrogen atom. At this point, it seemed that atoms were made up of equal numbers of electrons and protons. However, in 1910, Thomson discovered that neon consisted of atoms of two different masses. Because of this, scientists hypothesized that atoms contained still a third type of particle that accounted for these differences in mass. Calculations showed that this particle should have a mass equal to that of a proton but no electrical charge. The existence of this neutral particle, called a neutron, was confirmed in the early 1930s.

1. Democritus (not until around 460 B.C.)

-a greek philosopher -developed the idea of atoms. -asked the question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? -thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms. -Thought that there are many different kinds of atoms, each distinct in shape and size.

While all of these subatomic particles were being discovered, scientists were also trying to determine how the particles of an atom were arranged....... (under 5. Ernest Rutherford's Gold Foil Experiment:):):):)

After the discovery of the electron, scientists pictured an atom as tiny particles of negative electricity embedded in a ball of positive charge. Almost at the same time, a Japanese physicist, Hantaro Nagaoka, proposed a different model in which electrons orbited a central, positively charged nucleus.

Meaning of Robert Millikan's Oil Drop Experiment:

Because the droplets were charged, they could be made to rise and fall by changing the voltage across the plates. Millikan calculated the average charge on the droplet based on the electrical charge on the plates. He found that the charges on the droplets varied, but that each charge was a multiple of one small charge. He correctly concluded that this small charge must be the charge of an electron. This charge is now the standard unit of negative charge (-1). Electrically this charge is equal to -1.6x10^-19 Coulombs.

In 1909, a team of scientists led by _____________________ in England carried out the first of several important experiments that revealed an arrangement far different from what most scientists believed at the time. Rutherford's experimental set-up is shown on the top of the next page. (under 5. Ernest Rutherford's Gold Foil Experiment:):):):)

Ernest Rutherford

6. Robert Millikan's Oil Drop Experiment Description:

In 1909, Robert Millikan began experiments to try to determine the exact charge of an electron. His experimental apparatus looks like the figure below. As oil was sprayed from a brass atomizer, electrons were transferred from the atomizer to the oil droplets. These negatively charged droplets filled a vacuum and by the force of gravity fell through a hole to an area between two charged plates as seen in the figure below.

2. Dalton's Atomic Theory

John Dalton (1766-1844) was one of the first scientists to study chemistry on an atomic level. His various experiments allowed him to create his famous Atomic Theory. Dalton's Atomic Theory consists of the following statements: 1. All matter is made up of atoms. 2. Atoms are indestructible and cannot be divided into smaller particles. (Atoms are indivisible.) 3. All atoms of one element are exactly alike, but they are different from atoms of other elements. Dalton's atomic theory was almost true. Dalton had assumed that atoms are the ultimate particles of matter and can't be broken up into smaller particles and that all atoms of the same element are identical. However, his theory had to be modifies as new discoveries were made in the late 19th and early 20th centuries.

Findings of J.J. Thomson's Cathode Ray Experiment:

Thomson found that the rays bent toward a positively charged plate and away from a negatively charged plate. He know that objects with like charged repel each other, and objects with opposite charges attract each other. Thomson concluded that cathode rays are made up of invisible, negatively charged particles called electrons. These electrons had to come from the matter (atoms) of the negative electrode.

3. J.J. Thomson's Cathode Ray Experiment Description:

Thomson's experiments used a vacuum tube such as that shown in the figure below. A vacuum tube is one that has had all the gases pumped out of it. At each end of the tube is a metal plate called an electrode, which is connected through the glass to a metal terminal outside the tube. These electrodes become electrically charged when they are connected to a high-voltage electrical source. When the electrodes are charged, rays travel in the tube from the negative electrode, which is called the cathode, to the positive electrode, which is called the anode. Because the rays originate at the cathode (negative end), they are called cathode rays.

Meaning of the Ernest Rutherford's Gold Foil Experiment:

To explain the results of the experiment, Rutherford's team proposed a new model of the atom. Because most of the particles passed through the foil, they concluded that the atom is nearly all empty space. Because so few particles were deflected, they proposed that the atom has a small, dense, positively charged central core, called a nucleus.

Downfall to Democritus' findings/questions/ideas:

Unfortunately, the atomic ideas of Democritus had no lasting effects on other Greek philosophers, including Aristotle. In fact, Aristotle dismissed the atomic idea as worthless. People considered Aristotle's opinions very important and if Aristotle thought the atomic idea had no merit, then most other people thought the same also. For more than 2000 years nobody did anything to continue the explorations that the Greeks had started into the nature of matter. Not until the early 1800's did people begin again to question the structure of matter.

From Thomson's experiments, scientists had to conclude that atoms were _____ just neutral spheres, but somehow were composed of electrically charged particles. In other words, atoms were not indivisible but were composed of smaller particles, referred to as ______________ particles. Further experimentation showed that the mass of an electron was equal to 1/1837 the mass of a hydrogen atom. Reason should tell you that there must be a lot more to the atom than electrons. Matter is not negatively charged, so atoms can't be negatively charged either. If atoms contained extremely light, negatively charged particles, then they must also contain _________________ charged particles. Scientists immediately worked to discover such particles. (Was this done right after the experiment then?! Before the next step/experiment, and findings?!?!)

not; subatomic; positively

Because of Dalton's atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts. In 1897, a British physicist, ____________, discovered that this solid-ball model was not accurate.

J.J. Thomson

Ernest Rutherford's Gold Foil Experiment Description:

The experimenters set up a lead-shielded box containing radioactive polonium, which emitted a beam of positively charged subatomic particles through a small hole. Today, we know that the particles of the beam consisted of two protons and two neutrons and are called alpha particles. The sheet of gold foil was surrounded by a screen coated with zinc sulfide, which glows when struck by the positively charged particles of the beam. When the beam struck the sheet of gold foil, most of the particles passed straight through the foil as if it wasn't there. However, a few of the beam's particles were deflected. Some were deflected slightly, but a few bounced straight back.