chapter 2 student summer 2020

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Chapter 2: Homework 2.1 Atomic Theory of Matter The theory that atoms are the fundamental building blocks of matter reemerged in the early nineteenth century, championed by John Dalton. Law of Conservation of Mass : 2.2 The Discovery of Atomic Structure Particles with the same charge repel one another, whereas particles with unlike charges attract one another. The Electron J. J. Thomson (1897) Streams of negatively charged particles were found to emanate from cathode tubes, causing fluorescence. Thomson measured the charge/mass ratio of the electron to be 1.76 ´ 10 8 coulombs/gram (C/g). http://www.youtube.com/watch? v=XU8nMKkzbT8 1
Chapter 2: Homework Millikan Oil-Drop Experiment Robert Millikan (University of Chicago) determined the charge on the electron in 1909. http://www.youtube.com/watch?v=XMfYHag7Liw Radioactivity • Radioactivity • It was first observed by Henri Becquerel. • Marie and Pierre Curie also studied it. • Three types of radiation were discovered by Ernest Rutherford: 1. 2. 3. The Atom, circa 1900 Thomson’s “plum pudding” model a positive sphere of matter with negative electrons imbedded in it. Discovery of the Nucleus Rutherford shot a particles at a thin sheet of gold foil; observed the pattern of scatter of the particles. http://www.youtube.com/watch?v=5pZj0u_XMbc Since some particles were deflected at large angles, Thomson’s model could not be correct. Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom. Most of the volume of the atom is empty space. 2
Chapter 2: Homework 2.3 The Modern View of Atomic Structure • Protons - Rutherford in 1919 • Neutrons - James Chadwick in 1932 • Protons and electrons are the only particles that have a charge • Protons and neutrons have essentially the same mass • mass of an electron is so small we ignore it Symbols of Elements Elements are symbolized by one or two letters. atomic number , Z ,: mass number: Isotopes: Sample Exercise 2.1: How many protons, neutrons, and electrons are in (a) a 138 Ba atom, (b) an atom of phosphorus-31? Sample Exercise 2.2: Give the complete chemical symbol for the atom that contains 82 protons, 82 electrons, and 126 neutrons. 2.4 Atomic Weights Atomic and molecular masses can be measured with great accuracy using a mass spectrometer. http://www.youtube.com/watch?v=OKxRx0ctrl0 Average mass Atomic mass unit : amu – exactly 12 amu in 1 atom of 12 C 3
Chapter 2: Homework 1 amu = 1.66054 x 10 -24 g or 1g = 6.022x10 23 amu Sample Exercise 2.3: Naturally occurring chlorine is 75.78% 35 Cl (atomic mass 34.969 amu) and 24.22% 37 Cl (atomic mass 36.966 amu). Calculate the atomic weight of chlorine. 2.5 Periodic Table Elements are arranged in order of atomic number • The rows on the periodic chart are periods. • Columns are groups. • Elements in the same group have similar chemical properties. When one looks at the chemical properties of elements, one notices a repeating pattern of reactivities. 4
Chapter 2: Homework Sample Exercise 2.4: Which two of these elements would you expect to show the greatest similarity in chemical and physical properties: B, Ca, F, He, Mg, P? 2.6 Molecules and Molecular Compounds Chemical Formulas: subscript to the right of the symbol of an element tells the number of atoms of that element in one molecule of the compound. Diatomic Molecules (2 atoms) 7 naturally occurring: H 2 , O 2 , N 2 , F 2 , Cl 2 , Br 2 , I 2 Molecular compounds Types of Formulas • Empirical formulas: • Molecular formulas: • Structural formulas: Sample Exercise 2.5: Give the empirical formula for diborane , whose molecular formula is B 2 H 6 . 2.7 Ions and Ionic Compounds When atoms lose or gain electrons, they become ions . • Cations are ________ and are formed by elements on the _________ side of the periodic chart. • Anions are ________ and are formed by elements on the __________ side of the periodic chart. Ionic Bonds Ionic compounds (such as NaCl) are generally formed between metals and nonmetals. 5
Chapter 2: Homework Predicting Ionic Charges Try to achieve noble gas configurations Ionic Compounds Ionic compound-made of cations and anions; usually combinations of metals and nonmetals (NaCl) Molecular compounds – generally composed of nonmetals only, (H 2 O) Sample Exercise 2.6: Which of these compounds would you expect to be ionic? Which are molecular? (a) N 2 O _______ (b) Na 2 O_______ (c) CaCl 2 ______ (d) SF 4 _______ (e) CBr 4 ________ (f) FeS ________ (g) P 4 O 6 ________ (h) PbF 2 ______ Writing Formulas Because compounds are electrically neutral, one can determine the formula of a compound this way: • The charge on the cation becomes the subscript on the anion. • The charge on the anion becomes the subscript on the cation. • If these subscripts are not in the lowest whole-number ratio, divide them by the greatest common factor. Sample Exercise 2.7: Write the empirical formula for the compound formed by (a) Na + and PO 4 3– _____________ (b) Zn 2+ and SO 4 2– _____________ (c) Fe 3+ and CO 3 2– ______________ (d) Ca 2+ and O 2- ______________ 6
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