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Chapter 2: Atoms, Molecules and Ions CHEMICAL LAWS 2.1 Conservation of Mass and the Law of Definite Proportions 2.2 Dalton s Atomic Theory and the Law of Multiple Proportions THE ATOMIC STRUCTURE 2.3 The Structure of Atoms: Electrons 2.4 The Structure of Atoms: Protons and Neutrons THE ELEMENTS AND THEIR ATOMS 2.5 Atomic Number 2.6 Atomic Mass COMPOUNDS AND MOLECULES 2.7 Compounds and Mixtures 2.8 Molecules, Ions and Chemical Bonds 2.9 Acids and Bases 2.10 Naming Compounds Conservation of mass and Law of Definite Proportions Law of Mass Conservation: Mass is neither created nor destroyed in chemical reactions. Law of Definite Proportions: Different samples of a pure chemical substance always contain the same proportion of elements by mass. 1 Conservation of mass Dalton s Atomic Theory Elements are made of tiny particles called atoms Atoms of the same element have the same mass. Atoms of different elements have different masses. Chemical combination of elements to make different substances occurs when atoms join together in simple whole numbers. Atoms are chemically indestructible. 2 Dalton s Atomic Theory Law of Multiple Proportions When two elements form two different compounds, the mass ratios are related by small whole numbers. Examples Nitrogen & oxygen combine to form NO or NO2: In NO the N:O mass ratio is 7:8 In NO2 the N:O mass ratio is 7:16 Hydrogen & oxygen combine to form H2O or H2O2: In H2O the H:O mass ratio is 1:8 In H2O2 the H:O mass ratio is 1:16 3 The Structure of Atoms Thomson calculated the electron s charge to mass ratio as 1.758820 x 108 Coulombs per gram. Voltage on plates place 1.602176 x 10-19 C of charge on each oil drop. Millikan calculated the electron s mass as 9.109382 x 10-28 grams. 4 The Structure of Atoms Discovery of Nucleus (Rutherford, 1871 1937): Rutherford irradiated gold foil with a beam of alpha ( ) particles to search for positive charged particles. Atom must be mostly empty space except for a central positive mass concentration. 5 The Structure of Atoms 6 Atomic numbers, mass numbers & isotopes Atomic number, Z = # of protons & determines the element Mass number, A = # of protons + # of neutrons In a neutral atom # of electrons = # of protons = Atomic Number, Z Atoms with the same # of protons but with a different # of neutrons are called ISOTOPES One isotope of Carbon; Carbon -12 7 In Periodic Table of Elements Carbon is shown as : Atomic Mass: A weighted average of the isotopic masses of an element s naturally occurring isotopes. Carbon has two natural isotopes, C-12 and C-13 Isotopes Examples 8 Isotopes Q ? The isotope 75 34Se is used medically for diagnosis of pancreatic disorders. How many protons, neutrons, and electrons does an atom of this isotope have? Q? An atom of element X contains 47 protons and 62 neutrons. Identify the element, and write the symbol for the isotope in the standard format. 9 This document was created with Win2PDF available at http://www.daneprairie.com. The unregistered version of Win2PDF is for evaluation or non-commercial use only.