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Atomic Theory Models Review Questions

Democritus

Review

1.    Who was Democritus?

2.    How did Democritus reason for the existence of atoms?

3.    How did Democritus describe atoms?

4.    Discuss how well Democritus’ ideas about atoms have withstood the test of time.

5.    Iron and lead are both metals, but iron is much harder than lead. How do you think Democritus might have explained this difference?

Dalton

Review

1.    Who was John Dalton?

2.    What evidence did Dalton use to argue for the existence of atoms?

3.    State Dalton’s atomic theory.

4.    Explain how Dalton modeled atoms and compounds.

Thomson’s Atomic Model

Review

1.    Who was J. J. Thomson?

2.    Explain how Thomson discovered negatively charged particles smaller than atoms.

3.    Thomson compared his idea of atomic structure to a plum pudding. Invent an original analogy for Thomson’s plum pudding model of the atom.

4.    Why was Thomson’s model soon rejected?

Rutherford’s Atomic Model

Review

1.    How did Ernest Rutherford discover the nucleus of the atom?

2.    Place Rutherford’s discovery in the broader history of the atom. (Hint: See the timeline at the following URL for more information:http://www.clickandlearn.org/gr9_sci/atoms/modelsoftheatom.html.)

3.    Describe how you could make a three-dimensional version of Rutherford’s planetary model of the atom.

Bohr's Atomic Model

Review

1.    How does Bohr’s atomic model build on Rutherford’s model?

2.    Explain the connection between energy quanta and energy levels.

3.    How does Bohr’s work demonstrate the importance of communication in science?

Electron Cloud Atomic Model

Review

1.    What is the problem with Bohr’s model of the atom?

2.    How did Schrödinger resolve this problem?

3.    Describe orbitals.

4.    Outline the electron cloud model of the atom.

Democritus' Idea of the Atom

Introduces the idea of the atom as the basic, indivisible building block matter.

·         Review how and when Democritus arrived at his idea of the atom.

·         Describe the atom as Democritus understood it.

·         State how Democritus explained the diversity of matter.

·         Explain why Democritus’ idea was ignored until about 1800.

The man shown above has been called the “laughing philosopher” because of his cheerful disposition. He certainly looks cheerful in this picture. Why is a philosopher featured in a science text? He made an amazing contribution to science, although it was ridiculed by others and then ignored for more than 2000 years. His name was Democritus, and he introduced the idea of the atom as the basic building block of all matter. You can learn about Democritus’ place in the history of the atom at this URL:

http://www.youtube.com/watch?v=BhWgv0STLZs (9:03)

Cutting the Cheese

Democritus lived in Greece from about 460 to 370 B.C.E. Like many other ancient Greek philosophers, he spent a lot of time wondering about the natural world. Democritus wondered, for example, what would happen if you cut a chunk of matter—such as a piece of cheese into smaller and smaller pieces. He thought that a point would be reached at which the cheese could not be cut into still smaller pieces. He called these pieces atomos, which means “uncuttable” in Greek. This is where the modern term atom comes from. In the video at the following URL, Bill Nye the Science Guy demonstrates how Democritus arrived at his idea of the atom.

http://www.youtube.com/watch?v=cnXV7Ph3WPk (6:37)

Just a Guess

Democritus’ idea of the atom has been called “the best guess in antiquity.” That’s because it was correct in many ways, yet it was based on pure speculation. It really was just a guess. Here’s what Democritus thought about the atom:

·         All matter consists of atoms, which cannot be further subdivided into smaller particles.

·         Atoms are extremely small—too small to see.

·         Atoms are solid particles that are indestructible.

·         Atoms are separated from one another by emptiness, or “void.”

Q: How are Democritus’ ideas about atoms similar to modern ideas about atoms?

A: Modern ideas agree that all matter is made up of extremely small building blocks called atoms.

Q: How are Democritus’ ideas different from modern ideas?

A: Although atoms are extremely small, it is now possible to see them with very powerful microscopes. Atoms also aren’t the solid, uncuttable particles Democritus thought. Instead, they consist of several kinds of smaller, simpler particles as well as a lot of empty space. In addition, atoms aren’t really indestructible because they can be changed to other forms of matter or energy.

Keep on Moving

Did you ever notice dust motes moving in still air where a beam of sunlight passes through it? You can see an example in the forest scene in the Figurebelow. This sort of observation gave Democritus the idea that atoms are in constant, random motion. If this were true, Democritus thought, then atoms must always be bumping into each other. When they do, he surmised, they either bounce apart or stick together to form clumps of atoms. Eventually, the clumps could grow big enough to be visible matter.

