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Basic Structure of the Atom Evidence Supporting the Atomic Theory Building the Atomic Model Democritus (460-370 BC) John Dalton (1803) J. J. Thomson (1897) Ernest Rutherford (1909) Niels Bohr (1913) James Chadwick (1932) Democritus Ancient Greek First to propose matter was made up of tiny particles called atomos Stated atoms could not be created, destroyed, or further divided Dalton Proposed Atomic Theory in 1803 Half a Century until well accepted 100 years until proven Each element : Composed of indivisible and indestructible atoms Atoms of different elements are different; atoms of the same element are the same Atoms of different elements combine to form compound atoms (molecules) J. J. Thomson Discovered electrons in 1897 Plum Pudding Model – Positive and negative charges scattered randomly throughout atom Instantly accepted Rutherford Gold foil experiment 1909 – Mass concentrated in very small core at the atom’s center (nucleus) Nucleus positive, negative electrons moving around it Contributions: Field of Nuclear Physics (1898, alpha and beta particles) Radioactive decay Rectify Periodic Table Bohr Doctoral Thesis 1911– Theory of Electrons In 1913, theorized electrons revolve around the nucleus in energy levels (Einstein and Planck) Energy levels closest to nucleus have low energy Energy levels increase in energy with distance from the nucleus Electrons gain and lose energy by moving between energy levels (quantum) “This is an enormous achievement” Einstein James Chadwick (1932) Identified neutron Modern Atomic Model The atom consists of three main (charged) particles: Protons (positive) Neutrons (neutral) Electrons (negative) Two main parts: Nucleus Electron cloud Atomic Model (cont.) Nucleus contains: Protons (+) Neutrons (0) Nucleus surrounded by: electron cloud Negative charge due to electrons Mass of an atom measured in amu’s, or atomic mass units. Atomic Structure Atomic Structure Atomic Model (cont.) Protons and neutrons are found in the nucleus, so it has a positive charge Electrons are found in the electron cloud, so it has a negative charge The mass of an atom is measured in amu’s, or atomic mass units. It is a unit of measurement of protons and neutrons Atomic Mass Units 1 amu is defined as 1/12 the mass of a carbon atom containing 6 protons and 6 neutrons 1 amu is also the mass of 1 proton or 1 neutron An electron has a mass of 1/2000 amu Key Terms Atomic number: the number of protons in the nucleus of an atom. Mass number: the sum of the number of protons AND the number of neutrons in the nucleus. Mass # = # protons + # neutrons Atomic Symbol Isotopes Isotopes are atoms of the same element with differing numbers of neutrons. Isotopes have different masses Isotopes Isotopes of Carbon How do you record the mass of a group of isotopes? Because most elements have more than one isotope, each element is given an average atomic mass The average atomic mass is the average mass of the mixtures of its isotopes How do you calculate the average atomic mass of an atom? The number of naturally occurring isotopes, their masses, and their percent abundances must be known. Example: Lithium has 2 isotopes: Li-6 (mass 6.015 amu and 7.5% abundance), and Li-7 (mass 7.017 amu and 92.5% abundance). What is its average atomic mass? How do you calculate the average atomic mass of an atom? Calculate the average atomic mass of silicon. The three silicon atoms have masses of 27.98 amu, 28.98 amu, and 29.97 amu with relative abundances of 92.23%, 4.67%, and 3.10%, respectively. Charged Atoms In a neutral atom, the number of protons equals the number of electrons. The positive and negative charges balance out, leaving the atom with 0 net charge In a charged atom or ion, there is an uneven number of protons and electrons, so the atom will have either a positive or negative net charge Ions