ACS Chemistry Exam 1 Notes - ACS Chemistry Exam 1 Notes...

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ACS Chemistry Exam 1 Notes Atomic Structure Isotopes are forms of the same element that differ only in the number of neutrons in their nuclei. o 235 92 U and 238 92 U Particles are lost by nuclear changes, not chemical changes. o Neutrons do NOT result in change of atomic number. o A proton has one unit of positive charge. o An alpha particle has two units of positive charge. When an atom emits an alpha particle, the atom's mass number decreases by four due to the loss of the four nucleons in the alpha particle. The atomic number of the atom goes down by exactly two, as a result of the loss of two protons – the atom becomes a new element. Examples of this are when uranium becomes thorium, or radium becomes radon gas due to alpha decay. o A beta unit has one unit of negative charge. When a radioactive nucleus disintegrates by emitting a beta-particle, the atomic number decreases by one and the mass number remains the same. Ernest Rutherford alpha-particle-scattering experiment. o Prior to his experiment, it was believed that atoms consisted of a cloud of positive charge with electrons embedded throughout. o Rutherford discovered that the atoms of the metals were mostly empty space. Thus the mass of an atom is concentrated in a small, dense center. o It was necessary to bombard the nuclei at high speeds to overcome the force of repulsion of the nucleus.
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