Q: Which modern theory of matter is similar to Democritus’ ideas about the motion of atoms?

A: The modern kinetic theory of matter is remarkably similar to Democritus’ ideas about the motion of atoms. According to this theory, atoms of matter are in constant random motion. This motion is greater in gases than in liquids, and it is greater in liquids than in solids. But even in solids, atoms are constantly vibrating in place.

Why Matter Varies

Democritus thought that different kinds of matter vary because of the size, shape, and arrangement of their atoms. For example, he suggested that sweetsubstances are made of smooth atoms and bitter substances are made of sharp atoms. He speculated that atoms of liquids are slippery, which allows them to slide over each other and liquids to flow. Atoms of solids, in contrast, stick together, so they cannot move apart. Differences in the weight of matter, he argued, could be explained by the closeness of atoms. Atoms of lighter matter, he thought, were more spread out and separated by more empty space.

Q: Democritus thought that different kinds of atoms make up different types of matter. How is this similar to modern ideas about atoms?

A: The modern view is that atoms of different elements differ in their numbers ofprotons and electrons and this gives them different physical and chemical properties.

That’s Ridiculous!

Democritus was an important philosopher, but he was less influential than another Greek philosopher named Aristotle, who lived about 100 years after Democritus. Aristotle rejected Democritus’ idea of the atom. In fact, Aristotle thought the idea was ridiculous. Unfortunately, Aristotle’s opinion was accepted for more than 2000 years, and Democritus’ idea was more or less forgotten. However, the idea of the atom was revived around 1800 by the English scientist John Dalton. Dalton developed an entire theory about the atom, much of which is still accepted today. He based his theory on experimental evidence, not on lucky guesses.

Summary

·         Around 400 B.C.E., the Greek philosopher Democritus introduced the idea of the atom as the basic building block matter.

·         Democritus thought that atoms are tiny, uncuttable, solid particles that are surrounded by empty space and constantly moving at random.

·         Democritus surmised that different kinds of matter consist of different types or arrangements of atoms.

Explore More

Watch the video at the following URL, and then develop a hypothesis to explain the results of the two experiments. Relate your hypothesis to Democritus’ ideas about atoms.

http://www.youtube.com/watch?v=g7debF-bSj8 (3:32)

Review

1.    Who was Democritus?

2. How did Democritus reason for the existence of atoms?
3. How did Democritus describe atoms?
4. Discuss how well Democritus’ ideas about atoms have withstood the test of time.
5. Iron and lead are both metals, but iron is much harder than lead. How do you think Democritus might have explained this difference?

Dalton's Atomic Theory

Pioneering work in the development of modern understanding of the atom.

·         Explain why Dalton reintroduced the idea of the atom.

·         State Dalton’s atomic theory.

·         Describe Dalton’s billiard ball model of the atom.

You probably know what this sketch represents. It’s a model of an atom, one of the miniscule particles that make up all matter. The idea that matter consists of extremely tiny particles called atoms was first introduced about 2500 years ago by a Greek philosopher named Democritus. However, other philosophers considered Democritus’ idea ridiculous, and it was more or less forgotten for more than 2000 years.

Reintroducing the Atom

Around 1800, the English chemist John Dalton brought back Democritus’ ancient idea of the atom. You can see a picture of Dalton below. Dalton grew up in a working-class family. As an adult, he made a living by teaching and just did research in his spare time. Nonetheless, from his research he developed one of the most important theories in all of science. Based on his research results, he was able to demonstrate that atoms actually do exist, something that Democritus had only guessed.

Dalton’s Experiments

Dalton did many experiments that provided evidence for the existence of atoms. For example:

·         He investigated pressure and other properties of gases, from which he inferred that gases must consist of tiny, individual particles that are in constant, random motion.

·         He researched the properties of compounds, which are substances that consist of more than one element. He showed that a given compound is always comprised of the same elements in the same whole-number ratio and that different compounds consist of different elements or ratios. This can happen, Dalton reasoned, only if elements are made of separate, discrete particles that cannot be subdivided.

Atomic Theory

From his research, Dalton developed a theory about atoms. Dalton’s atomic theory consists of three basic ideas:

·         All substances are made of atoms. Atoms are the smallest particles of matter. They cannot be divided into smaller particles, created, or destroyed.

·         All atoms of the same element are alike and have the same mass. Atoms of different elements are different and have different masses.

