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 couldnt 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 Figure 1.1. 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.  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 () 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 Rutherfords experiment in the Figure 1.2. 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 the Figure 1.2. Most of the alpha particles passed straight through the gold foil as though it wasnt 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! 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.  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 predictedand soon discoveredthat the nucleus contains positively charged particles, which he named protons. Rutherford also predicted the existence of neutral nuclear particles called neutrons, but he failed to find them. However, his student James Chadwick discovered them several years later.  Rutherfords discoveries meant that Thomsons 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 Rutherfords model of the atom, which is shown in the Figure 1.3, the electrons move around the massive nucleus like planets orbiting the sun. Thats why his model is called the planetary model. Rutherford didnt know exactly where or how electrons
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 could nt 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 Figure 1.1 . 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 . 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 ( ) 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 Rutherfords experiment in the Figure 1.2 . 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 the Figure 1.2 . Most of the alpha particles passed straight through the gold foil as though it was nt 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 ! 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 . 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 predictedand soon discoveredthat the nucleus contains positively charged particles , which he named protons . Rutherford also predicted the existence of neutral nuclear particles called neutrons , but he failed to find them . However , his student James Chadwick discovered them several years later . Rutherfords discoveries meant that Thomsons 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 Rutherfords model of the atom , which is shown in the Figure 1.3 , the electrons move around the massive nucleus like planets orbiting the sun . That s why his model is called the planetary model . Rutherford did nt know exactly where or how electrons
in rutherfords experiments, most of the alpha particles
in rutherfords experiments , most of the alpha particles
passed straight through the gold foil.
passed straight through the gold foil.