High School Chemistry/Further Understanding of the Atom

In the last lesson, you learned about the atom, and the early experiments that led to the development of Dalton's Atomic Theory. But Dalton's Atomic Theory isn't the end of the story. Do you remember the scientific method introduced in early on in the book? Chemists using the scientific method make careful observations and measurements and then use these measurements to propose theories. That’s exactly what Dalton did. Dalton used the following observations:

1. Mass is neither created nor destroyed during a chemical reaction.
2. Elements always combine in the same proportions by mass when they form a given compound.
3. When elements form more than one compound, the different masses of one element that are combined with the same mass of the other element are in a ratio of small whole numbers.

With these observations (which came from careful measurement), Dalton proposed his Atomic Theory – a model which suggested how the underlying structure of matter might lead to the three observations above. The scientific method, though, doesn't stop once a theory has been proposed. Instead, the theory should suggest new experiments that can be performed to test whether or not the original theory is accurate and complete.

Dalton's Atomic Theory held up well to a lot of the different chemical experiments that scientists performed to test it. In fact, for almost 100 years, it seemed as if Dalton's Atomic Theory was the whole truth. However, in 1897, a scientist named J. J. Thompson conducted some research which suggested that Atomic Theory wasn’t the entire story. As it turns out, Dalton had a lot right. He was right in saying matter is made up of atoms; he was right in saying there are different kinds of atoms with different mass and other properties; he was "almost" right in saying atoms of a given element are identical; he was right in saying during a chemical reaction, atoms are merely rearranged; he was right in saying a given compound always has atoms present in the same relative numbers. But he was wrong in saying atoms were indivisible or indestructible. As it turns out, atoms are divisible. In fact, atoms are composed of smaller subatomic particles.

Lesson Objectives

• Explain the experiment that led to Thomson's discovery of the electron.
• Describe Thomson's "plum pudding" model of the atom.
• Describe Rutherford's Gold Foil experiment, and explain how this experiment proved that the "plum pudding" model of the atom was incorrect.

Thomson Discovered Electrons Were Part of the Atom

In the mid-1800s, scientists were beginning to realize that the study of chemistry and the study of electricity were actually related. First, a man named Michael Faraday showed how passing electricity through mixtures of different chemicals could cause chemical reactions. Shortly after that, scientists found that by forcing electricity through a tube filled with gas, the electricity made the gas glow! Scientists didn't, however, understand the relationship between chemicals and electricity until a British physicist named J. J. Thomson (Figure 4.7) began experimenting with what is known as a cathode ray tube.

Figure 4.7: A portrait of J. J. Thomson.

The below figure shows a basic diagram of a cathode ray tube like the one J. J. Thomson would have used. A cathode ray tube is a small glass tube with a cathode (a negatively charged metal plate) and an anode (a positively charged metal plate) at opposite ends. By separating the cathode and anode by a short distance, the cathode ray tube can generate what are known as "cathode rays" – rays of electricity that flowed from the cathode to the anode, J. J. Thomson wanted to know what cathode rays were, where cathode rays came from and whether cathode rays had any mass or charge. The techniques that J. J. Thomson used to answer these questions were very clever and earned him a Nobel Prize in physics. First, by cutting a small hole in the anode J. J. Thomson found that he could get some of the cathode rays to flow through the hole in the anode and into the other end of the glass cathode ray tube. Next, J. J. Thomson figured out that if he painted a substance known as "phosphor" onto the far end of the cathode ray tube, he could see exactly where the cathode rays hit because the cathode rays made the phosphor glow.

A cathode ray tube.

J. J. Thomson must have suspected that cathode rays were charged, because his next step was to place a positively charged metal plate on one side of the cathode ray tube and a negatively charged metal plate on the other side of the cathode ray tube, as shown in the below figure. The metal plates didn't actually touch the cathode ray tube, but they were close enough that a remarkable thing happened! The flow of the cathode rays passing through the hole in the anode was bent upwards towards the positive metal plate and away from the negative metal plate.

A cathode ray was attracted to the positively charged metal plate placed above the tube, and repelled from negatively charged metal plate placed below the tube.

In other words, instead of the phosphor glowing directly across from the hole in the anode (as in the first figure), the phosphor now glowed at a spot quite a bit higher in the tube (as in the second figure).

J. J. Thomson thought about his results for a long time. It was almost as if the cathode rays were attracted to the positively charged metal plate above the cathode ray tube, and repelled from the negatively charged metal plate below the cathode ray tube. J. J. Thomson knew that charged objects are attracted to and repelled from other charged objects according to the rule: opposites attract, likes repel. This means that a positive charge is attracted to a negative charge, but repelled from another positive charge. Similarly, a negative charge is attracted to a positive charge, but repelled from another negative charge. Using the "opposites attract, likes repel" rule, J. J. Thomson argued that if the cathode rays were attracted to the positively charged metal plate and repelled from the negatively charged metal plate, they themselves must have a negative charge!

