history of the atom

daltons research

Dalton did many experiments that provided evidence for atoms. For example, he studied the pressure of gases. He concluded that gases must consist of tiny particles in constant motion. Dalton also researched the properties of compounds. He showed that a compound always consists of the same elements in the same ratio. On the other hand, different compounds always consist of different elements or ratios. This can happen, Dalton reasoned, only if elements are made of tiny particles that can combine in an endless variety of ways. From his research, Dalton developed a theory of the atom. You can learn more about Dalton and his research by watching the video at this URL: (9:03). MEDIA Click image to the left or use the URL below. URL:

dalton brings back the atom

Around 1800, a British chemist named John Dalton revived Democrituss early ideas about the atom. Dalton is pictured in Figure 5.8. He made a living by teaching and just did research in his spare time. Nonetheless, from his research results, he developed one of the most important theories in science.

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daltons atomic models

Dalton incorrectly thought that atoms are tiny solid particles of matter. He used solid wooden balls to model them. The sketch in the Figure 5.9 shows how Daltons model atoms looked. He made holes in the balls so they could be joined together with hooks. In this way, the balls could be used to model compounds. When later scientists discovered subatomic particles (particles smaller than the atom itself), they realized that Daltons models were too simple. They didnt show that atoms consist of even smaller particles. Models including these smaller particles were later developed.

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daltons atomic theory

The atomic theory Dalton developed consists of three ideas: All substances are made of atoms. Atoms are the smallest particles of matter. They cannot be divided into smaller particles. They also cannot be 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. A given compound always consists of the same kinds of atoms in the same ratio. Daltons theory was soon widely accepted. Most of it is still accepted today. The only part that is no longer accepted is his idea that atoms are the smallest particles. Scientists now know that atoms consist of even smaller particles.

thomsons vacuum tube experiments

Thomson was interested in electricity. He did experiments in which he passed an electric current through a vacuum tube. The experiments are described in Figure 5.10. Thomsons experiments showed that an electric current consists of flowing, negatively charged particles. Why was this discovery important? Many scientists of Thomsons time thought that electric current consists of rays, like rays of light, and that it is positive rather than negative. Thomsons experiments also showed that the negative particles are all alike and smaller than atoms. Thomson concluded that the negative particles couldnt be fundamental units of matter because they are all alike. Instead, they must be parts of atoms. The negative particles were later named electrons.

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thomson adds electrons

The next major advance in the history of the atom was the discovery of electrons. These were the first subatomic particles to be identified. They were discovered in 1897 by a British physicist named J. J. Thomson. You can learn more about Thomson and his discovery at this online exhibit: .

rutherford finds the nucleus

A physicist from New Zealand named Ernest Rutherford made the next major discovery about atoms. He discovered the nucleus. You can watch a video about Rutherford and his discovery at this URL: MEDIA Click image to the left or use the URL below. URL:

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thomsons plum pudding model

Thomson knew that atoms are neutral in electric charge. So how could atoms contain negative particles? Thomson thought that the rest of the atom must be positive to cancel out the negative charge. He said that an atom is like a plum pudding, which has plums scattered through it. Thats why Thomsons model of the atom is called the plum pudding model. You can see it in Figure 5.11. It shows the atom as a sphere of positive charge (the pudding) with negative electrons (the plums) scattered through it.

the nucleus and its particles

Based on his results, Rutherford concluded that all the positive charge of an atom is concentrated in a small central area. He called this area the nucleus. Rutherford later discovered that the nucleus contains positively charged particles. He named the positive particles protons. Rutherford also predicted the existence of neutrons in the nucleus. However, he failed to find them. One of his students, a physicist named James Chadwick, went on to discover neutrons in 1932. You learn how at this URL: .

democritus introduces the atom

The history of the atom begins around 450 B.C. with a Greek philosopher named Democritus (see Figure 5.7). Democritus wondered what would happen if you cut a piece of matter, such as an apple, into smaller and smaller pieces. He thought that a point would be reached where matter could not be cut into still smaller pieces. He called these "uncuttable" pieces atomos. This is where the modern term atom comes from. Democritus was an important philosopher. However, he was less influential than the Greek philosopher Aristotle, who lived about 100 years after Democritus. Aristotle rejected Democrituss idea of atoms. In fact, Aristotle thought

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rutherfords atomic model

Rutherfords discoveries meant that Thomsons plum pudding model was incorrect. Positive charge is not spread out everywhere in an atom. It is all concentrated in the tiny nucleus. The rest of the atom is empty space, except for the electrons moving randomly through it. In Rutherfords model, electrons move around the nucleus in random orbits. He compared them to planets orbiting a star. Thats why Rutherfords model is called the planetary model. You can see it in Figure 5.13.

