Chapter 2 Notes Atoms, Molecules, and Ions |
The atomic theory of matter Democritus Atomos – indivisible Plato and Aristotle No indivisible particles Continuous The atomic theory of matter Dalton Atomic Theory Each element is composed of extremely small particles called atoms. All atoms of a given element are identical; the atoms of different elements are different and have different properties. Atoms of an element are not changed into different types of atoms by chemical changes; atoms are neither created nor destroyed in chemical reactions. Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms. Father of modern atomic theory The atomic theory of matter Dalton’s theory Explained the law of constant composition(Proust). Explained the law of conservation of mass(Lavoisier) Helped deduce the law of multiple proportions(Dalton). The discovery of atomic structure Crooke - cathode rays(1850’s) (-) to (+) Could be deflected in electric or magnetic fields Thomson(1897) Nature of the rays was identical regardless of cathode material Concluded that rays are not waves but particles with mass Calculated a value of 1.76 x 108 coulombs per gram for charge to mass ratio of electron Millikan(1909) Oil drop experiment to find the charge of an electron(1.6 x 10-19 Coulombs) Radioactivity Becquerel(1896) Found that uranium minerals emit high energy radiation spontaneously. Rutherford(1909) Found
3 types of radiation Alpha – helium nuclei, massive, slow, paper deters it Beta – high speed electrons, fast, negative, metal foil deters it Gamma – similar to x-rays, no particles, no charge, very harmful to living tissue Rutherford’s gold foil experiment 1910 Used alpha particles shot at gold foil. Some passed through the foil. Some were deflected at an angle. Some bounced back. Major contribution to understanding the atom. The atom is mostly space. There is a dense, central core called the nucleus. The nucleus is positive. Further information Rutherford
discovered the proton in 1919 in the cathode ray tube in much the same way that Thomson discovered the electron. Chadwick discovered the neutron in 1932. They were more difficult to find. Models of the atom Dalton – spherical but no charges Thomson – plum
pudding model; positive sphere with negative electrons in it. Rutherford – nuclear atom with lots of empty space; nucleus is positive and dense; electrons are randomly spaced outside the nucleus. Bohr – nuclear atom but placed electrons in energy levels outside the nucleus. No electrons anywhere but in the energy levels. Only certain numbers of electrons in each level. The modern view of the atom Schoedinger model Has all the parts that the Bohr atom had but has fine structure added to explain bright line spectra. Has protons, electrons, and neutrons. Discusses the position of electrons in terms of probability of finding them in a location. Has sublevels(s, p, d, f, g, h…) inside the main energy levels Atoms are very small. Masses are expressed in amu and radii are expressed in Angstroms(non-SI) or picometers. The modern view of the atom With the discovery of the neutron, Dalton’s atomic theory
had to be revised. Now there was the possibility that two atoms of the same kind could have different masses(isotopes) and that two different atoms could have the same masses. Isotopes have the same number of protons(atomic number; identity) but different numbers of neutrons. Thus a nuclide would be written 12C for carbon-12 and 13C for carbon-13. The superscript is called the mass number(p + n). The Periodic Table Families(groups) Representative groups 1A Alkali metals(active at the bottom of column) 2A Alkaline Earth metals 3A(13) Boron group 4A(14) Carbon group 5A(15) Nitrogen group 6A(16) Chalcogens 7A(17) Halogens(active at the top of column) 8A(18) Noble gases(inert) The Periodic Table Transition metals Include lanthanides and actinides B-groups Have more than one oxidation
number Metals Nonmetals Metalloids Compounds Molecules and Formula Units Types of bonding Ionic bonding occurs when two or more ions join. It occurs between a metal and a nonmetal. Ions are formed when an atom loses or gains an electron. Oppositely charged ions are attracted to one another and a formula unit is made. Covalent bonding occurs when two nonmetals share electrons and a molecule is made. Types of formulas Molecular formulas are those that indicate the actual numbers of each kind of atom present. Empirical formulas are those that indicate the relative numbers of atoms
present. They may or may not indicate an actual compound. Structural formulas indicate the way in which atoms are attached to one another. Two compounds may have the same formula but may have differing structures and hence will have different properties. Ions and Ionic Compounds Ionic compounds are formed from cations(positive ions) and anions(negative ions). A cation is formed when a metal atom loses one or more electrons. An anion is formed when a nonmetal atom gains one or more electrons. Representative elements form predictable ions whereas transition elements do not. (Exceptions: Cd, Zn, and Ag) Molecular Compounds Molecular compounds are made when two nonmetal atoms share electrons. This forms a covalent bond. Pure covalent Polar covalent Coordinate covalent Metallic covalent |