·         Atoms join together to form compounds, and a given compound always consists of the same kinds of atoms in the same proportions.

Dalton’s atomic theory was accepted by many scientists almost immediately. Most of it is still accepted today. However, scientists now know that atoms are not the smallest particles of matter. Atoms consist of several types of smaller particles, including protons, neutrons, and electrons. For the back story on Dalton’s ideas about atoms and his place in the history of the atom, watch this video: http://www.youtube.com/watch?v=BhWgv0STLZs (9:03).

For a musical version of the history of the atom, listen to “Atom Song” at this URL:http://www.youtube.com/watch?v=0P3m_L-myog (3:29).

The Billiard Ball Model

Because Dalton thought atoms were the smallest particles of matter, he envisioned them as solid, hard spheres, like billiard (pool) balls, so he used wooden balls to model them. Three of his model atoms are pictured in the Figurebelow. Do you see the holes in the balls? Dalton added these so the model atoms could be joined together with hooks and used to model compounds.

Q: When scientists discovered smaller particles inside the atom, they realized that Dalton’s atomic models were too simple. How do modern atomic models differ from Dalton’s models?

A: Modern atomic models, like the one pictured at the top of this article, usually represent subatomic particles, including electrons, protons, and neutrons. For a fuller comparison of Dalton’s ideas and modern ideas, go to this URL:

http://www.youtube.com/watch?v=Qk1JdUyW-Xo (5:15)

Summary

·         Around 1800, the English chemist John Dalton reintroduced the idea of the atom, which was first introduced by the ancient Greek philosopher named Democritus.

·         Dalton did many experiments with gases and compounds that provided evidence for the existence of atoms.

·         Dalton developed an atomic theory that is still mostly accepted today. It is one of the most important theories in all of science.

·         Dalton thought individual atoms were solid, hard spheres, so he modeled them with wooden balls.

Explore More

Read the article about Dalton’s atomic theory at the following URL. Then take the online quiz. Be sure to check your answers and read the explanations for any incorrect answers.

http://antoine.frostburg.edu/chem/senese/101/atoms/dalton.shtml

Review

5.    Who was John Dalton?

6.    What evidence did Dalton use to argue for the existence of atoms?

7.    State Dalton’s atomic theory.

8.    Explain how Dalton modeled atoms and compounds.

Thomson's Atomic Model

Discovery of the electron and the plum pudding model of the atom.

·         Explain how J. J. Thomson discovered the electron.

·         Describe Thomson’s plum pudding model of the atom.

You probably know that the wires strung between these high towers carry electricity. But do you know what electricity is? It actually consists of a constant stream of tiny particles called electrons. Electrons are negatively chargedfundamental particles inside atoms. Atoms were discovered around 1800, but almost 100 years went by before electrons were discovered.  

Thomson Discovers Electrons

John Dalton discovered atoms in 1804. He thought they were the smallest particles of matter, which could not be broken down into smaller particles. He envisioned them as solid, hard spheres. It wasn’t until 1897 that a scientist named Joseph John (J. J.) Thomson discovered that there are smaller particles within the atom. Thomson was born in England and studied at Cambridge University, where he later became a professor. In 1906, he won the Nobel Prize in physics for his research on how gases conduct electricity. This research also led to his discovery of the electron. You can see a picture of Thomson below. You can take a museum tour of his discovery at this online exhibit:http://www.aip.org/history/electron/.

Thomson’s Experiments

In his research, Thomson passed current through a cathode ray tube, similar to the one seen in the Figure below. A cathode ray tube is a glass tube from which virtually all of the air has been removed. It contains a piece of metal called an electrode at each end. One electrode is negatively charged and known as a cathode. The other electrode is positively charged and known as an anode. When high-voltage electric current is applied to the end plates, a cathode ray travels from the cathode to the anode.

What is a cathode ray? That’s what Thomson wanted to know. Is it just a ray ofenergy that travels in waves like a ray of light? That was one popular hypothesisat the time. Or was a cathode ray a stream of moving particles? That was the other popular hypothesis. Thomson tested these ideas by placing negative and positive plates along the sides of the cathode ray tube to see how the cathode ray would be affected. The cathode ray appeared to be repelled by the negative plate and attracted by the positive plate. This meant that the ray was negative in charge and that is must consist of particles that have mass. He called the particles “corpuscles,” but they were later renamed electrons. For a video demonstration of Thomson’s experiment, go to this URL:http://ericsaltchemistry.blogspot.com/2010/10/jj-thomsons-experiments-with-cathode.html.

