thomsons atomic model

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 wasnt 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 1.1.

thomsons experiments

In his research, Thomson passed current through a cathode ray tube, similar to the one seen in the Figure 1.2. 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? Thats what Thomson wanted to know. Is it just a ray of energy that travels in waves like a ray of light? That was one popular hypothesis at 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. 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 the voltage. 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 Daltons 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 werent discovered until a couple of decades after Thomson discovered electrons.

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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 1.3mostly positive in charge (the pudding) with negative electrons (the plums) scattered through it. Q: How is our modern understanding of atomic structure different from Thomsons 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 1.4. The nucleus was discovered just a few years after Thomson discovered the electron, so the plum pudding model was soon rejected.

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instructional diagrams

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questions

thomson won a nobel prize for his discovery of protons.

a. true

-->  b. false

thomson knew that the particles he discovered were smaller than atoms because of their

a) charge.

b) color.

-->  c) mass.

d) volume.

thomson thought that the positive charges in an atom were

a) concentrated in the nucleus.

b) clumped into small clusters like plums.

-->  c) spread throughout the atom.

d) none of the above

thomsons research proved daltons earlier claim about the smallest particles of matter.

a. true

-->  b. false

diagram questions

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