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Electron Arrangement Subatomic Particles • All atoms consist of the same three subatomic particles: protons, neutrons, and electrons. • Only the number of each varies from element to element. • Atoms are neutral, so the number of protons (positively charged) must be equal to the number of electrons (negatively charged). • Based on Rutherford’s Gold Foil Experiment, it was determined that the protons are located in the nucleus and the electrons are moving about the nucleus. Planetary Model • According to the planetary model, electrons orbit the nucleus much like planets orbit the sun. • The different “orbits” the electrons occupy are referred to as energy levels. • The lowest energy level is the orbit closest to the nucleus. Energy Levels • The lowest energy level (energy level 1) can hold a maximum of 2 electrons. • Energy level 2 can hold a maximum of 8 electrons. Energy level 2 is farther from the nucleus than energy level 1 and is therefore larger in size. • Energy level 3 can hold a maximum 18 electrons. • The equation 2n2 can be used to determine the maximum number of electrons in any energy level. (n is the energy level number) • Note: There is never more than 8 electrons in the outer level. (We will learn why in the next unit) • Electrons found in the outer level are called valence electrons. Bohr Diagrams • A Bohr Diagram is a method for representing the arrangement of electrons within an atom. Atomic Number • Moseley, in an experiment called the X-ray tube experiment, determined that each element has a different and unique number of protons. • The number of protons is referred to as the element’s atomic number. • The atomic number can be used to identify the element. • Atomic Number = # of protons= # of electrons. Mass Number • The mass number is the total number of particles located in the nucleus. • Mass Number = # of protons + # of neutrons • To determine the number of neutrons, subtract the atomic number from the mass number. • Mass # - atomic # = # of neutrons Nuclear Symbol • A nuclear symbol can be used to represent the number of subatomic particles found in an atom. • Example: 35 17 Mass Number Cl Atomic Number How many protons, neutrons, and electrons are found in this atom? Bohr Diagram • Draw the Bohr diagram for the atom of chlorine. 17 p 18 n More Bohr Diagrams • Draw the Bohr diagrams for the elements represented by the following nuclear symbols: Isotopes • In an experiment working with neon atoms, Thomson discovered some of the atoms had different masses. • Because they all were neon, the number of protons and electrons were all the same. • Therefore, the difference in mass was due to different numbers of neutrons. • He called these different atoms isotopes. • This discovery led to a revision of what part of Dalton’s theory? Examples of Isotopes Determine the number of protons, electrons, and neutrons for each of the isotopes of carbon. The atomic number identifies the element. The mass number identifies the isotope. Therefore, when identifying an isotope by name, the mass number is shown. Example: carbon-12