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“Atomic Structure” Essential Question: Describe how the model of the atom has changed since the Greek idea of “Atomos” 1 Defining the Atom • Democritus – He believed that atoms were indivisible and indestructible “Atomos” 2 Dalton’s Atomic Theory (experiment based!) 1) All elements are composed of tiny indivisible particles called atoms 2) Atoms of the same element are identical. John Dalton 3) Atoms combine in whole-number ratios to form compounds 4) In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element. 3 Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron 4 Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. 5 Gold Foil Experiment - 1911 6 The Rutherford Atomic Model • Based on his experimental evidence: – The atom is mostly empty space – All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus” – The neutron was discovered by Chadwick in 1932 7 Bohr Model • Bohr refined Rutherford's model of the atom by finding that electrons were found in distinct energy levels around the nucleus. When electrons move to different energy levels energy is released. 8 Quantum Atomic Model • This is the current view of atoms, it is a mathematical model rather than a physical model. It describes the electrons as being found in regions of probability around the nucleus, an “electron cloud” 9 Changes in Atomic model 10 Democritus/Dalton Thompson(plum pudding) Rutherford Schrödinger (quantum Model) Bohr 11 Subatomic Particles Particle Charge Mass (g) Location Electron (e-) -1 negligible Electron cloud Proton (p+) +1 1 Nucleus Neutron (no) 0 1 Nucleus 12 Atomic Number Mass Number The atomic number of an element is the number of protons in the nucleus. The protons determine the identity of the element The mass number is the number of protons and neutrons in the nucleus of an isotope. Carbon-14 indicates the mass is 14 Carbon-13 indicates the mass is 13 These are isotopes of carbon 13 How to find number of protons The number of protons is the same as the atomic number. Atomic number = number of protons How to find number of electrons The number of electrons is the same as the number of protons. Number of electrons = number of protons How to find number of neutrons 1. Round mass number to the ones place. 35.453 = 35 24.805 = 25 2. Then subtract atomic number from mass number. Mass # - Atomic # = number of neutrons Mass number Complete Symbols Superscript → Subscript → Mass number Atomic number X 17 Symbols Find each of these: a) number of protons b) number of neutrons c) number of electrons d) Atomic number e) Mass Number 80 35 Br 18 p+ n0 e- Mass # Oxygen - 18 8 10 8 18 Arsenic - 75 33 42 33 75 Phosphorus - 31 15 16 15 31 Nuclide 19 Isotopes and Atomic Mass Notes Essential Question: Why is the mass listed on the periodic table an average? 20 Isotopes are…. • Atoms of the same element that have a different numbers of neutrons. • Thus, different mass numbers. 21 • To show different isotopes the mass # is written after element: –carbon-12 carbon-14 Elements occur in nature as mixtures of isotopes. 22 Isotope Examples 23 Examples of Isotopes: Isotope Hydrogen–1 (protium) Hydrogen-2 (deuterium) Hydrogen-3 (tritium) Protons Electrons Neutrons 1 1 0 1 1 1 1 1 2 Nucleus 24 Average Atomic Mass This is based on the abundance (percentage) of each isotope of that element in nature. Instead of grams, the unit we use is the Atomic Mass Unit (amu) 25 • It is defined as one-twelfth the mass of a carbon-12 atom. – Carbon-12 chosen because of its isotope purity. • Each isotope has its own atomic mass, thus we determine the average from percent abundance. 26 To calculate the average: • Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results. • If not told otherwise, the mass of the isotope is expressed in atomic mass units (amu) 27 Atomic Masses Atomic mass is the average of all the naturally occurring isotopes of that element. Mass % in nature (mass ) x (% as a decimal) Carbon-12 12 98.89% 12 x .9889 =11.87 Carbon-13 13 1.11% 13 x .0111 = 0.14 Carbon-14 14 0.01% 14 x .0001 = .0014 Isotope Carbon = 11.87 + 0.14 + .0014 =12.011 28 The Periodic Table: A Preview Each horizontal row (there are 7 of them) is called a period Each vertical column is called a group, or family 29