1. Based on Dalton s evidence, circle the drawing that demonstrates Dalton s model.

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1 Various models of the ATOM Dalton Model John Dalton developed the first atomic model in Before him people, mostly philosophers, had speculated about the smallest unit of matter and two theories prevailed. Aristotle s Theory Matter could always be divided into smaller and smaller pieces; there was not a smallest piece of matter An analogy would be that a piece of paper can be cut in half multiple times and still looks like and has the properties of a piece of paper Democritus Theory Matter could not be divided into smaller pieces for ever; called these hard, round particles atomos An analogy would be that from a distance a beach is smooth and solid but up close, it can be seen that it is made of grains of sand Dalton noticed that during chemical reactions, certain elements combined in specific proportions. For example, when hydrogen and oxygen combined to form water, there was always two parts hydrogen to one part oxygen. 1. Based on Dalton s evidence, circle the drawing that demonstrates Dalton s model. 2. Draw a diagram to illustrate that when hydrogen and oxygen react, there are always two parts hydrogen and one part oxygen that make up water. 3. Which theory does Dalton s evidence support: Aristotle or Democritus theory? Why? Later, scientists were able to show that the atom is made of smaller subatomic particles. 4. Does Dalton s model account for these smaller particles? Why or why not?

2 Thomson Model J.J. Thomson developed his own model of the atom after experimenting with cathode rays in His model was based off of Dalton s model, which was developed in Dalton s Atomic Theory Atoms are the smallest form of matter, generally depicted as hard spheres. 1. Atoms are tiny, indivisible particles. 2. Atoms of the same element are identical. 3. Atoms of different elements combine in whole number ratios to form compounds. 4. Chemical reactions involve the rearrangement of atoms. (No new atoms created or destroyed) A cathode ray is produced when a high voltage is applied to an electrode in a closed tube with a partial vacuum. A glowing beam travels from the cathode to the anode. J.J. Thomson discovered that cathode rays could be deflected by magnets and/or charged plates. The set-up is shown below. First, he generated the rays, which move in a straight line from one end of the tube to the other, and cannot move around objects. Then, he put an electric field around the tube, causing the ray to bend. Thomson deduced that the particles that made up the ray were from atoms and were negatively charged. He knew that the atom was overall neutral. Thomson viewed the atom as a sphere of uniform positive charge with negative electrons embedded throughout. 1. Based on Thomson s evidence, circle the drawing that demonstrates Thomson s model. 2. Was the particle that made up the ray positively or negatively charged? How do you know? 3. How does Thomson s theory contradict Dalton s theory? This model is often called the plum pudding model. Plum pudding has raisins studded within a pudding. 4. How does this model resemble plum pudding with raisins? What do the raisins represent?

3 Rutherford Model In 1896 Henri Becquerel discovered radioactivity. Working with radium (a radioactive element) Rutherford discovered that the invisible rays were actually composed of three different rays: alpha (positively charged), beta (negatively charged), and gamma (uncharged) rays. These experiments were performed while Rutherford was the head of the physics department at McGill University in Montreal. Ernest Rutherford performed an elegant experiment in 1911 to clarify the structure of the atom. Because it used gold foil as the source of atoms, the experiment has become known as the gold foil experiment. He built his assumptions on the most current model, the Thomson model, which was developed in Thomson s Atomic Theory Atoms had a small, negatively charged particle called an electron. Because the atom is overall neutral, the atom must also have a positively charged area. Thomson envisioned an atom where the negatively charged electrons were embedded within a positively-charged sphere. Rutherford shot positively charged particles (called alpha particles) at a thin layer of gold atoms. If the positive charge of the atom is evenly distributed, and the electrons are spread out, and have little mass, there should be nothing to prevent the fast alpha particles from passing through the gold foil unchanged. Many of the alpha particles went straight through the foil. However, some of the particles were slightly deflected or bounced straight backwards. 1. If most of the alpha particles went through, what does this tell you about the empty space in an atom? (Is there a lot of empty space in an atom?) Why did you choose your answer? 2. If the alpha particles were deflected, what does that tell you about the charge of the middle of the atom? (The middle of the atom is called the nucleus.) Is it positive or negative? How do you know? 3. If the nucleus has the charge you chose in question 3, where must the electrons be? 4. Based on Rutherford s evidence, circle the drawing that demonstrates Rutherford s model.

4 Bohr Model Niels Bohr developed his atomic model in The current model was the Rutherford model, which stated the following criteria. Rutherford s Atomic Theory Atoms are mostly empty space. Most of the mass of the atom is concentrated in a dense positivelycharged nucleus in the middle of the atom. The electrons were supposed to circle around the nucleus. The problem with this model is that because an orbiting electron is always accelerating it should be losing energy and would eventually collapse into the nucleus. (attraction of opposite charges) Additionally, all energies values are lost resulting in a continuous light spectrum. Bohr studied the line spectrum of hydrogen. Hydrogen gas was first excited (forced into a higher energy state) by an electric current, it will emit energy in the form of visible light. If this light is passed through a prism, it separates into its component colorus or frequencies. The line spectrum of hydrogen at the atomic emission spectra of different elements. These spectra resulted when an electrical current is run through a chamber filled with a gas. Unlike regular white light, these elements give characteristic lines that showed the electrons moving between set energy levels. The line spectrum of hydrogen consists of four prominent colored lines, each of which corresponds to a characteristic energy. Bohr believed that these lines were produced when an electron in the hydrogen atom dropped from a higher energy level to a lower energy level, releasing energy in the form of light. He believed that the electrons orbit at fixed distances from the nucleus. This distance is known as an energy level. Higher energy levels are located further from the nucleus. An electron 1. Based on Bohr s evidence, circle the drawing that demonstrates Bohr s model. 2. Show an electron absorbing energy and moving to a higher energy level. 3. A photon of light is given off when the electron drops back down to a lower energy level. Why are only certain light energies represented in the spectrum? (What does this tell you about the energy levels?))

5 Bohr s model was one of the first models to use mathematics to predict the behavior of electrons. However, Bohr s model could not explain the lines of even slightly more complex atoms (like helium). This led to the development of the modern model, called the quantum mechanical model.

6 Names: Extension: Match the key words with the atomic model (some may be used twice): 1. orbits 2. orbitals 3. clouds 4. solid sphere 5. plum pudding 6. dense, positively-charged nucleus 7. gold foil experiment A. Rutherford Model B. J.J. Thompson C. Dalton s model D. Bohr Model (planetary model) E. wave-mechanical model Write the correct atomic model underneath the diagram that represents it: