2.2 dalton s atomic theory - chemistry...

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CHAPTER 2 History of the AtomicTheory

Chapter Outline2.1 DEMOCRITUS’ IDEA OF THE ATOM

2.2 DALTON’S ATOMIC THEORY

2.3 THOMSON’S ATOMIC MODEL

2.4 RUTHERFORD’S ATOMIC MODEL

2.5 BOHR’S ATOMIC MODEL

2.6 QUANTUM MECHANICAL ATOMIC MODEL

2.7 REFERENCES

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www.ck12.org Chapter 2. History of the Atomic Theory

2.1 Democritus’ Idea of the Atom

• Describe how the Greek philosophers approached nature.• Describe the discussion about matter.• Describe the contribution Democritus made to our understanding of matter.

What would the philosophers do?

People enjoy getting together to discuss things, whether it is how your favorite sports team is doing, what the bestnew movie is, the current politics, or any number of other topics. Often the question is raised about who is rightand who is wrong. If the football game is to be played this coming weekend, all we can do is offer opinions as to itsoutcome. The game has not been played yet, so we don’t know who will actually win.

The ancient Greek philosophers did a lot of discussing, with part of their conversations concerning the physicalworld and its composition. There were different opinions about what made up matter. Some felt one thing wastrue while others believed another set of ideas. Since these scholars did not have laboratories and had not developedthe idea of the experiment, they were left to debate. Whoever could offer the best argument was considered right.However, often the best argument had little to do with reality.

One of the on-going debates had to do with sand. The question posed was: into how small of pieces can you dividea grain of sand? The prevailing thought at the time, pushed by Aristotle, was that the grain of sand could be divided

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2.1. Democritus’ Idea of the Atom www.ck12.org

indefinitely, that you could always get a smaller particle by dividing a larger one and there was no limit to how smallthe resulting particle could be.

FIGURE 2.1Into how small of pieces can you divide agrain of sand?

Since Aristotle was such an influential philosopher, very few people disagreed with him. However, there weresome philosophers who believed that there was a limit to how small a grain of sand could be divided. One ofthese philosophers was Democritus (~460-~370 B.C.), often referred to as the “laughing philosopher” because ofhis emphasis on cheerfulness. He taught that there were substances called atoms and that these atoms made up allmaterial things. The atoms were unchangeable, indestructible, and always existed.

FIGURE 2.2Democritus.

The word “ atom” comes from the Greek atomos and means “indivisible.” The atomists of the time (Democritusbeing one of the leading atomists) believed there were two realities that made up the physical world: atoms and void.There was an infinite number of atoms, but different types of atoms had different sizes and shapes. The void was theempty space in which the atoms moved and collided with one another. When these atoms collided with one another,they might repel each other or they might connect in clusters, held together by tiny hooks and barbs on the surfaceof the atoms.

Aristotle disagreed with Democritus and offered his own idea of the composition of matter. According to Aristotle,

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everything was composed of four elements: earth, air, fire, and water. The theory of Democritus explained thingsbetter, but Aristotle was more influential, so his ideas prevailed. We had to wait almost two thousand years beforescientists came around to seeing the atom as Democritus did.

How right was Democritus?

It is very interesting that Democritus had the basic idea of atoms, even though he had no experimental evidence tosupport his thinking. We now know more about how atoms hold together in “clusters” (compounds), but the basicconcept existed over two thousand years ago. We also know that atoms can be further subdivided, but there is stilla lower limit to how small we can break up that grain of sand.

Summary

• Greek philosophers debated about many things.• Aristotle and others believed that a grain of sand could be divided indefinitely.• Democritus believed there was a lower limit to the dividing of a grain of sand.

Practice

Questions

Use the link below to answer the following questions:

http://plato.stanford.edu/entries/democritus/

1. Who influenced the thinking of Democritus?2. Who were the atomists?3. How did Democritus explain how we saw objects?4. What type of atom did Democritus believe the soul was composed of?

Review

Questions

1. How did the ancient Greek philosophers spend their time?2. What approach did they not have for studying nature?3. Who was the most influential philosopher of that time?4. What was the major contribution Democritus made to the thinking of his day?5. List characteristics of atoms according to Democritus.

• philosopher: People who do a lot of discussing and debate, with part of their conversations concerning thephysical world and its composition.

• atom: The philosopher Democritus (~460-~370 B.C.), taught that there were substances called atoms and thatthese atoms made up all material things. The atoms were unchangeable, indestructible, and always existed.

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2.2. Dalton’s Atomic Theory www.ck12.org

2.2 Dalton’s Atomic Theory

• List the components of Dalton’s atomic theory.

Pick a little, talk a little, pick a little, talk a little,

Cheep cheep cheep, talk a lot, pick a little more

These lyrics from the musical “Music Man” summed up the way science was done for centuries. OK, the lyricsreferred to a group of gossiping ladies, but the outcome was the same. The Greek and Roman philosophersdebated, discussed, and sometimes even attacked one another. But the mode of discovery was talk. There wasno experimentation –the idea had not been thought of yet. So science did not develop very far and there was noreliable way to establish what was true and what was false.

John Dalton

While it must be assumed that many more scientists, philosophers and others studied the composition of matter afterDemocritus, a major leap forward in our understanding of the composition of matter took place in the 1800s withthe work of the British scientist John Dalton. He started teaching school at age twelve, and was primarily known as

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a teacher. In his twenties, he moved to the growing city of Manchester, where he was able to pursue some scientificstudies. His work in several areas of science brought him a number of honors. When he died, over 40,000 peoplein Manchester marched at his funeral.

Dalton studied the weights of various elements and compounds. He noticed that matter always combined infixed ratios based on weight, or volume in the case of gases. Chemical compounds always contain the sameproportion of elements by mass, regardless of amount, which provided further support for Proust’s law of definiteproportions. Dalton also observed that there could be more than one combination of two elements.

FIGURE 2.3Dalton.

Dalton’s Atomic Theory (1804)

From his experiments and observations, as well as the work from peers of his time, Dalton proposed a new theory ofthe atom. This later became known as Dalton’s atomic theory. The general tenets of this theory were as follows:

• All matter is composed of extremely small particles called atoms.• Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ

in size, mass, and other properties.• Atoms cannot be subdivided, created, or destroyed.• Atoms of different elements can combine in simple whole number ratios to form chemical compounds.• In chemical reactions, atoms are combined, separated, or rearranged

Dalton’s atomic theory has been largely accepted by the scientific community, with the exception of three changes.We know now that (1) an atom can be further sub-divided, (2) all atoms of an element are not identical in mass, and(3) using nuclear fission and fusion techniques, we can create or destroy atoms by changing them into other atoms.

Summary

• Dalton proposed his atomic theory in 1804.• The general tenets of this theory were as follows:

– All matter is composed of extremely small particles called atoms.– Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements

differ in size, mass, and other properties.– Atoms cannot be subdivided, created, or destroyed.– Atoms of different elements can combine in simple whole number ratios to form chemical compounds.– In chemical reactions, atoms are combined, separated, or rearranged

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2.2. Dalton’s Atomic Theory www.ck12.org

FIGURE 2.4Dalton’s symbols.

Practice

Use the link below to do the exercise. Read the sections and take the quiz at the end.

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

Review

Questions

1. How did the Greek and Roman philosophers study nature?2. When did John Dalton start teaching school?3. Did Dalton believe that atoms could be created or destroyed?4. List the basic components of Dalton’s atomic theory.5. What parts of the theory are not considered valid any more?

• atom: The smallest unit of an element.• atomic theory: The general tenets of Dalton’s atomic theory were as follows:

– All matter is composed of extremely small particles called atoms.– Atoms of a given element are identical in size, mass, and other properties.– Atoms of different elements differ in size, mass, and other properties.– Atoms cannot be subdivided, created, or destroyed.– Atoms of different elements can combine in simple whole number ratios to form chemical compounds.– In chemical reactions, atoms are combined, separated, or rearranged

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2.3 Thomson’s Atomic Model

• Explain what a model is.• Describe the “plum pudding” model of the atom.