Thomson also measured the mass of the particles he had identified. He did this by determining how much the cathode rays were bent when he varied thevoltage. He found that the mass of the particles was 2000 times smaller than the mass of the smallest atom, the hydrogen atom. In short, Thomson had discovered the existence of particles smaller than atoms. This disproved Dalton’s claim that atoms are the smallest particles of matter. From his discovery, Thomson also inferred that electrons are fundamental particles within atoms.

Q: Atoms are neutral in electric charge. How can they be neutral if they contain negatively charged electrons?

A: Atoms also contain positively charged particles that cancel out the negative charge of the electrons. However, these positive particles weren’t discovered until a couple of decades after Thomson discovered electrons.

The Plum Pudding Model

Thomson also knew that atoms are neutral in electric charge, so he asked the same question: How can atoms contain negative particles and still be neutral? He hypothesized that the rest of the atom must be positively charged in order to cancel out the negative charge of the electrons. He envisioned the atom as being similar to a plum pudding, like the one pictured in the Figure below—mostly positive in charge (the pudding) with negative electrons (the plums) scattered through it.

Q: How is our modern understanding of atomic structure different from Thomson’s plum pudding model?

A: Today we know that all of the positive charge in an atom is concentrated in a tiny central area called the nucleus, with the electrons swirling through empty space around it, as in the Figure below. The nucleus was discovered just a few years after Thomson discovered the electron, so the plum pudding model was soon rejected.

Summary

·         In 1897, J. J. Thomson discovered the first subatomic particle, the electron, while researching cathode rays.

·         To explain the neutrality of atoms, Thomson proposed a model of the atom in which negative electrons are scattered throughout a sphere of positive charge. He called his atom the plum pudding model.

Explore More

Watch this detailed presentation about J. J. Thomson’s discovery of the electron, and then answer the question below.

http://www.youtube.com/watch?v=SZSEnCydeb0 (12:30)

1.    Thomson not only discovered that a cathode ray consists of flowing negatively charged particles that are smaller than atoms. He also made the logical leap that these particles help make up atoms. What reasoning did Thomson use to make this inference?

Review

5.    Who was J. J. Thomson?

6.    Explain how Thomson discovered negatively charged particles smaller than atoms.

7.    Thomson compared his idea of atomic structure to a plum pudding. Invent an original analogy for Thomson’s plum pudding model of the atom.

8.    Why was Thomson’s model soon rejected?

Rutherford's Atomic Model

Discovery of the nucleus and the planetary model of the atom.

·         Explain how Ernest Rutherford discovered the nucleus of the atom.

·         Describe Rutherford’s planetary model of the atom.

Thom is shooting baskets. He’s trying to hit the backboard so the ball will bounce off it and into the basket. If only the backboard was bigger! It would be a lot easier to hit. If the ball misses the backboard, it will just keep going and Thom will have to run after it. Believe it or not, the research that led to the discovery of the nucleus of the atom was a little like shooting baskets.  

Narrowing Down the Nucleus

In 1804, almost a century before the nucleus was discovered, the English scientist John Dalton provided evidence for the existence of the atom. Dalton thought that atoms were the smallest particles of matter, which couldn't be divided into smaller particles. He modeled atoms with solid wooden balls. In 1897, another English scientist, named J. J. Thomson, discovered the electron. It was first subatomic particle to be identified. Because atoms are neutral in electric charge, Thomson assumed that atoms must also contain areas of positive charge to cancel out the negatively charged electrons. He thought that an atom was like a plum pudding, consisting mostly of positively charged matter with negative electrons scattered through it.

The nucleus of the atom was discovered next. It was discovered in 1911 by a scientist from New Zealand named Ernest Rutherford, who is pictured in Figurebelow. Through his clever research, Rutherford showed that the positive charge of an atom is confined to a tiny massive region at the center of the atom, rather than being spread evenly throughout the “pudding” of the atom as Thomson had suggested. You can watch a video about Rutherford and his discovery at this URL:

http://www.youtube.com/watch?v=wzALbzTdnc8 (3:28)

Go for the Gold!

The way Rutherford discovered the atomic nucleus is a good example of the role of creativity in science. His quest actually began in 1899 when he discovered that some elements give off positively charged particles that can penetrate just about anything. He called these particles alpha (α) particles (we now know they were helium nuclei). Like all good scientists, Rutherford was curious. He wondered how he could use alpha particles to learn about the structure of the atom. He decided to aim a beam of alpha particles at a sheet of very thin gold foil. He chose gold because it can be pounded into sheets that are only 0.00004 cm thick. Surrounding the sheet of gold foil, he placed a screen that glowed when alpha particles struck it. It would be used to detect the alpha particles after they passed through the foil. A small slit in the screen allowed the beam of alpha particles to reach the foil from the particle emitter. You can see the setup for Rutherford’s experiment in the Figure below.

