Name: ___________________________________ Per: ______ Date: ___________ History of the Atom Video Questions Video: The 2,400 year search for the atom Democritus – (460-370 BCE) 1. 1st to propose that matter is made of tiny particles surrounded by empty space. 2. He called these particles _____________. 3. ________________disagreed and said that matter was composed of 4 elements: earth, wind, water, and fire. 4. Atoms were largely forgotten until _____________________’s theory in the 19th century. Video: Chem & Physics: History of the atom Dalton – (1803) 1. Everything is composed of _______________and helped to explain what occurred in __________________ ________________________. 2. Different _________________________always combined to form chemical __________________in amounts that were simple, whole number ______________________. 3. Each __________________ had its own unique type of atom with its own characteristic ___________________. 4. Atoms were ________________________. 5. This model of the atom lasted almost ________________years. 6. The discovery of ___________________particles meant Dalton’s model needed some work. Thomson – (1897) 1. Thomson was the 1st to discover a subatomic particle – the _____________________. 2. His experiments used _______________________rays. 3. Thomson used _____________________plates to deflect cathode rays and estimate the mass. 4. He showed that cathode rays must be made up of ___________________charged particles that were over ____________ _______________smaller than the lightest atom (Hydrogen). 5. Thomson’s model is known as the ____________ ________________model. 6. Draw a picture of Thomson’s model. Rutherford – (1909) 1. Alpha particles are a type of __________________charged radiation. 2. When shot at a piece of gold foil, most alpha particles pass ________________ ____________________. But occasionally, an alpha particle would be violently _____________________back, as if it has hit something ____________________. 3. The Gold Foil experiment led to the discovery of the _____________________, which is the center of the atom, where most of the mass of an atom is concentrated. 4. The core of an atom is ______________________charged. 5. Draw a picture of the Rutherford model of the atom. 6. What was the problem with Rutherford’s model of the atom? Bohr – (1913) 1. Draw a picture of Bohr’s model of the atom. 2. 3. 4. 5. Bohr proposed that electrons are found in circular ______________around the nucleus. Bohr’s model is known as the ______________________ model of the atom. According to Bohr’s model, electrons are found in fixed _______________ levels. The path closest to the nucleus has the ________________energy. The energy is generally _________________ as electrons are farther away from the nucleus. 6. Electrons can __________________ from one energy level to another, but that are not found ___________ levels. 7. Therefore, electrons gain or lose a __________________amount of energy every time they change energy levels ________________definition = separate and distinct, not continuous 8. This change in energy level is called a ____________________ of energy. 9. The ______________electrons are in the outermost orbits and are involved in chemical reactions. They determine the chemical __________________of an atom. Quantum Mechanical Model – (1920) Replaced the Bohr Model of the atom. History of the Atom WS 1. Complete after watching the videos 2. Read each summary before answering the questions Democritus (460 – 370 B.C.) Democritus was a Greek philosopher who proposed the idea that matter is made of tiny, indivisible, indestructible particles that he called atomos, from which the English word atom (meaning indivisible) is derived. He did not have the necessary equipment to test his ideas by using controlled experiments, and his ideas were eventually rejected. 1. Describe Democritus’s atom. ________________________________________________________ 2. Why were Democritus’s ideas rejected? ________________________________________________ ________________________________________________________________________________ 3. Draw a picture of Democritus’s atom. John Dalton (1766 – 1844): John Dalton was an English chemist. He studied numerous chemical reactions, making careful measurements and observations. He determined mass ratios of elements involved in those reactions. From analyzing his results, he came up with a theory called Dalton’s atomic theory. Here are his ideas. All elements are composed of atoms. It is impossible to divide or destroy an atom. All atoms of the same elements are alike. (One atom of oxygen is like another atom of oxygen.) Atoms of different elements are different. (An atom of oxygen is different from an atom of hydrogen.) Atoms of different elements combine to form a compound. These atoms have to be in definite whole number ratios. For example, water is a compound made up of 2 atoms of hydrogen and 1 atom of oxygen (a ratio of 2:1). Three atoms of hydrogen and 2 atoms of oxygen cannot combine to make water. 1. According to Dalton, what are elements made of? _____________ Can they be divided? ________ 2. An atom of hydrogen and an atom of carbon are _______________________. 3. Two atoms of hydrogen are __________________. 4. What are compounds made of? _____________________________________________________ 5. How do elements combine? ___________________________________________________ 6. The ratio of atoms in H2O is: 7. Draw a picture of Dalton’s atom. a) 1:1 b) 2:1 c) 1:2 J. J. Thomson (Late 1800s): J. J. Thomson was an English scientist. He was experimenting with gas discharge tubes when he noticed a movement in the tube. He called the movement cathode rays. The rays moved from the negative end of the tube to the positive end. When he held a negative metal plate near the cathode ray, the ray was deflected away. No matter which gas he put into the tube, he always made the same observations. He realized that the rays were made of negatively charged particles – electrons. His plum pudding model describes the atom as a sphere of positively charged “pudding”, with negative electrons evenly distributed throughout, like plums in the pudding. 1. Why are cathode rays deflected away from a negatively charged metal plate? ______________________________________________________________________________ 2. Why do cathode rays move from the negative end of the tube to the positive end? _______________________________________________________________________________ 3. What are cathode rays made of? __________________. What charge do they have? ________ 4. Why did Thomson see cathode rays with every element tested? _______________________________________________________________________________ 5. Draw a picture of Thomson’s atom. Ernest Rutherford (1871 – 1937): Ernest Rutherford conducted a famous experiment called the gold foil experiment. He used a thin sheet of gold foil. He also used special equipment to shoot alpha particles (positively charged particles) at the gold foil. Most particles passed straight through the foil like the foil was not there. Some particles went straight back or were deflected as if they had hit something. The experiment shows: Atoms are made of a small positive nucleus; positive nucleus repels positive alpha particles Atoms are mostly empty space 1. What is the charge of an alpha particle? _______________________________________ 2. Why is Rutherford’s experiment called the gold foil experiment? _______________________________________________________________________________ 3. How did he know that an atom was mostly empty space? _______________________________________________________________________________ 4. Why did Rutherford conclude that atoms contain a nucleus? _______________________________________________________________________________ 5. How did he know that the nucleus was positively charged? _______________________________________________________________________________ 6. Rutherford’s atom is called the nuclear atomic model. Draw a picture of it. The proton and the neutron: By 1932, scientists had discovered that the nucleus of an atom contains two types of particles: protons, which have a positive charge, and neutrons, which are neutral (have no charge). Both of these particles have about the same mass, which is much larger than the mass of an electron. Since the masses are so tiny, we use usually talk about their masses relative to each other. Since the proton and neutron have approximately the same masses, we say their mass is 1, and the mass of the electron is so small in comparison, we say it is 0. Complete the chart below. Subatomic Particle Relative Mass Charge Location Proton Neutron electron Draw a picture of an atom, showing the location of the protons, neutrons, and electrons. Make sure the parts of the atom are labeled.