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Atoms, Molecules, and Ions British chemist, John Dalton (1766 – 1844) provided basic theory for modern chemistry. All matter, whether element, compound, or mixture is composed of small particles called atoms. Postulates of Dalton's Atomic Theory: 1. All matter is composed of indivisible atoms. 2. An element is a type of matte composed of only one kind of atom. 3. A compound is a type of matter composed of 2 or more elements combine in definite arrangements. 4. A chemical reaction consists of the rearrangement of the atoms present in the reacting substances to give new chemical combinations present in the substances formed in the reaction. Atomic symbols a one or two letter notation used to represent an atom corresponding to a particular element. Law of Multiple Proportions: If 2 elements A and B combine to form more than one compound, then the masses of B that can combine with a given mass of A are in a ratio of small whole numbers. Example: Structure of the Atom Basic rule for the behavior of electrically charged particles – Like charges repel each other; unlike charges attract. Nucleus – +charged contains most of the mass of an atom. Electron – - charged particle exists in the region around the nucleus. British scientist JJ Thomson measured the cathode rays. He determined the chg to mass ratio of 1.76 x108 C/g. Robert Millikan succeeded in measuring the chg of an electron in the Millikan oil drop experiment. Chg of an e’ = 1.602 x10-19 C. Ernest Rutherford working w/ alpha particles bombarded golf & metal foil. Atom – 3 subatomic particles, p, e, and neutron Chg E’ = -1.602 x10E-19 C Chg P = +1.602 x10-19 C Chg N = 0 Nucleus consist of protons and neutrons What makes an atom different in different elements? Isotope - atoms of a given element that differ in the number of neutrons and therefore in mass. Atomic Mass Units (AMU) - a mass unit equal to exactly 1/12 the mass of a carbon-12 atom. Atomic Mass - average atomic mass for the naturally occurring element. Isotope abundance Atomic weight for an element is the average weight of all of the isotopes for that element. = weighted average. Chlorine (Cl) - 2 major isotopes 35 Cl and 37Cl Atomic wt = (% abundance isotope#1/100) (mass of #1) + (% abundance isotope#2/100) (mass of #2) % abundance = (# of atoms of a given isotope) / (total # atoms of all isotopes) Example: Period Table of the Elements 1869 Russian chemist, Demitri Mendeleev and German chemist Lothar Meyer, working independently Periodic Table- the arrangement of elements in order of increasing atomic # with elements having similar properties placed in vertical rows. Period - the elements in any horizontal row Group - the elements in a vertical column also known as families. Main Group elements (A group) Metal Nonmetals Transition elements (B group) Metalloids Chemical Formulas Notation that is used to convey relative proportions of atoms in the substance. Molecule group of atoms that are chemically bonded together Molecular formula gives the exact number of different atoms of an element in a molecule. Empirical formula is a chemical formula that gives only the relative number (ratio) of atoms for each element in a molecule - simplest formula. Ionic substances Ion - an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing electrons. Cations vs. Anions Ionic compound - composed of cations and anions. Formula unit - the group of atom or ions explicitly symbolized in the formula Naming compounds - chemical nomenclature Naming Ionic compounds - first name the cation then follow with the anion Common monatomic ions Rules for Ionic compound 1. Monatomic cations are named after the element if there is only one such ion. 2. If there is more than one monatomic cation, the cation is named after the element followed by a roman numeral. 3. The name of the monatomic anions are obtained from the stem name of the element followed by the suffix -ide. Polyatomic ion - an ion consisting of two or more atoms chemically bonded together and carrying a net electric charge. Binary compound - compound of only two elements Rules for naming binary compounds 1. Name the first element using the exact element name 2. Name the second element by writing the stem name and adding suffix - ide 3. Add a prefix to each element name to denote the subscript of the element (Table 2.6) Oxoacid - an acid containing hydrogen, oxygen and another element. Hydrates - a compound that contains water molecule weakly bound in its crystal Chemical Equations - the symbolic representation of a chemical reaction in terms of chemical formulas. 2Na + Cl2 2NaCl Balancing Chemical Equations Reactants - starting substances Products - substances that result from a reaction Strategy: Balance first the atom for elements that occur in only one substance on each side of the equation. Example: CH4 + O2 H3PO3 CO2 + H2O H3PO4 +PH3