Q: What would you expect to happen when the alpha particles strike the gold foil?

A: The alpha particles would penetrate the gold foil. Alpha particles are positive, so they might be repelled by any areas of positive charge inside the gold atoms.

Assuming a plum pudding model of the atom, Rutherford predicted that the areas of positive charge in the gold atoms would deflect, or bend, the path of all the alpha particles as they passed through. You can see what really happened in theFigure above. Most of the alpha particles passed straight through the gold foil as though it wasn’t there. The particles seemed to be passing through empty space. Only a few of the alpha particles were deflected from their straight path, as Rutherford had predicted. Surprisingly, a tiny percentage of the particles bounced back from the foil like a basketball bouncing off a backboard! You can see an animation of this experiment at the following URL:http://www.clickandlearn.org/gr9_sci/atoms/modelsoftheatom.html.

Q: What can you infer from these observations?

A: You can infer that most of the alpha particles were not repelled by any positive charge, whereas a few were repelled by a strong positive charge.

The Nucleus Takes Center Stage

Rutherford made the same inferences. He concluded that all of the positive charge and virtually all of the mass of an atom are concentrated in one tiny area and the rest of the atom is mostly empty space. Rutherford called the area of concentrated positive charge the nucleus. He predicted—and soon discovered—that the nucleus contains positively charged particles, which he named protons. Rutherford also predicted the existence of neutral nuclear particles calledneutrons, but he failed to find them. However, his student James Chadwick discovered them several years later. You can learn how at this URL:http://www.light-science.com/chadwick.html.

The Planetary Model

Rutherford’s discoveries meant that Thomson’s plum pudding model was incorrect. Positive charge is not spread evenly throughout an atom. Instead, it is all concentrated in the tiny nucleus. The rest of the atom is empty space except for the electrons scattered through it. In Rutherford’s model of the atom, which is shown in the Figure below, the electrons move around the massive nucleus like planets orbiting the sun. That’s why his model is called the planetary model. Rutherford didn’t know exactly where or how electrons orbit the nucleus. That research would be undertaken by later scientists, beginning with Niels Bohr in 1913. New and improved atomic models would also be developed. Nonetheless, Rutherford’s model is still often used to represent the atom. You can see an animated version of the model at this URL:http://www.clickandlearn.org/gr9_sci/atoms/modelsoftheatom.html.

Summary

·         Ernest Rutherford discovered the nucleus of the atom in 1910. He sent a beam of alpha particles toward gold foil and observed the way the particles were deflected by the gold atoms. From his results, he concluded that all of the positive charge and virtually all of the mass of an atom are concentrated in one tiny area, called the nucleus, and the rest of the atom is mostly empty space.

·         In Rutherford’s planetary model of the atom, the electrons move through empty space around the tiny positive nucleus like planets orbiting the sun.

Explore More

Watch this video about Rutherford’s gold foil experiment, and then answer the questions below.

http://www.youtube.com/watch?v=XBqHkraf8iE (4:06)

1.    How did Rutherford observe alpha particles in his experiment? In the modern version of Rutherford’s experiment, which is shown in the video, how are alpha particles observed? Which way do you think is more accurate?

2.    Based on the animation in the video, draw a sketch showing what happens to alpha particles as they pass through gold atoms.

3.    How has Rutherford’s gold foil experiment been adopted by modern researchers?

Review

4.    How did Ernest Rutherford discover the nucleus of the atom?

5.    Place Rutherford’s discovery in the broader history of the atom. (Hint: See the timeline at the following URL for more information:http://www.clickandlearn.org/gr9_sci/atoms/modelsoftheatom.html.)

6.    Describe how you could make a three-dimensional version of Rutherford’s planetary model of the atom.

Bohr's Atomic Model

Model of the atom that introduces energy levels where electrons are found.

·         Describe how Bohr modified Rutherford’s atomic model.

·         Explain energy levels in atoms in terms of energy quanta.

Look at the people in the picture. Do you see how they are standing on different rungs of the ladder? When you stand on a ladder, you can stand on one rung or another, but you can never stand in between two rungs. A ladder can be used to model parts of an atom. Do you know how? Read on to find out.  

