Early Atomic Theory and Structure. Beginning Days (Theory of an Atom) Beginning Days 8/14/2011. Chapter 5

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1 Early Atomic Theory and Structure Chapter 5 Beginning Days (Theory of an Atom) 5 th century B.C., Greek philosophers Empedocles and Democritus proposed their own theories about an atom. Empedocles stated that elements composed primarily of 4 elements: earth, air, water, and fire Beginning Days (Theory of an Atom) 5 th century B.C., Greek philosophers Empedocles and Democritus proposed their own theories about an atom. Democritus claimed that: all forms of matter are composed of tiny, indivisible particles called atoms 1

2 Beginning Days (Theory of an Atom) 5 th century B.C., Greek philosophers Empedocles and Democritus proposed their own theories about an atom. Democritus theory would be tested by Aristotle around 4 th century B.C. and he opposed his claim but not on scientific observations and agreed with Empedocles claim. FINAL Conclusion (Theory of an Atom) Dalton s Model of the Atom Around the 18 th century, John Dalton revised Democritus theory by proposing his own model about an atom and summarized it in six statements: 1. Elements are composed of small, indivisible particles called atoms. 2. Atoms of the same element are alike in mass and size. 3. Atoms of different elements have different masses and sizes. FINAL Conclusion (Theory of an Atom) Dalton s Model of the Atom Around the 18 th century, John Dalton revised Democritus theory by proposing his own model about an atom and summarized it in six statements: 4. Chemical compounds are formed by the union of two or more atoms of different elements. 5. Atoms combined to form compounds in simple numerical ratios, such as 1 to 1, 1 to 2, 2 to 3, and so on. 6. Atoms of two elements may combine in different ratios to form more than one compound. 2

3 Law of Definite Composition This law states that studying the composition of a compound, the ratio between two elements is proportional to their masses. Let s look at this in more detail!!! Law of Definite Composition Suppose I want to see if two samples of carbon dioxide are decomposed in ratio to their corresponding elements. 1. SAMPLE #1: 4.00 grams of carbon produced with 1.50 grams of oxygen. 2. SAMPLE #2: 3.34 grams of carbon produced with 1.25 grams of oxygen. Are they the same??? Law of Multiple Composition (cont d) The law of multiple proportions indicates that atoms of two or more elements can combine to result in different ratios lead to producing more than 1 compound. Let s look at water and hydrogen peroxide!!! 3

4 H 2 O There are 8.0 grams of oxygen per gram of hydrogen. COMPARISON H 2 O 2 There are 16.0 grams of oxygen per gram of hydrogen. mo (H2O) 16.00g 8 = = mh (H2O) 2.02g 1 mo (H2O2) 32.00g 16 = = mh (H2O2) 2.02g 1 Electric CHARGE (PROPERTIES) Do you ever consider why hair attracts a hair dryer? Why clothes attract dryer sheets? ANSWER: There is an electrical charge that s occurring at the present moment!!! Electric CHARGE (PROPERTIES) Actually, the key points about electrical charge are as follows: 1. Charges come in two forms (+ and -). 2. Like charges repel and unlike charge attract one another. 3. Charge can be transferred from one object to another. 4. The amount of attractionalforce can be measured between atoms using the following formula: kq q F = r

5 History The discovery of ions was first studied by Michael Faraday. His study was on interacting certain substances when dissolved in water. In substances, certain atoms will attract the positive electrode and others will attract the negative electrode. Additional STUDIES (IONS) SvanteArrhenius additionally studied that an ion is an atom can carry a positive or negative charge. If a substance has ions, conductivity is present. NaCl(aq) = > Na + (aq) + Cl - (aq) CONCLUSIONS (Ions) The negative end of an electrode attracts Na + (known as the cathode). The positive end of an electrode attracts Cl - (known as the anode). Both Faraday s and Arrhenius study led to Stoney s conclusion that there must exist a fundamental unit known as electrons. However, it wasn t valid until Thomson s studied the electron. 5

6 Atom What you should know about an atom? 1. It is the smallest, identifiable particle of an element. 2. The diameter is about nm in length. 3. There are smaller particles within an atom (SUBATOMIC Particles). Atomic THEORY (cont d) First study about subatomic particles was performed by Sir William Crooks later known as the CROOKS Tube. Atomic THEORY (cont d) The CROOKS tube determined the following about cathode rays: 1. They travel in straight lines. 6

