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Chemistry Chapter 4 Atoms: The Building Blocks of Matter Law of Conservation of Mass Mass is neither created nor destroyed during chemical or physical reactions. Total mass of reactants = Total mass of products Antoine Lavoisier http://www.gre atscientists.net /antoinelavoisier/ Proust’s Law of Definite Proportions • In 1770s a French apothecary discovered the Law of Constant Composition: A given compound always contains the same proportions (by mass) of elements regardless of the source For example water H2O must Always have 8 g oxygen Combining with 1 g Hydrogen Hydrogen Peroxide H2O2 must Have 16 g Oxygen combining with 1 g Hydrogen Dalton’s Atomic Theory (1808) All matter is composed of extremely small particles called atoms Atoms of a given element are identical in size, mass, and other properties; atoms of different John Dalton elements differ in size, mass, and other properties Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in simple whole-number ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce that the electrons are the subatomic negatively charged particles carrying electricity in a cathode ray tube. Thompson measured the mass of cathode rays, showing they were made of particles that were around 1800 times lighter than the lightest atom, Hydrogen Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. Some Modern Cathode Ray Tubes Conclusions from the Study of the Electron Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons Electrons have so little mass that atoms must contain other particles that account for most of the mass Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. Rutherford’s Gold Foil Experiment Alpha particles are helium nuclei Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded http://videos.howstuffworks.com/hsw/27126matter-and-energy-ernest-rutherfordsexperiment-video.htm Try it Yourself! In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target? The Answers Target #1 Target #2 Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected “Like bullets bouncing off of tissue paper!” Conclusions: The nucleus is small The nucleus is dense The nucleus is positively charged The Atomic Scale Most of the mass of the atom is in the nucleus (protons and neutrons) Electrons are found outside of the nucleus (the electron cloud) Most of the volume of the atom is empty space “q” is a particle called a “quark” Atomic Mass Scale 1 atomic mass unit (u) has a mass of 1/12 of the mass of carbon-12 atom or 1.66054 *10-24g (1g= 6.02214*1023 u) Particle Charge Mass (u) Proton Positive (1+) 1.0073 Neutron None (neutral) 1.0087 Electron Negative (1-) 5.486*10-4 Atomic Number Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. Element # of protons Atomic # (Z) 6 6 Phosphorus 15 15 Gold 79 79 Carbon Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope. Mass # = p+ + n0 Nuclide p+ n0 e- Mass # Oxygen - 18 8 10 8 18 Arsenic - 75 33 42 33 75 Phosphorus - 31 15 16 15 31 Isotopes Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons • THERE ARE TWO WAYS TO REPRESENT THE • DIFFERENT TYPES OF ISOTOPES Mass number Atomic number 1 1 H 0 Or H-1 Mass number Charge (is included for ions Charged atoms and is not Included for neutral atoms • An atom becomes an ion when it loses or gains electrons. Now the electrons and protons are note balanced. 27 13 Al 13p 13p 13e 27 13 Al 3 -3e13p 10e Modern Atomic Theory Several changes have been made to Dalton’s theory. Dalton said: Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties Modern theory states: Atoms of an element have a characteristic average mass which is unique to that element. Average Atomic Mass (Atomic Weight) • Average atomic mass is the average of all the naturally isotopes of that element Atomic mass= ∑ (mass number ∙ % abundance) Example Find the average atomic mass of Carbon. Isotope Symbol Composition of the nucleus % in nature Carbon-12 12C 6 protons 6 neutrons 98.89% Carbon-13 13C 6 protons 7 neutrons 1.11% Carbon-14 14C 6 protons 8 neutrons <0.01% AAM= (.9889*12)+ (.0111*13)+ (.0001*14)= AAM (Carbon)= 12.011 Modern Atomic Theory #2 Dalton said: Atoms cannot be subdivided, created, or destroyed Modern theory states: Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions! Mass spectrometer • The MS works by ionizing chemical compounds to generate charged molecules or molecule fragments and measurement of their mass to charge ratios