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Nivaldo J. Tro http://academic.cengage.com/chemistry/tro Chapter 3 Atoms and Elements Deborah Koeck • Texas State University, San Marcos Understanding the Macroscopic World • The connection between the microscopic atom and the macroscopic element is the key to understanding the chemical world. Atoms • An atom is the smallest identifiable unit of an element. • Similarities and differences on the atomic scale correlate with similarities and differences on the macroscopic scale. Atoms Protons Determine the Element • The atomic number, Z, represents the number of protons in the nucleus of an atom. • The charge of a proton is assigned numerical value of +1. • Protons constitute a significant part of the mass of an atom. – Mass of a proton is 1.0 amu Chemical Symbols • Symbols are an abbreviation of the chemical name which can be based on: – The English name for the element: hydrogen à H – The Greek or Latin name: ferrum (iron) à Fe – Their place of discovery: europium à Eu – The scientist honored by the naming: curium à Cm Periodic Table of Elements Electrons • A neutral atom has as many electrons outside its nucleus as protons within its nucleus. • The opposing charges of protons and electrons hold electrons within a spherical region surrounding the nucleus. • Atoms can lose and gain one or more electrons. Determining Protons and Electrons Neutrons • Neutrons are almost as massive as protons but carry no electrical charge. • The number of neutrons in the atoms of an element can vary resulting in ISOTOPES. – Can be naturally occurring or man-made • The sum of neutrons and protons in an atom is called the mass number of the atom. Atomic Mass • Atomic masses listed on the periodic table are weighted averages of the masses of each naturally occurring isotope for that element. • Silver has two naturally occurring isotopes. One has a mass of about 107 amu and the other has a mass of about 109 amu. • Silver -107 has an abundance in nature of 51.84%. • Silver -109 has an abundance in nature of 48.16%. Atomic Mass Calculate the average atomic mass of silver. Silver has two naturally occurring isotopes. • One has a mass of 106.90509 amu (abundance in nature of 51.84%.) • The other has a mass of 108.90476 amu (abundance in nature of 48.16%.) 106.90509 amu ( 0.5184) = 55.42 amu 108.90476 amu (0.4816) = 52.45 amu 107.87 amu Molar Mass Calculate the Molar Mass of Calcium carbonate The mass of one mole of a compound. • Calcium carbonate • Chemical Formula à CaCO3 Calcium 40.08 grams X 1 mole = 40.08 grams Carbon 12.01 grams X 1 mole = 12.01 grams Oxygen 16.00 grams X 3 moles = 48.00 grams 100.09 grams Periodic Law • Mendeleev listed the known elements in order of increasing atomic mass, grouping those with similar properties. • He noticed that certain similar properties would recur in a periodic fashion. • His tabulation is the precursor to our modern periodic table. Mendeleev and the Periodic Table • Proposed that elements would be discovered to fill in gaps in the table. • Proposed that some measured atomic masses were in error. • Summarized a large number of observations – The underlying reasons for periodic behavior were as yet unknown – The Bohr model and the quantum mechanical model were needed to explain the observations. The Bohr Model • This is a MODEL of the atom that links electron behavior (microscopic) to the periodic law (macroscopic). Bohr Diagrams/Electron Configurations • Based on the behavior of electrons as particles • Electron orbits – Specified with n, the orbit’s quantum number • Fixed energies • Atoms with full outer orbits are extremely stable. • Fixed radii • Maximum number of electrons • Atoms with outer orbits based on n that are not full are • This model is an unstable and will undergo chemical reactions oversimplification (as are most attempting to fill the outer models). orbit. The Quantum Mechanical Model • Based on the behavior of electrons as waves • Replace Bohr’s orbits with orbitals, a representation of electron location as the probability of finding it in a certain region of space. • Orbitals are grouped into shells and fill similarly to Bohr’s orbits. Determinism and Quantum Mechanics • Specifics of orbital filling are beyond the scope of this text…BUT – The statistical nature of the quantum mechanical model rocked the scientific establishment. • An indeterminate universe • Both models are useful even though Bohr’s, by experiment, has been shown to be invalid. Families of Elements • Also called groups • Based on outer electron configurations • Vertical columns – Alkali metals – Alkaline earth metals – Chalcogens – Halogens – Noble gases Element Families Metals, Nonmetals, and Metalloids Molecular Elements • Some elements occur as diatomic molecules. The Mole Concept • Counting particles as small as atoms is impractical. • The mole concept allows us to relate the mass of a sample of an element to the number of atoms within it. • The unit of comparison is called a mole and corresponds to 6.022 x 1023. Avogadro’s Number • Determined such that the numerical value of the atomic mass of an element in amu is equal to the molar mass of that element in grams per mole. • These relationships of grams, moles, and atoms can be used as conversion factors. Chapter Summary Molecular Concept Societal Impact • Atomic Number and Mass • Chemical processes that Number cause change are caused by • Isotopes changes in atoms or • Molar Mass molecules. • Bohr Model • Pollution results from “misplaced” atoms. Atoms • Quantum Mechanical that are not where they are Model supposed to be located. • Orbitals • Models are used to describe nature.
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