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Slide 1 ___________________________________ 5 Early Atomic Theory and Structure ___________________________________ ___________________________________ ___________________________________ ___________________________________ Lightning occurs when electrons move to neutralize charge difference between the storm clouds and Earth. ___________________________________ Foundations of College Chemistry, 14th Ed. Morris Hein and Susan Arena ___________________________________ Copyright © 2014 John Wiley & Sons, Inc. All rights reserved. Slide ___________________________________ Chapter Outline 2 5.1 Dalton’s Model of the Atom 5.2 Electric Charge ___________________________________ ___________________________________ A. Discovery of Ions 5.3 Subatomic Parts of the Atom 5.4 The Nuclear Atom 5.5 Isotopes of the Elements 5.6 Atomic Mass 3 ___________________________________ ___________________________________ ___________________________________ © 2014 John Wiley & Sons, Inc. All rights reserved. Slide ___________________________________ ___________________________________ Early Theories on the Structure of Matter ___________________________________ Early models of the atom were developed by the Greeks. Empedocles proposed matter was composed of four basic elements: earth, air , water and fire. ___________________________________ Democritus proposed matter was composed of small, indivisible particles he called atoms. ___________________________________ Atoms could combine in different ways, giving rise to the diversity of compounds we observe. ___________________________________ ___________________________________ Aristotle, an influential philosopher, supported Empedocles’ theory, so atomic theory was not fully accepted until 2000 years later. © 2014 John Wiley & Sons, Inc. All rights reserved. ___________________________________ Slide 4 ___________________________________ Dalton’s Model of the Atom ___________________________________ Dalton’s theory of atoms, proposed in the early 1800s, states: 1. Elements are composed of small, indivisible particles called atoms. 2. Atoms of the same element are identical in mass and size. ___________________________________ ___________________________________ 3. Atoms of different elements differ in their mass and size. 4. Compounds are formed by combining two or more atoms of different elements. 5. Atoms combine to form compounds in simple whole number ratios. ___________________________________ 6. Atoms of two elements may combine in different ratios, leading to formation of different compounds. ___________________________________ © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 5 ___________________________________ ___________________________________ Dalton’s Model of the Atom ___________________________________ ___________________________________ H2 O ___________________________________ H2 O 2 a. Atoms are individual particles which are different for each element. b./c. Atoms combine in fixed ratios to form compounds. Two elements can combine in varying ratios to give different compounds. ___________________________________ ___________________________________ Most of Dalton’s theory remains valid today. © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 6 ___________________________________ ___________________________________ Dalton’s Model of the Atom ___________________________________ Revisions to Dalton’s Theory ___________________________________ 1. Elements can be decomposed under certain conditions. ___________________________________ 2. Not all atoms of the same element have identical mass. These are called isotopes. ___________________________________ ___________________________________ 3. Atoms are not indivisible. Atoms are composed of subatomic particles. ___________________________________ © 2014 John Wiley & Sons, Inc. All rights reserved. Slide ___________________________________ Electric Charge 7 ___________________________________ Properties of Electric Charge 1. Charge may be either positive or negative. 2. Opposite charges (positive and negative) attract while like charges (i.e. negative and negative) repel. ___________________________________ 3. Charge may be transferred from one object to another, by contact or induction. ___________________________________ 4. The force of attraction between charges (F) is related to the distance between charges by: ___________________________________ F= kq1q2 ___________________________________ r2 where q1 and q2 are the charges, r is the distance between charges, and k is a constant. © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 8 ___________________________________ ___________________________________ Discovery of Ions Michael Faraday: English scientist who discovered electrolytes (compounds that conduct electricity when dissolved in water). ___________________________________ ___________________________________ Faraday also discovered that some compounds decompose in water into their elements. ___________________________________ These elements were attracted to either negatively or positively charged electrodes in the solution, meaning they were no longer neutral. ___________________________________ These charged elements are called ions. ___________________________________ A light bulb glows when ions are present in a saltwater solution when current is passed through it. © 2014 John Wiley & Sons, Inc. All rights reserved. Slide ___________________________________ ___________________________________ The Nature of Ions 9 ___________________________________ Arrhenius extended Faraday’s work. He proposed ions are atoms (or groups of atoms) that carry a positive or negative charge. Ex. NaCl in water dissociates into two ions, The Na+ and ___________________________________ Cl–. ___________________________________ Na+ (cation) produced is attracted to the negatively charged electrode (cathode). – The Cl (anion) produced is attracted to the positively charged electrode (anode). ___________________________________ Based on Faraday’s and Arrhenius’ work, Stoney proposed the electron was a fundamental unit of electricity associated with atoms. J. J. Thomson later experimentally confirmed the existence of electrons. © 2014 John Wiley & Sons, Inc. All rights reserved. ___________________________________ ___________________________________ Slide 10 ___________________________________ Subatomic Parts of the Atom ___________________________________ A single atom is tiny (diameter of 0.1 to 0.5 nm). Because