J. J. Thomson then did some rather complex experiments with magnets, and used his results to prove that cathode rays were not only negatively charged, but also had mass. Remember that anything with mass is part of what we call matter. In other words, these cathode rays must be the result of negatively charged "matter" flowing from the cathode to the anode. But there was a problem. According to J. J. Thomson's measurements, either these cathode rays had a ridiculously high charge, or else had very, very little mass – much less mass than the smallest known atom. How was this possible? How could the matter making up cathode rays be smaller than an atom if atoms were indivisible? J. J. Thomson made a radical proposal: maybe atoms are divisible. J. J. Thomson suggested that the small, negatively charged particles making up the cathode ray were actually pieces of atoms. He called these pieces "corpuscles", although today we know them as "electrons". Thanks to his clever experiments and careful reasoning J. J. Thomson is credited with the discovery of the electron.

Protons Were Thought to Exist but Discovered Much Later

In the last section, we learned that atoms are, in fact, divisible, and that one of the subatomic particles making up an atom is a small, negatively charged entity called an "electron". Now imagine what would happen if atoms were made entirely of electrons. First of all, electrons are very, very small; in fact, electrons are about 2000 times smaller than the smallest known atom, so every atom would have to contain a whole lot of electrons. But there's another, even bigger problem: electrons are negatively charged. Therefore, if atoms were made entirely out of electrons, atoms would be negatively charged themselves… and that would mean all matter was negatively charged as well.

Of course, matter isn't negatively charged. In fact, most matter is what we call neutral – it has no charge at all. If matter is composed of atoms, and atoms are composed of negative electrons, how can matter be neutral? The only possible explanation is that atoms consist of more than just electrons. Atoms must also contain some type of positively charged material which balances the negative charge on the electrons. Negative and positive charges of equal size cancel each other out, just like negative and positive numbers of equal size. What do you get if you add +1 and −1? You get 0, or nothing. That's true of numbers, and that's also true of charges. If an atom contains an electron with a −1 charge, but also some form of material with a +1 charge, overall the atom must have a (+1) + (−1) = 0 charge – in other words, the atom must be neutral, or have no charge at all.

Based on the fact that atoms are neutral, and based on J. J. Thomson's discovery that atoms contain negative subatomic particles called "electrons", scientists assumed that atoms must also contain a positive substance. It turned out that this positive substance was another kind of subatomic particle, known as the "proton". Although scientists knew that atoms had to contain positive material, protons weren't actually discovered, or understood, until quite a bit later.

Thomson's Model of the Atom

Figure 4.8: Thomson's "plum-pudding" model for the atom. Notice how the "plums" are the negatively charged electrons, while the positive charge is spread throughout the entire pudding batter.

When Thomson discovered the negative electron, he realized that atoms had to contain positive material as well – otherwise they wouldn't be neutral overall. As a result, Thomson formulated what's known as the "plum pudding" model for the atom. According to the "plum pudding" model, the negative electrons were like pieces of fruit and the positive material was like the batter or the pudding. This made a lot of sense given Thomson's experiments and observations. Thomson had been able to isolate electrons using a cathode ray tube; however he had never managed to isolate positive particles. As a result, Thomson theorized that the positive material in the atom must form something like the "batter" in a plum pudding, while the negative electrons must be scattered through this "batter". (If you've never seen or tasted a plum pudding, you can think of a chocolate chip cookie instead. In that case, the positive material in the atom would be the "batter" in the chocolate chip cookie, while the negative electrons would be scattered through the batter like chocolate chips.)

Figure 4.8 shows a "plum pudding" and a "plum pudding" model for the atom. Notice how easy it would be to pick the pieces of fruit out of a plum pudding. On the other hand, it would be a lot harder to pick the batter out of the plum pudding, because the batter is everywhere. If an atom were similar to a plum pudding in which the electrons are scattered throughout the "batter" of positive material, then you'd expect it would be easy to pick out the electrons, but a lot harder to pick out the positive material.

Everything about Thomson's experiments suggested the "plum pudding" model was correct – but according to the scientific method, any new theory or model should be tested by further experimentation and observation. In the case of the "plum pudding" model, it would take a man named Ernest Rutherford to prove it wrong. Rutherford and his experiments will be the topic of the next section.

Rutherford's Model of the Atom

Disproving Thomson's "plum pudding" model began with the discovery that an element known as uranium emitted positively charged particles called alpha particles as it underwent radioactive decay. Radioactive decay occurs when one element decomposes into another element. It only happens with a few very unstable elements. This involves some difficult concepts so, for now, just accept the fact that uranium decays and emits alpha particles in the process. Alpha particles themselves didn't prove anything about the structure of the atom. In fact, a man named Ernest Rutherford (see the figure at right) proved that alpha particles were nothing more than helium atoms that had lost their electrons. Think about why an atom that has lost electrons will have a positive charge. Alpha particles could, however, be used to conduct some very interesting experiments.