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rutherfords gold foil experiments

In 1899, Rutherford discovered that some elements give off positively charged particles. He named them alpha particles (a). In 1911, he used alpha particles to study atoms. He aimed a beam of alpha particles at a very thin sheet of gold foil. Outside the foil, he placed a screen of material that glowed when alpha particles struck it. If Thomsons plum pudding model were correct, the alpha particles should be deflected a little as they passed through the foil. Why? The positive "pudding" part of gold atoms would slightly repel the positive alpha particles. This would cause the alpha particles to change course. But Rutherford got a surprise. Most of the alpha particles passed straight through the foil as though they were moving through empty space. Even more surprising, a few of the alpha particles bounced back from the foil as though they had struck a wall. This is called back scattering. It happened only in very small areas at the centers of the gold atoms.

instructional diagrams

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questions

Which statement is not part of the original atomic theory?

-->  a. Atoms consist of smaller particles of matter.

b. Atoms cannot be created or destroyed.

c. All atoms of the same element have the same mass.

d. Atoms join together to form compounds.

John Dalton thought that an atom is like a(n)

a. plum pudding.

b. solar system.

-->  c. hard solid ball.

d. vacuum tube.

The scientist who discovered protons was

a. John Dalton.

b. J. J. Thomson.

-->  c. Ernest Rutherford.

d. James Chadwick.

Aristotle thought that

a. atoms exist.

b. atoms are uncuttable.

c. all matter consists of atoms.

-->  d. the idea of atoms is ridiculous.

Rutherford concluded from his experiments that

a. all the positive charge of an atom is concentrated in the center.

b. positive charge is spread evenly throughout an atom.

c. electrons orbit the positively charged nucleus.

-->  d. two of the above

scientist who discovered electrons.

a. Democritus

b. Aristotle

c. John Dalton

-->  d. J. J. Thomson

e. Ernest Rutherford

f. plum pudding model

g. planetary model

philosopher who thought the idea of the atom was ridiculous

a. Democritus

-->  b. Aristotle

c. John Dalton

d. J. J. Thomson

e. Ernest Rutherford

f. plum pudding model

g. planetary model

Thomsons atomic model

a. Democritus

b. Aristotle

c. John Dalton

d. J. J. Thomson

e. Ernest Rutherford

-->  f. plum pudding model

g. planetary model

Thomson aimed a beam of alpha particles at gold foil.

a. true

-->  b. false

philosopher who introduced the idea of the atom

-->  a. Democritus

b. Aristotle

c. John Dalton

d. J. J. Thomson

e. Ernest Rutherford

f. plum pudding model

g. planetary model

The plums in the plum pudding model represent protons.

a. true

-->  b. false

Rutherfords atomic model

a. Democritus

b. Aristotle

c. John Dalton

d. J. J. Thomson

e. Ernest Rutherford

f. plum pudding model

-->  g. planetary model

scientist who developed atomic theory

a. Democritus

b. Aristotle

-->  c. John Dalton

d. J. J. Thomson

e. Ernest Rutherford

f. plum pudding model

g. planetary model

The planets in the planetary model represent electrons.

-->  a. true

b. false

scientist who discovered the nucleus

a. Democritus

b. Aristotle

c. John Dalton

d. J. J. Thomson

-->  e. Ernest Rutherford

f. plum pudding model

g. planetary model

The first subatomic particle to be discovered was the proton.

a. true

-->  b. false

Dalton thought that all substances are made of atoms.

-->  a. true

b. false

The history of the atom began almost

-->  a. 2500 years ago.

b. 700 years ago.

c. 500 years ago.

d. 100 years ago.

Daltons research provided evidence that

a. atoms exist.

b. gases consist of tiny particles in constant motion.

c. a compound always consists of the same elements in the same ratio.

-->  d. all of the above

Which statement is part of Daltons atomic theory?

-->  a. All substances are made of atoms.

b. Atoms can be divided into smaller particles.

c. Atoms form when compounds join together.

d. All atoms of the same element have the same number of protons.

Daltons atomic models were most similar to

-->  a. bowling balls.

b. plum puddings.

c. planetary orbits.

d. blades of a fan.

Thomsons research involved

a. gold foil and alpha particles.

-->  b. electric current and a vacuum tube.

c. gases and pressure.

d. neutrons and back scattering.

In the plum pudding model of the atom, the plums represent

a. protons.

b. neutrons.

c. nuclei.

-->  d. electrons.

In the planetary model, the planets represent

a. alpha particles.

b. gold atoms.

-->  c. electrons.

d. positive charges.

Aristotle rejected Democrituss idea of the atom.

-->  a. true

b. false

Dalton thought that atoms could be created or destroyed.

a. true

-->  b. false

Daltons atomic theory was later completely rejected.

a. true

-->  b. false

Ernest Rutherford discovered neutrons.

a. true

-->  b. false

Thomson showed that electric charge is carried by particles of matter.

-->  a. true

b. false

The pudding in the plum pudding model represents positive charge.

-->  a. true

b. false

Democritus represented atoms with solid wooden balls.

a. true

-->  b. false

In the gold foil experiments, most of the alpha particles were deflected backward from the gold foil.

a. true

-->  b. false

Dalton was the first scientist to observe atoms with a microscope.

a. true

-->  b. false

Electrons flow through a vacuum tube from the negative end to the positive end.

-->  a. true

b. false

diagram questions

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