7 Atomic THEORY (cont d) The CROOKS tube determined the following about cathode rays: 2. They are negative in charge. Atomic THEORY (cont d) The CROOKS tube determined the following about cathode rays: 3. They are deflected by electric and magnetic fields. Atomic THEORY (cont d) The CROOKS tube determined the following about cathode rays: 4. They produce sharp shadows. 7

8 Atomic THEORY (cont d) The CROOKS tube determined the following about cathode rays: 5. They can move a small, paddle wheel. These studies led to the development of the electron!!! Components (ATOM) The three main components in an atom are the following: 1. Proton (GOLDSTEIN/THOMSON) 2. Neutron (CHADWICK) 3. Electron (CROOKS) All of these are also known as subatomic particles. 8

9 Question Calculate number of atoms in 25 g of hydrogen, if each hydrogen atom has a mass of g. THOMSON S Model (Electron) His model illustrates that while most of the atom comprises of a positive charge, there are electrons embedded. NICKNAME: Plumpudding model THOMSON S Model (Ions) Additional facts from this study: 1. Positive ions loses electrons from its atom. 2. Negative ions gain electrons from its atom. Let s look at a couple of examples!!! 9

10 EXAMPLE #1 EXAMPLE #2 Rutherford s MODEL (CONCEPT) Rutherford continued the study by applying Thomson s model to verify his claim about an atom. Let s look at his experiment!!!! 10

11 Figure-Illustration of Rutherford s alpha particle experiment COMPARISON PLUM Pudding Model Rutherford s Gold Foil Experiment Rutherford s actual result protons neutrons ATOMIC Number What is the atomic number? It is the number of protons the atom (or ion) has present in its nucleus. If it s an atom: # of protons = # of electrons If it s an ion: # of protons # of electrons 11

12 Definition Many elements can have the same atomic number but different masses. This is the primary characteristic in: ISOTOPES Let s look at its notation!!! Isotopes NOTATION One can write out the notation of an isotope using the following: Mass number (sum of protons and neutrons in the nucleus) A Z E Symbol of element Atomic number (number of protons in the nucleus) Solving (Protons, Neutrons, and Electrons) Mass Number = # protons + # neutrons Atomic Number = # protons NOTE: for a neutral element, # protons = # electrons 12

13 Example Isotopes of OXYGEN O 8 O 17 8 O What are the important things that needs to be known? PROBLEM Isotopes Determine the number of protons, neutrons, and electrons in the following isotopes C 17 8 O 15 7 N PROBLEM Identification IDENTITY Determine the identity of element(s) which have the following characteristics. a) 14 protons and 15 neutrons b) 10 electrons c) 209 X 83 13

14 Question How many protons, neutrons, and electrons are in this ion? 17 O 2- Complete the table: Isotopes Element Symbol Atomic No. Mass No. No. of Protons No. of Electrons No. Of Neutrons chlorine lead Pb 82 argon Ar 18 Question What is the mass number of an atom that contains 30 protons, 30 electrons, and 35 neutrons? a. 35 b. 30 c. 65 d

15 Question 79 Approximately 50.70% of all atoms of bromine Br 35 are atoms. How many neutrons, protons and electrons does this isotope of bromine have? a. 79 neutrons, 35 protons and 35 electrons b. 44 neutrons, 35 protons and 35 electrons c. 35 neutrons, 79 protons and 35 electrons d. 44 neutrons, 35 protons and 44 electrons e. 79 neutrons, 35 protons and 44 electrons ATOMIC Mass The atomic mass of any element is determined by the mass of each isotope involved. Typically as seen earlier, the atomic mass unit (AMU) is in correspondence to 1/12 the mass of a carbon-12 atom. Actual mass of carbon-12 atom is x g. 1/12 of this mass is x g. Analyzing ATOMIC Mass A typical reading from a mass spectrometer. The two principal isotopes of copper are shown with the abundance (%) given. 15

16 EXAMPLE #1 Cu Average atomic mass is amu. EXAMPLE #2 N Average atomic mass is amu. EXAMPLE #3 As Average atomic mass is amu. 16

17 Solving ATOMIC Mass How does one calculate the atomic mass of any element? SOLUTION: Atomic Mass = fraction of isotope 1 x mass + fraction of isotope 2 x mass + fraction of isotope 3 x mass + PROBLEM Practice 5.5 Silver occurs as two isotopes with atomic masses and amu respectively. The first isotope represents 51.82% and the second 48.18%. Determine the average atomic mass of silver. 17