Rutherford's gold foil experiment.

Ernest Rutherford was fascinated by all aspects of alpha particles. For the most part, though, he seemed to view alpha particles as tiny bullets that he could use to fire at all kinds of different materials. One experiment in particular, however, surprised Rutherford, and everyone else. Rutherford found that when he fired alpha particles at a very thin piece of gold foil, an interesting thing happened. Almost all of the alpha particles went straight through the foil as if they'd hit nothing at all. Every so often, though, one of the alpha particles would be deflected slightly as if it had bounced off of something hard. Even less often, Rutherford observed alpha particles bouncing straight back at the "gun" from which they had been fired! It was as if these alpha particles had hit a wall "head-on" and had ricocheted right back in the direction that they had come from.

Rutherford thought that these experimental results were rather odd. Rutherford described firing alpha particles at gold foil like shooting a high-powered rifle at tissue paper. Would you ever expect the bullets to hit the tissue paper and bounce back at you? Of course not! The bullets would break through the tissue paper and keep on going, almost as if they'd hit nothing at all. That's what Rutherford had expected would happen when he fired alpha particles at the gold foil. Therefore, the fact that most alpha particles passed through didn't shock him. On the other hand, how could he explain the alpha particles that got deflected? Even worse, how could he explain the alpha particles that bounced right back as if they'd hit a wall?

Rutherford decided that the only way to explain his results was to assume that the positive matter forming the gold atoms was not, in fact, distributed like the batter in plum pudding, but rather, was concentrated in one spot, forming a small positively charged particle somewhere in the center of the gold atom. We now call this clump of positively charged mass the nucleus. According to Rutherford, the presence of a nucleus explained his experiments, because it implied that most alpha particles passed through the gold foil without hitting anything at all. Once in a while, though, the alpha particles would actually collide with a gold nucleus, causing the alpha particles to be deflected, or even to bounce right back in the direction they came from.

Rutherford Suggested Electrons "Orbited"

While Rutherford's discovery of the positively charged atomic nucleus offered insight into the structure of the atom, it also led to some questions. According to the "plum pudding" model, electrons were like plums embedded in the positive "batter" of the atom. Rutherford's model, though, suggested that the positive charge wasn't distributed like batter, but rather, was concentrated into a tiny particle at the center of the atom, while most of the rest of the atom was empty space. What did that mean for the electrons? If they weren't embedded in the positive material, exactly what were they doing? And how were they held in the atom? Rutherford suggested that the electrons might be circling or "orbiting" the positively charged nucleus as some type of negatively charged cloud, but at the time, there wasn't much evidence to suggest exactly how the electrons were held in the atom.

Despite the problems and questions associated with Rutherford's experiments, his work with alpha particles definitely seemed to point to the existence of an atomic "nucleus". Between J. J. Thomson, who discovered the electron, and Rutherford, who suggested that the positive charges in an atom were concentrated at the atom's center, the 1890s and early 1900s saw huge steps in understanding the atom at the "subatomic" (or smaller than the size of an atom) level. Although there was still some uncertainty with respect to exactly how subatomic particles were organized in the atom, it was becoming more and more obvious that atoms were indeed divisible. Moreover, it was clear that the pieces an atom could be separated into negatively charged electrons and a nucleus containing positive charges. In the next lesson, we'll look more carefully at the structure of the nucleus, and we'll learn that while the atom is made up of positive and negative particles, it also contains neutral particles that neither Thomson, nor Rutherford, were able to detect with their experiments.

Lesson Summary

• Dalton's Atomic Theory wasn't entirely correct. It turns out that atoms can be divided into smaller subatomic particles.
• A cathode ray tube is a small glass tube with a cathode and an anode at one end. Cathode rays flow from the cathode to the anode.
• When cathode rays hit a material known as "phosphor" they cause the phosphor to glow. J. J. Thomson used this phenomenon to reveal the path taken by a cathode ray in a cathode ray tube.
• J. J. Thomson found that the path taken by the cathode ray could be bent towards a positive metal plate, and away from a negative metal plate. As a result, he reasoned that the particles in the cathode ray were negative.
• Further experiments with magnets proved that the particles in the cathode ray also had mass. Thomson's measurements indicated, however, that the particles were much smaller than atoms.
• J. J. Thomson suggested that these small, negatively charged particles were actually subatomic particles. We now call them "electrons".
• Since atoms are neutral, atoms that contain negatively charged electrons must also contain positively charged material.
• According to Thomson's "plum pudding" model, the negatively charged electrons in an atom are like the pieces of fruit in a plum pudding, while the positively charged material is like the batter.
• When Ernest Rutherford fired alpha particles at a thin gold foil, most alpha particles went straight through; however, a few were scattered at different angles, and some even bounced straight back.
• In order to explain the results of his gold foil experiment, Rutherford suggested that the positive matter in the gold atoms was concentrated at the center of the gold atom in what we now call the nucleus of the atom.
• Rutherford's model of the atom didn't explain where electrons were located in an atom.

