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10/2/2013 5 Early Atomic Theory and Structure Chapter Outline 5.1 Dalton’s Model of the Atom 5.2 Electric Charge A. Discovery of Ions 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 5.3 Subatomic Parts of the Atom 5.4 The Nuclear Atom 5.5 Isotopes of the Elements 5.6 Atomic Mass Copyright © 2014 John Wiley & Sons, Inc. All rights reserved. © 2014 John Wiley & Sons, Inc. All rights reserved. Early Theories on the Structure of Matter Dalton’s Model of the Atom 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. 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. © 2014 John Wiley & Sons, Inc. All rights reserved. Dalton’s Model of the Atom Dalton’s Model of the Atom Revisions to Dalton’s Theory H2O H2O2 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. 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. Most of Dalton’s theory remains valid today. © 2014 John Wiley & Sons, Inc. All rights reserved. © 2014 John Wiley & Sons, Inc. All rights reserved. 1 10/2/2013 Electric Charge 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. 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. 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. A scanning tunneling microscope (STM) image shows an array of Cu atoms. © 2014 John Wiley & Sons, Inc. All rights reserved. © 2014 John Wiley & Sons, Inc. All rights reserved. Subatomic Parts of the Atom Electrons and Protons A Crooks tube permits generation of cathode rays, which are streams of electrons. 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 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. 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. © 2014 John Wiley & Sons, Inc. All rights reserved. The Effect of Subatomic Particles 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. Atoms can become ions by gaining or losing electrons from this sphere. Thomson’s Model of the Atom Electrons are negatively charged particles which are embedded in a positively charged atomic sphere. Electrons are lost from atoms to give cations. Electrons + charged sphere Thomson’s “plum pudding” model of the atom. Electrons are gained from atoms to give anions. © 2014 John Wiley & Sons, Inc. All rights reserved. © 2014 John Wiley & Sons, Inc. All rights reserved. 2 10/2/2013 Neutrons The last subatomic particle was discovered by Chadwick in 1932. Summary of Subatomic Particles Atoms are composed of three smaller, subatomic particles: electrons, protons and neutrons. Neutrons (n) A particle with no electrical charge. Neutrons have a mass similar to that of a proton. 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. © 2014 John Wiley & Sons, Inc. All rights reserved. Nuclear Model of the Atom Nuclear Model of the Atom In 1911, Ernest Rutherford established the nuclear model of the atom by bombarding gold atoms with α particles. Because most of the particles were not deflected, this suggested most of the atom is empty space. Most of the particles passed through the gold foil, but some were deflected and some even bounced back! 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. 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. © 2014 John Wiley & Sons, Inc. All rights reserved. Atomic Number Isotopes of the Elements Atomic Number: Number of protons in the nucleus of an atom. 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. 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. 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. 3 10/2/2013 Isotopes of the Elements Isotopes of the Elements Example: Isotopes of Hydrogen Protium 1 proton 0 neutrons Deuterium Tritium 1 proton 1 neutron 1 proton 2 neutrons Standard Isotopic Notation Mass Number A Atomic Number Z E Element Symbol Practice: How many protons, neutrons, and electrons are found in each of the following isotopes? 64 Cu 29 Mass number: Total number of protons and neutrons for an element. Atomic Number: 29 protons (therefore 29 electrons) # Neutrons = Mass Number – Atomic Number 64 – 29 = 35 neutrons © 2014 John Wiley & Sons, Inc. All rights reserved. © 2014 John Wiley & Sons, Inc. All rights reserved. Let’s Practice! Atomic Mass Which isotope corresponds to an element with 24 protons and 28 neutrons? 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. a. 28 52 Cr Solution: b. 52 24 Cr c. 52 28 Ni # protons = Atomic Number = 24 Element: Cr d. 128 Te 52 e. 24 52 Mass Number = protons + neutrons = 24 + 28 = 52 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. Cr © 2014 John Wiley & Sons, Inc. All rights reserved. © 2014 John Wiley & Sons, Inc. All rights reserved. Atomic Mass and Isotope Distribution 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. 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. 4 10/2/2013 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 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 Atomic Mass % Abundance Atomic Mass % Abundance 9.046 + (a) x (0.7553) = 35.46 amu (a) x (0.7553) = 26.41 amu a = 34.97 amu © 2014 John Wiley & Sons, Inc. All rights reserved. © 2014 John Wiley & Sons, Inc. All rights reserved. Learning Objectives 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. 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. 5