Millions of children over the years have enjoyed building models - this model airplane is one example of the typesthat can be constructed. Perhaps sixty years ago the models were made of balsa wood, a very light material. Partswould be cut by hand, carefully glued together, and then covered with paper or other fabric. The development ofplastics made the construction of model aircraft must simpler in many respects and the end-product is more durableand damage-proof.

A model serves a useful purpose –it gives us an idea of what the real thing is like. The model plane seen above haswings, a tail, and an engine just like the real thing. This model also has a propeller, as is the case with most smallplanes and some smaller passenger planes. However, the model is not the real thing. We certainly cannot fly peopleor cargo in the model, but we can get some idea of what a real plane looks like and how it works.

Science uses many models to explain ideas. We model the electron as a very small particle with a negativecharge. That gives us a picture, but a very incomplete one. This picture works fine for most chemists, but isinadequate for a physicist. Models give us a start toward understanding structures and processes, but certainly arenot a complete representation of the entity we are examining.

Atomic Models

The electron was discovered by J.J. Thomson in 1897. Protons were also known, as was the fact that atoms wereneutral in charge. Since the intact atom had no net charge and the electron and proton had opposite charges, the nextstep after the discovery of subatomic particles was to figure out how these particles were arranged in the atom. This

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2.3. Thomson’s Atomic Model www.ck12.org

is a difficult task because of the incredibly small size of the atom. Therefore, scientists set out to design a model ofwhat they believed the atom could look like. The goal of each atomic model was to accurately represent all of theexperimental evidence about atoms in the simplest way possible.

Following the discovery of the electron, J.J. Thomson developed what became known as the “ plum pudding” modelin 1904. Plum pudding is an English dessert similar to a blueberry muffin. In this model, the electrons were stuckinto a uniform lump of positive charge like blueberries in a muffin. In Thomson’s plum pudding model of the atom,the electrons were embedded in a uniform sphere of positive charge. The positive matter was thought to be jelly-like or a thick soup. The electrons could move around somewhat. As they got closer to the outer portion of the atom,the positive charge in the region was greater than the neighboring negative charges and the electron would be pulledback more toward the center region of the atom.

FIGURE 2.5

This model of the atom soon gave way, however, to a new model developed by New Zealander Ernest Rutherford(1871-1937) about five years later. Thomson received many honors during his lifetime, including being awardedthe Nobel Prize in Physics in 1906 and a knighthood in 1908

Summary

• A model gives an idea of what something looks like, but is not the real thing.• The “plum pudding” model of the atom consisted of a uniform sphere of positive charge with negative

electrons imbedded in the sphere.

Practice

Use the link below to answer the following questions:

http://www.universetoday.com/38326/plum-pudding-model/

1. In the plum pudding model of the atom, what are the plums?2. In this model, what is the dough?3. What was the major purpose of the plum pudding model?4. How is this model different from modern modes of the atom?

Review

1. What is a model?2. Why are models useful in science?3. In Thomson’s model of the atom, where were the electrons?4. What was the positive charge in this model?

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5. What kept the electrons in the atom?6. Whose model replaced Thomson’s?7. What awards did Thomson receive?

• atomic model: When scientists set out to design a model of what they believed the atom could look like, thegoal of each atomic model was to accurately represent all of the experimental evidence about atoms in thesimplest way possible.

• plum pudding: In 1904 J.J. Thomson developed this model. The electrons were stuck into a uniform lump ofpositive charge like blueberries in a muffin. The positive matter was thought to be jelly- like or a thick soup.The electrons could move around somewhat. As they got closer to the outer portion of the atom, the positivecharge in the region was greater than the neighboring negative charges and the electron would be pulled backmore toward the center region of the atom.

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2.2 dalton s atomic theory - chemistry...

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