Review Questions

1. Decide whether each of the following statements is true or false.

   (a) Cathode rays are positively charged.

   (b) Cathode rays are rays of light, and thus they have no mass.

   (c) Cathode rays can be repelled by a negatively charged metal plate.

   (d) J.J. Thomson is credited with the discovery of the electron.

   (e) Phosphor is a material that glows when struck by cathode rays.

2. Match each observation with the correct conclusion.

   (a) Cathode rays are attracted to a positively charged metal plate. - i. Cathode rays are positively charged. - ii. Cathode rays are negatively charged. - iii. Cathode rays have no charge.

   (b) Electrons have a negative charge. - i. atoms must be negatively charged. - ii. atoms must be positively charged. - iii. atoms must also contain positive subatomic material.

   (c) Alpha particles fired at a thin gold foil are occasionally scattered back in the direction that they came from - i. the positive material in an atom is spread throughout like the “batter” in pudding - ii. atoms contain neutrons - iii. the positive charge in an atom is concentrated in a small area at the center of the atom.

3. Alpha particles are:

   (a) Helium atoms that have extra electrons.

   (b) Hydrogen atoms that have extra electrons.

   (c) Hydrogen atoms that have no electrons.

   (d) Electrons.

   (e) Helium atoms that have lost their electrons.

   (f) Neutral helium atoms.

4. What is the name given to the tiny clump of positive material at the center of an atom?
5. Choose the correct statement.

   (a) Ernest Rutherford discovered the atomic nucleus by performing experiments with aluminum foil.

   (b) Ernest Rutherford discovered the atomic nucleus using a cathode ray tube.

   (c) When alpha particles are fired at a thin gold foil, they never go through.

   (d) Ernest Rutherford proved that the "plum pudding model" was incorrect.

   (e) Ernest Rutherford experimented by firing cathode rays at gold foil.

6. Answer the following questions:

   (a) Will the charges +2 and −2 cancel each other out?

   (b) Will the charges +2 and −1 cancel each other out?

   (c) Will the charges +1 and +1 cancel each other out?

   (d) Will the charges −1 and +3 cancel each other out?

   (e) Will the charges +9 and −9 cancel each other out?

7. Electrons are ______ negatively charged metals plates and ______ positively charged metal plates?
8. What was J. J. Thomson's name for electrons?
9. A "sodium cation" is a sodium atom that has lost one of its electrons. Would the charge on a sodium cation be positive, negative or neutral? Would sodium cations be attracted to a negative metal plate, or a positive metal plate? Would electrons be attracted to or repelled from sodium cations?
10. Suppose you have a cathode ray tube coated with phosphor so that you can see where on the tube the cathode ray hits by looking for the glowing spot. What will happen to the position of this glowing spot if:

    (a) a negatively charged metal plate is placed above the cathode ray tube

    (b) a negatively charged metal plate is placed to the right of the cathode ray tube

    (c) a positively charged metal plate is placed to the right of the cathode ray tube

    (d) a negatively charged metal plate is placed above the cathode ray tube, and a positively charged metal plate is placed to the left of the cathode ray tube

    (e) a positively charged metal plate is placed below the cathode ray tube, and a positively charged metal plate is also placed to the left of the cathode ray tube.

Vocabulary

alpha (α) particles

Helium atoms that have lost their electrons. They are produced by uranium as it decays.

anode

A positively charged metal plate.

cathode

A negatively charged metal plate.

cathode rays

Rays of electricity that flow from the cathode to the anode. J.J. Thomson proved that these rays were actually negatively charged subatomic particles (or electrons).

cathode ray tube

A glass tube with a cathode and anode, separated by some distance, at one end. Cathode ray tubes generate cathode rays.

nucleus

The small central core of the atom where most of the mass of the atom (and all of the atoms positive charge) is located.

phosphor

A chemical that glows when it is hit by a cathode ray.

plum pudding model

A model of the atom which suggested that the negative electrons were like plums scattered through the positive material (which formed the batter).

subatomic particles

Particles that are smaller than the atom. The three main subatomic particles are electrons, protons, and neutrons.

This material was adapted from the original CK-12 book that can be found here. This work is licensed under the Creative Commons Attribution-Share Alike 3.0 United States License