Modeling the Atom

The existence of the atom was first demonstrated around 1800 by John Dalton. Then, close to a century went by before J.J. Thomson discovered the first subatomic particle, the negatively charged electron. Because atoms are neutral in charge, Thomson thought that they must consist of a sphere of positive charge with electrons scattered through it. In 1910, Ernest Rutherford showed that this idea was incorrect. He demonstrated that all of the positive charge of an atom is actually concentrated in a tiny central region called the nucleus. Rutherford surmised that electrons move around the nucleus like planets around the sun. Rutherford’s idea of atomic structure was an improvement on Thomson’s model, but it wasn’t the last word. Rutherford focused on the nucleus and didn’t really clarify where the electrons were in the empty space surrounding the nucleus.

The next major advance in atomic history occurred in 1913, when the Danish scientist Niels Bohr published a description of a more detailed model of the atom. His model identified more clearly where electrons could be found. Although later scientists would develop more refined atomic models, Bohr’s model was basically correct and much of it is still accepted today. It is also a very useful model because it explains the properties of different elements. Bohr received the 1922 Nobel prize in physics for his contribution to our understanding of the structure of the atom. You can see a picture of Bohr below.

On the Level

As a young man, Bohr worked in Rutherford’s lab in England. Because Rutherford’s model was weak on the position of the electrons, Bohr focused on them. He hypothesized that electrons can move around the nucleus only at fixed distances from the nucleus based on the amount of energy they have. He called these fixed distances energy levels, or electron shells. He thought of them as concentric spheres, with the nucleus at the center of each sphere. In other words, the shells consisted of sphere within sphere within sphere. Furthermore, electrons with less energy would be found at lower energy levels, closer to the nucleus. Those with more energy would be found at higher energy levels, farther from the nucleus. Bohr also hypothesized that if an electron absorbed just the right amount of energy, it would jump to the next higher energy level. Conversely, if it lost the same amount of energy, it would jump back to its original energy level. However, an electron could never exist in between two energy levels. These ideas are illustrated in the Figure below.

Q: How is an atom like a ladder?

A: Energy levels in an atom are like the rungs of a ladder. Just as you can stand only on the rungs and not in between them, electrons can orbit the nucleus only at fixed distances from the nucleus and not in between them.

Energy by the Spoonful

Bohr’s model of the atom is actually a combination of two different ideas: Rutherford’s atomic model of electrons orbiting the nucleus and German scientist Max Planck’s idea of a quantum, which Planck published in 1901. A quantum(plural, quanta) is the minimum amount of energy that can be absorbed or released by matter. It is a discrete, or distinct, amount of energy. If energy werewater and you wanted to add it to matter in the form of a drinking glass, you couldn’t simply pour the water continuously into the glass. Instead, you could add it only in small fixed quantities, for example, by the teaspoonful. Bohr reasoned that if electrons can absorb or lose only fixed quantities of energy, then they must vary in their energy by these fixed amounts. Thus, they can occupy only fixed energy levels around the nucleus that correspond to quantum increases in energy.

Q: The idea that energy is transferred only in discrete units, or quanta, was revolutionary when Max Planck first proposed it in 1901. However, what scientists already knew about matter may have made it easier for them to accept the idea of energy quanta. Can you explain?

A: Scientists already knew that matter exists in discrete units called atoms. This idea had been demonstrated by John Dalton around 1800. Knowing this may have made it easier for scientists to accept the idea that energy exists in discrete units as well.

Summary

·         In Bohr’s atomic model, electrons move around the nucleus only at fixed distances from the nucleus based on the amount of energy they have. The fixed distances where electrons may orbit are called energy levels.

·         Bohr arrived at his model by applying Planck’s idea of energy quanta to Rutherford’s atomic model of electrons orbiting the nucleus.

Explore More

Read the article at the following URL. Be sure to click through the animation at the bottom of the Web page. Then answer the questions below.

http://www.brooklyn.cuny.edu/bc/ahp/SDPS/SD.PS.electrons.html

1.    What are atomic orbitals? How many electrons can each orbital hold?

2.    How are orbitals related to energy levels?

3.    How many electrons can be found at energy level 1? At energy level 2?

Review

4.    How does Bohr’s atomic model build on Rutherford’s model?

5.    Explain the connection between energy quanta and energy levels.

6.    How does Bohr’s work demonstrate the importance of communication in science?

Electron Cloud Atomic Model

Electrons travel in waves, which means their exact positions cannot be determined.