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Chapter 7 Models of Atomic Structure Grade 9 Science Class Notes For Chapter 7 Atom - Is the smallest particle of an element that is still that element. - contains subatomic particles called protons and electrons. Nucleus - a tiny core that is very small in volume dense compared to the rest of the atom and intensely positive. An Electron Cloud - “envelope” that is very large in volume. Light compared to the nucleus and negatively charged. The electron cloud id the region of the atom where the electrons move rapidly around the nucleus. An Atom is the smallest particle of an element that is still that element. Rutherford’s model of an atom: Rutherford’s model describes a nucleus which is a tiny core that is very small in volume, dense compared to the rest of the atom, and positively charged. Also that there is an “envelope” that is very large in volume, light compared to the nucleus, and negatively charged. Bohr model of the atom: Bohr’s model compared the nucleus to the Sun, and the electrons to the planets . The Sun exerts an enormous gravitational pull on the planets, but they do not spiral inward and crash. Why? Because they revolve at just the right speed to remain in their orbits. Similarly, the atom’s positive nucleus exerts a strong force of attraction on the negative electrons. The electrons do not spiral inward and crash, however, because they are moving rapidly in fixed regions around the nucleus. These regions are threedimensional and sphere-like. For this reason, they are called electron shells. (page 242-textbook). Bohr felt the nuclear model was accurate as far as it went, but was incomplete. Therefore, scientists today agree upon the Bohr-Rutherford Model as a representative model for the structure of an atom. The Bohr-Rutherford model of an atom uses aspects of both models. The Bohr-Rutherford Model of an atom can be called the first “modern” view of the atom. The central part of an atom is called the Nucleus. The nucleus is made up of protons and neutrons. Protons - have a positive charge Neutrons - have no charge Electrons - negatively charged particles The Atomic Number of an atom is the number of protons it has. The Atomic Mass or Weight of the atom is total number of protons and neutrons in atoms nucleus. The number of neutrons of an atom can be determined by subtracting the atomic number from the atomic mass (which is the total number of proton and neutrons in the nucleus) of an element. What is radioactivity? The spontaneous emission of radiation by an atom. Three types of radiation were discovered by Ernest Rutherford: a particles b particles g rays Protons and electrons are the only particles that have a charge. Protons and neutrons have essentially the same mass. The mass of an electron is so small we ignore it. What are isotopes? Atoms of the same element with different masses due to the fact Isotopes have different numbers of neutrons. For example: 1 H 1 2H 1 3H 1 This Hydrogen atom has one proton, 1 electron and 0 Neutrons. This Hydrogen atom has one proton, 1 electron and 1 Neutrons. This Hydrogen atom has one proton, 1 electron and 2 Neutrons. Charges on Atoms: An ion is an atom, or group of atoms, that has a net positive or negative charge. cation – ion with a positive charge If a neutral atom loses one or more electrons it becomes a cation. For Example: Na 11 protons 11 electrons Na+ 11 protons 10 electrons Cl 17 protons 17 electrons Cl17 protons 18 electrons Al 13 protons 13 electrons Al 3+ 13 protons 10 electrons Remember: The number of electrons is the same as the number of protons. When there is a charge present that indicates either more electrons (a negative charge) or less electrons (a positive charge). Be sure to Read through the Power Point Presentation available on the school home site, teachers page: Tlomond, for Bonus question material. Bohr Model or Electron Shell diagram: Be able to draw an electron shell diagram (Bohr Model) for any given element on the periodic table up to Ar with an atomic number of 18. Be able to explain the 2-8-8 Rule for drawing an electron shell diagram (Bohr Model). The 2-8-8 Rule states that there can only be a maximum of two electrons in the first shell of an atom, then there can only be a maximum of eight electrons in the second shell of an atom and there only be a maximum of eight electrons in the third shell of an atom. To help further your understanding of the material covered in chapter 7. Refer to your class notes, the notes available here as well as the pages in the text, SciencePower 9, 229, 234, 235, 237, 242-243, 246-248.