atoms are so small, determining the presence of subatomic particles was very difficult. ___________________________________ New instruments in the early 1900s permitted detection of these particles. ___________________________________ ___________________________________ ___________________________________ ___________________________________ A scanning tunneling microscope (STM) image shows an array of Cu atoms. © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 11 ___________________________________ Subatomic Parts of the Atom ___________________________________ A Crooks tube permits generation of cathode rays, which are streams of electrons. ___________________________________ ___________________________________ ___________________________________ ___________________________________ A Crooks (cathode) ray tube. The stream of electrons passes between the electrodes. The electron beam is deflected by both electric and magnetic fields, indicating it has charge. © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 12 ___________________________________ ___________________________________ Electrons and Protons ___________________________________ Electrons (e–): A particle with negative electrical charge (assigned a relative charge of –1). Electrons have a very small mass (9.110 x 10–28 g) and size (<10–12 cm). ___________________________________ Protons (p): ___________________________________ ___________________________________ A particle with positive electrical charge (assigned a relative charge of +1). Protons have a much larger mass (~1837 times the mass of an electron). © 2014 John Wiley & Sons, Inc. All rights reserved. ___________________________________ ___________________________________ Slide 13 ___________________________________ The Effect of Subatomic Particles ___________________________________ Thomson’s work demonstrated the atom is composed of smaller, charged particles. Dalton’s theory of the atom then had to be revised. ___________________________________ Thomson’s Model of the Atom ___________________________________ Electrons are negatively charged particles which are embedded in a positively charged atomic sphere. ___________________________________ Electrons + charged sphere ___________________________________ Thomson’s “plum pudding” model of the atom. ___________________________________ © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 14 ___________________________________ The Effect of Subatomic Particles ___________________________________ Atoms can become ions by gaining or losing electrons from this sphere. ___________________________________ ___________________________________ Electrons are lost from atoms to give cations. ___________________________________ ___________________________________ ___________________________________ Electrons are gained from atoms to give anions. © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 15 ___________________________________ Neutrons ___________________________________ The last subatomic particle was discovered by Chadwick in 1932. ___________________________________ Neutrons (n) ___________________________________ A particle with no electrical charge. Neutrons have a mass similar to that of a proton. ___________________________________ ___________________________________ ___________________________________ © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 16 ___________________________________ Summary of Subatomic Particles ___________________________________ Atoms are composed of three smaller, subatomic particles: electrons, protons and neutrons. ___________________________________ ___________________________________ ___________________________________ Chemical properties of atoms can be described based on the electrons, protons and neutrons. ___________________________________ Though other subatomic particles are now known, the theories of atomic structure are based only on these 3 subatomic particles. © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 17 ___________________________________ ___________________________________ Nuclear Model of the Atom In 1911, Ernest Rutherford established the nuclear model of the atom by bombarding gold atoms with α particles. ___________________________________ ___________________________________ ___________________________________ ___________________________________ Most of the particles passed through the gold foil, but some were deflected and some even bounced back! ___________________________________ This suggested the gold atoms must have a densely, positively charged nucleus to affect the path of an α particle (a positively charged He atom). © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 18 ___________________________________ ___________________________________ Nuclear Model of the Atom ___________________________________ Because most of the particles were not deflected, this suggested most of the atom is empty space. ___________________________________ ___________________________________ ___________________________________ Protons and neutrons are located in the nucleus. Electrons are dispersed throughout the remainder of the atom (mainly open space). Neutral atoms contain the same number of protons and neutrons to maintain charge balance. © 2014 John Wiley & Sons, Inc. All rights reserved. ___________________________________ ___________________________________ Slide 19 ___________________________________ Atomic Number ___________________________________ Atomic Number: Number of protons in the nucleus of an atom. ___________________________________ The atomic number determines the identity of the atom. ___________________________________ Atomic numbers for every element are above the element’s symbol in the periodic table. ___________________________________ 27 ___________________________________ Atomic Number Co ___________________________________ © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 20 ___________________________________ Isotopes of the Elements ___________________________________ After discovery of the nuclear model of the atom, the mass of almost all atoms was found to be larger than expected, based on the number of protons and electrons. This led to the discovery of neutrons. ___________________________________ Though all atoms of the same element have the same number of protons, atoms of the same element may have different numbers of neutrons. ___________________________________ ___________________________________ ___________________________________ Isotopes: atoms of an element with