In many ways, Dalton's ideas are still useful today. For example, they help us to understand elements, compounds, and molecules.

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1 History of the Atom Name: Reading excerpt from Absorb Chemistry for GCSE by Lawrie Ryan Introduction Our understanding of the physical world has grown at an incredible rate in the last 200 years. The key to the advances made in chemistry has been our growing knowledge about atoms. In this unit we will look at some of the early historical discoveries that helped us build up a useful 'working model' of the atom. John Dalton Ancient Greeks had ideas about particles and atoms. But it wasn't until the start of the nineteenth century that a theory of atoms became linked to strong experimental evidence. It was then that an English scientist called John Dalton put forward his ideas about atoms. From his experiments and observations, he suggested that atoms were like tiny, hard balls. Each chemical element had its own atoms that differed from others in mass. Dalton believed that atoms were the fundamental building blocks of nature and could not be split. In chemical reactions, the atoms would rearrange themselves and combine with other atoms in new ways. In many ways, Dalton's ideas are still useful today. For example, they help us to understand elements, compounds, and molecules. 1. Which of Dalton's ideas below do we no longer believe? Elements contain only 1 type of atom Atoms rearrange in chemical reactions. Atoms are solid masses that can't be split into smaller particles. 2. Dalton made a list of substances that he believed were elements. Decide whether the following substances from his list are elements or compounds. Soda Oxygen Carbon Gold Lime J.J. Thomson At the end of the nineteenth century, a scientist called J.J. Thomson discovered the electron. This is a tiny negatively charged particle that is much, much smaller than any atom. When he discovered the electron, Thomson was experimenting by applying high voltages to gases at low pressure. He noticed an interesting effect. This is shown in the experiment in Fig.2 below: Thomson did experiments on the beams of particles in his tube. They were attracted to a positive charge, so Thomson correctly concluded that they must be negatively charged themselves. Other experiments showed that it would take about 2000 electrons to weigh the same as the lightest atom, hydrogen. He called the tiny, negatively charged particles electrons.

2 But where had these tiny particles come from? Since they were so small, Thomson suggested that they could only have come from inside atoms. So Dalton's idea of the indestructible atom had to be revised. Thomson proposed a different model for the atom. He said that the tiny negatively charged electrons must be embedded in a cloud of positive charge (after all, atoms themselves carry no overall charge, so the charges must balance out). Thomson imagined the electrons as the bits of plum in a plum pudding (rather like currants spread through a Christmas pudding but with lots more space in between) 3. Which statement about Thomson's model of the atom is true? A) The charge on the electrons is far greater than the positive charge in the atom. B) The total charge carried by the electrons equals the positive charge in the atom. Ernest Rutherford The next development came about 10 years later. Two of Ernest Rutherford's students, Hans Geiger and Ernest Marsden, were doing an experiment at Manchester University with radiation. They were using the dense, positively charged particles (called alpha particles) as 'bullets' to fire at a very thin piece of gold foil. They expected the particles to barge their way straight through the gold atoms unimpeded by the diffuse positive charge spread throughout the atom that Thomson's model described. However, they got a big surprise. Look at their experiment below: In 1911, Ernest Rutherford interpreted these results and suggested a new model for the atom. He said that Thomson's model could not be right. The positive charge must be concentrated in a tiny volume at the center of the atom, otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source. On this model, the electrons orbited around the dense nucleus (center of the atom). Here is Rutherford's model:

3 4. In which 2 ways did Rutherford change Thomson's model of the atom? A) He said that the electrons were concentrated in the center of the atom (nucleus) B) He said that the positive charge was concentrated in the center of the atom (nucleus) C) He said that the electrons were orbiting the center of the atom (nucleus) Niels Bohr The next important development came in 1914 when Danish physicist Niels Bohr revised the model again. It had been known for some time that the light given out when atoms were heated always had specific amounts of energy, but no one had been able to explain this. Bohr suggested that the electrons must be orbiting the nucleus in certain fixed energy levels (or shells). The energy must be given out when 'excited' electrons fall from a high energy level to a low one. Figure 6: Bohr's Model of the Atom 5. How did Bohr change Rutherford's model of the atom? A) He said that the electrons were concentrated in the center of the atom. B) He said that electrons could only occupy specific energy levels as they orbit the nucleus. C) He said that the electrons could orbit the nucleus in a completely random way. Additional information: Erwin Shrodinger Because Bohr's model was incorrect for atoms other than hydrogen, scientists needed a new approach. In the mid 1920s Erwin Shrodinger suggested that electrons might have the properties of both particles and waves. Shrodinger's mathematic applications lead to a model of the atom that is called the Modern Atomic Model. This new model does not put electrons in specific energy orbits similar to planets revolving around the sun. The Modern Atomic Model states that it is impossible to know the exact location of an electron. Orbitals (or electron clouds) are used to show a region around the nucleus where the electron is likely to exist most of the time. 6. How did Schrodinger change Bohr's model of the atom? A) He said that it is not possible to know the exact location of protons. B) He discovered neutrons and placed them in clouds or orbitals. C) He said that we can only know the probability of finding an electron in a certain region around the nucleus.

4 Practice Questions: Name: 6. Which of the following ideas of John Dalton are still believed to be correct? A) Atoms are solid B) Atoms in an element are all the same type of atom. C) Atoms cannot be split into smaller particles. D) Compounds contain different types of atoms. 7. Who proposed the 'Plum Pudding' model of the atom? 8. What is the charge on an electron? A) positive B) negative C) neutral 9. Who is given credit to figuring out this charge on the electron? 9. Who is given credit to figuring out that the atom is mostly made up of empty space? 10. Which statement below is correct? A) Most of the volume of an atom is taken up by a large nucleus? B) Most of the volume of an atom is taken up by the space that the electrons move around the nucleus. 11. How did Rutherford revise ideas about the distribution of positive charge in an atom compared with the 'plum pudding' model? A) Rutherford suggested that the charge was evenly spread throughout the atom. B) Rutherford suggested there was no positive charge in the atom. C) Rutherford suggested that the positive charge was concentrated in the center of the atom. 12. Who first suggested that the electrons orbited the nucleus in fixed energy levels (shells)?. 13. The Modern Atomic Model (Electron Cloud Model, Wave Mechanical Model) show electrons existing: A) in specific energy levels around the nucleus. B) in the nucleus at the center of the atom. C) in a region where there are most likely to be found.

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