4.9: Protons

Figure \(\PageIndex{1}\) (Credit: Adrian Pingstone (User:Arpingstone/Wikimedia Commons); Source: http://commons.wikimedia.org/wiki/File:Tvr.280i.arp.jpg(opens in new window); License: Public Domain)

Can you name this car?

Describing what we can see is a fairly easy matter. If we are asked to describe the sports car illustrated below, we could all quickly come up with a fairly accurate description. A person knowledgeable about cars would include more details, but everyone would have the basic information in their description.

What makes the description easy to come up with? We can see it, we have a common language to describe it (size, color, construction), and we have a basic idea of what it is (a car, not a house or a tree). Scientists have far more difficulty in describing things they cannot see. There is no way to look directly at an atom and see its detailed structure. When a discovery is first made, there is often no language to use to tell others exactly what it is. This was the problem in talking about the atom and its structure. 

Putting the Puzzle Pieces Together

Research builds upon itself – one piece connects to another. Sometimes the puzzle doesn't seem to make sense because some of the pieces are missing at the moment. Each finding gives a clearer picture of the whole and also raises new questions. The detective work that led to the discovery of the proton was built upon finding pieces to the puzzle and putting them together in the right way.

The electron was discovered using a cathode ray tube. An electric current was passed from the cathode (the negative pole) to the anode (positive pole). Several experiments showed that particles were emitted at the cathode and that these particles had a negative charge. These experiments demonstrated the presence of electrons.

Figure \(\PageIndex{2}\): JJ Thomson's experiment with cathode rays. (Credit: User:Kurzon/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:JJ_Thomson_Cathode_Ray_2_explained.svg(opens in new window); License: Public Domain)

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 is the simplest kind of atom with only one electron. When that electron is removed, a positively charged particle should remain.

Discovery of the Proton

In 1886, Eugene Goldstein (1850-1930) 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 the positively charged subatomic particle present in all atoms. The mass of the proton is about 1840 times the mass of the electron.

Summary

• When an electron is removed from a hydrogen atom, a proton remains.
• Goldstein observed rays traveling in the opposite direction of the cathode rays in a cathode ray tube.
• He demonstrated that these rays were positive particles and called the canal rays.

Review

1. Why is it easy to describe things we can see?
2. Why did researchers believe that the particle left after electrons were emitted as cathode rays had to be positive?
3. Atoms, which are always neutral in electric charge, contain electrons as well as protons and neutrons. An electron has an electrical charge of -1. If an atom has three electrons, infer how many protons it has.
4. How many electrons does it take to weight the same as one proton?