the same atomic number but different numbers of neutrons. © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 21 ___________________________________ ___________________________________ Isotopes of the Elements Example: Isotopes of Hydrogen Protium Deuterium ___________________________________ Tritium ___________________________________ ___________________________________ 1 proton 0 neutrons 1 proton 1 neutron 1 proton 2 neutrons ___________________________________ Standard Isotopic Notation Mass Number A Atomic Number Z E ___________________________________ Element Symbol Mass number: Total number of protons and neutrons for an element. © 2014 John Wiley & Sons, Inc. All rights reserved. ___________________________________ Slide ___________________________________ Isotopes of the Elements 22 ___________________________________ ___________________________________ ___________________________________ Practice: How many protons, neutrons, and electrons are found in each of the following isotopes? ___________________________________ 64 Cu 29 ___________________________________ Atomic Number: 29 protons (therefore 29 electrons) # Neutrons = Mass Number – Atomic Number 64 – 29 = 35 neutrons ___________________________________ © 2014 John Wiley & Sons, Inc. All rights reserved. Slide ___________________________________ Let’s Practice! 23 ___________________________________ Which isotope corresponds to an element with 24 protons and 28 neutrons? ___________________________________ a. 28 52 Cr Solution: ___________________________________ b. 52 24 Cr c. 52 28 Ni # protons = Atomic Number = 24 Element: Cr ___________________________________ Mass Number = protons + neutrons = 24 + 28 = 52 ___________________________________ d. 128 Te 52 e. 24 52 Cr ___________________________________ © 2014 John Wiley & Sons, Inc. All rights reserved. Slide ___________________________________ Atomic Mass 24 ___________________________________ Because the mass of a single atom is so small, it is inconvenient to use this as a mass unit. ___________________________________ Instead, relative atomic mass units (amu) are used. ___________________________________ 12 Using carbon-12, 6 C , as a standard, 1 atomic mass unit is equal to 1/12th the mass of a carbon-12 atom. ___________________________________ 1 amu = 1.6606 x 10-24 g ___________________________________ All periodic tables use atomic masses based on the carbon-12 isotope. © 2014 John Wiley & Sons, Inc. All rights reserved. ___________________________________ Slide 25 ___________________________________ Atomic Mass and Isotope Distribution ___________________________________ Since most elements are a mixture of isotopes, the atomic mass for an element is the weighted average of all naturally occurring isotopes of the element. Example: The atomic mass of Cu is 63.546 amu. Cu exists as 2 major isotopes, Cu-63 and Cu-65. ___________________________________ Cu-63 is more abundant, as the atomic mass is very close to 63 amu. ___________________________________ Calculating average atomic mass: ___________________________________ Sum of the atomic mass of each isotope multiplied by its % abundance. ___________________________________ © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 26 ___________________________________ ___________________________________ Atomic Mass and Isotope Distribution ___________________________________ ___________________________________ Average atomic mass of Cu: ___________________________________ (62.9298) x (0.6909) + (64.9278) x (0.3091) = 63.55 amu Atomic Mass % Abundance ___________________________________ Atomic Mass % Abundance ___________________________________ Measuring Cu isotope abundances by using mass spectrometry. © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 27 ___________________________________ ___________________________________ Atomic Mass Practice ___________________________________ Silver exists as two isotopes with atomic masses of 106.9041 and 108.9047 amu. Determine the average atomic mass for silver if the % abundance for each isotope is 51.82 and 48.18%, respectively. ___________________________________ ___________________________________ ___________________________________ Average atomic mass of Ag: (106.9041) x (.5182) + (108.9047) x (0.4818) = 107.8680 amu ___________________________________ Atomic Mass % Abundance Atomic Mass % Abundance © 2014 John Wiley & Sons, Inc. All rights reserved. ___________________________________ Slide 28 ___________________________________ Let’s Practice! Chlorine exists as two isotopes, Cl-37 (36.96590 amu) and Cl-35. If the percent abundance of each isotope is 24.47 % and 75.53 %, what is the atomic mass of Cl-35 if the average atomic mass is 35.46 amu? ___________________________________ a. 36.95690 d. 34.96885 b. 36.57823 e. 33.56438 c. 35.64544 Solution: ___________________________________ ___________________________________ ___________________________________ Solve for a: ___________________________________ (36.96590) x (.2447) + (a) x (0.7553) = 35.46 amu 9.046 + (a) x (0.7553) = 35.46 amu (a) x (0.7553) = 26.41 amu © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 29 ___________________________________ a = 34.97 amu ___________________________________ Learning Objectives ___________________________________ 5.1 Dalton’s Model of the Atom Describe Dalton’s model of the atom and compare it to the earlier concepts of matter. ___________________________________ 5.2 Electric Charge ___________________________________ Use Coulomb’s Law to calculate the force between particles and distinguish between a cation and anion. ___________________________________ 5.3 Subatomic Parts of the Atom ___________________________________ Describe the three basic subatomic particles and how they changed Dalton’s model of the atom. ___________________________________ © 2014 John Wiley & Sons, Inc. All rights reserved. Slide 30 ___________________________________ Learning Objectives ___________________________________ 5.4 The Nuclear Atom ___________________________________ Explain how the nuclear model of the atom differs from the Dalton and Thomson models. ___________________________________ 5.5 Isotopes of the Elements Define the terms atomic number, mass number and isotope. ___________________________________ 5.6 Atomic Mass ___________________________________ Define the relationship between the atomic mass of an element and the masses of its isotopes. © 2014 John Wiley & Sons, Inc. All rights reserved. ___________________________________