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Explore the evolution of atomic theories from Democritus to modern atomic structure. Learn about subatomic discoveries, isotopes, and counting atomic particles. Discover how the atomic mass is calculated and understand the significance of isotopes in defining elements.
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The Structure of the Atom Chapter 3 Chemistry: Matter and Change
Early Theories of Matter (3.1) Democritus’s Ideas • Matter is composed of empty space through which atoms move • Atoms are solid, homogeneous, indestructible, indivisible • Atoms come in different sizes and shapes which creates different properties • Changes in matter result from changes in groupings of atoms, not in the atoms themselves Dalton’s Atomic Theory (not completely true today!) 1. Elements are made of tiny particles called atoms 2. All atoms of a given element are identical 3. Atoms of a given element are different from those of any other element 4. Atoms of one element can combine with atoms of other elements to form compounds 5. Atoms are indivisible and can not be created or destroyed in a chemical reaction http://web.visionlearning.com/dalton_playhouse/ad_loader.html
Subatomic Discoveries (3.2) • JJ Thomson: Plum Pudding Model • Uniform spherical cloud of positive charge with enough negative electrons • Robert Millikan: Electron Charge • Rutherford: Nucleus and Protons • Gold foil experiment – alpha (+) particles shot at gold foil surrounded by a detection field, most of the particles passed through, but others were deflected by the center of concentrated positive charge • Rutherford and Chadwick: Neutrons • http://www.brainpop.com/science/matterandchemistry/ Atomic Model • http://phet.colorado.edu/en/simulation/rutherford-scattering
Subatomic Particles and the Nuclear Atom (3.2-3.3) • Atom consists of: 1. nucleus – protons (+) and neutrons (0) 2. electrons (-) outside of nucleus http://phet.colorado.edu/en/simulation/build-an-atom Particle Relative Mass Relative Charge Electron 1 1- Proton 1836 1+ Neutron 1839 0 **It’s the number and arrangement of electrons that gives atoms their different chemical properties**
Isotopes (3.2-3.3) http://www.brainpop.com/science/ matter and chemistry/isotopes • http://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass • Atoms of the same element that contain the same number of protons but have different mass numbers due to different number of neutrons-isotope
Isotope Notations (3.2-3.3) A +/- X • Nuclear • X = element symbol • A = mass number (protons +neutrons) • Z = atomic number (protons) • +/- = charge on the isotope (not on all isotopes) • Hyphen Element name/symbol – mass number Example: Uranium-235 or U-235 Z
Counting Atomic Particles (3.2) • # protons = atomic number OR • # protons = mass number - # neutrons • # neutrons = mass number - # protons • Mass number = # protons + # neutrons • (do NOT use the decimal number on the periodic table, that is the average atomic mass based on the average of all the isotopes of that element) • Electrons • No charge: # electrons = # protons • (+) charge: # electrons = # protons – charge • (-) charge: # electrons = # protons + charge
The atomic mass of an element is the weighted average mass of the isotopes of that element. One atomic mass unit, or amu, is defined as 1/12 the mass of a carbon-12 atom. Atomic Mass (4.3)
