Ch. 2 Atoms and Elements. Early Atomic Theory

Transcription

1 Ch. 2 Atoms and Elements Chapter Overview Introduction to atomic theory [2.1-5] Properties of atomic particles, elements [2.6] Your friend, the Periodic Table! [2.7] Atomic symbols [2.8] Isotopes [2.8] Average atomic mass [2.8] Ions [2.7] The mole, and calculations with mole [2.9] You are expected to know the names and symbols for elements 1-20 (Hydrogen, H through Calcium, Ca) CHEM& 141 F08 1 Early Atomic Theory Four elements from the ancient Greeks: Air, Earth, Fire and Water. Largely based in astrology. Ideas present for how these elements combine to affect life! CHEM& 141 F08 2 1

2 Modern Atomic Theory (~ s) Dalton s Postulates 1. Each element is composed of tiny indestructable particles, called atoms. 2. All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements. 3. Atoms combine in simple, whole-number ratios to form compounds. CHEM& 141 F08 3 Modern Atomic Theory Through a series of experiments by Thompson, Becquerel, Rutherford, and Millikan, the structure of an atom and the characteristics of fundamental particles are determined. Nucleus in center, surrounded by an electron cloud. CHEM& 141 F08 4 2

3 Sub-Atomic Particles Particle Symbol Charge (C) Charge Mass (kg) Mass (amu) Electron e x x Proton p, p +, H x x Neutron n x p = 1 n = 1 amu; 1 e - = 1/1836 amu 1 amu = x kg CHEM& 141 F08 5 Properties of Atoms All atoms of the same element have identical numbers of protons. All carbon atoms have 6 protons, all chromium atoms have 24 protons. The mass of 1 p + 1 n >> 1 e - The total number of p + + n = mass of an atom Neutral atoms are charge neutral: # p + = # e - This is because protons and electrons have equal and opposite charges! And opposites attract! Neutrons dilute the protons in the nucleus. CHEM& 141 F08 6 3

4 CHEM& 141 F08 7 Periodic Table Properties A Period: A horizontal row on the periodic table. A Family or Group: A vertical column on the periodic table. Metals: conductors, malleable and ductile; mainly solids at room temp. Metalloids: semiconductors; mainly solids at room temp. Nonmetals: poor conductors; few solids, liquids, mainly gases at room temp. Each time you step along the periodic table, you add one proton and one electron to create a new element. CHEM& 141 F08 8 4

5 Atomic Symbols A Z X X = Element Symbol Z = Atomic Number A = Mass Number CHEM& 141 F08 9 Practice Protons Neutrons Symbol A B C D What is the relationship between A and D? CHEM& 141 F

6 More Practice Name Z A p + n e CHEM& 141 F08 11 Atomic masses The masses on the periodic table do not match the mass numbers! Atomic Mass = a weighted average of the masses of the isotopes. Atomic Mass =!( relative abundance of isotope x exact mass of isotope) CHEM& 141 F

7 Atomic Masses Example: What is the atomic mass of carbon? 12 C: 98.89% abundance, amu 13 C: 1.11% abundance, amu 12 C: x amu = amu 13 C: x amu = amu mass = amu CHEM& 141 F08 13 Problem Iron(Fe): 54 Fe 56 Fe 57 Fe 5.845% % 2.119% amu amu amu Determine the average atomic mass of iron. CHEM& 141 F

8 Ions Ions occur when there is an imbalance between protons and electrons. Protons > electrons: the resulting ion is positive; this is called a cation. Protons < electrons: the resulting ion is negative; this is called an anion. Ion charge = # p + - # e - Examples: A calcium ion has 20 p + and 18 e -. The charge on the ion is = +2 Ca +2. A fluoride ion has 9 p + and 10 e -. The charge on the ion is 9-10 = -1 F - or F -1. How many electrons are in a phosphide ion, P -3? -3 = 15 - # e - # e - = 18 e - CHEM& 141 F08 16 Predicting Ion Charges We can use the Periodic Table to predict many (but not all) ion charges. Cations: Group # = charge Anions: 8 - (group #) = charge CHEM& 141 F

9 Predicting Ions Determine the typical charge, the number of protons and the number of electrons for the following ions. Element Charge # p + # e- Aluminum Beryllium Oxygen Bromine Potassium CHEM& 141 F08 18 The Mole Concept In the lab, balances measure amounts of chemicals in grams, but reactions happen on the atomic scale. How do we reconcile between these units? The MOLE!!!! Definition: the number of atoms in exactly 12 g of 12 C. Abbreviation: mol Avogadro s Number x things = 1 mole What are the things? Atoms, ions, molecules, electrons, pennies, chickens. CHEM& 141 F

10 The Mole Concept In your lifetime, you may own 400 shoes. How many moles of shoes is that? 1 mole = x shoes 400 shoes x 1 mole x shoes = 7 x10-22 moles of shoes During a reaction, mol of oxygen gas react. How many molecules react? Atoms? mol O 2 x x 1023 molecules = 3.39 x molecules 1 mol 2 atoms 3.39 x molecules x 1 molecule = 6.78 x 1023 atoms CHEM& 141 F08 20 Problems How atoms of carbon are in moles MSG: NaC 5 H 8 NO 4? Step 1: Do you have the formula? The formula provides a ratio of atoms in a molecule Step 2: Start with the number of moles, use N A to convert to molecules Step 3: Use the formula to convert molecules to atoms moles MSG x x 1023 molecules 1 mole x 5 atoms C 1 molecule MSG = 2.26 x 1024 atoms C Step 2 Step 3 CHEM& 141 F

11 Problems How many atoms of oxygen are in moles of aluminum sulfate? CHEM& 141 F08 22 Molar Masses Conveniently, 1 amu = 1 g/mol. The atomic masses found on the periodic table are now the number of grams of the element in one mole. The same relation can be made for formula weights as well. Problem: Determine the molar masses for the following pure substances. Mo N 2 Cu 2 SO 4 C 6 H 12 O 6 CHEM& 141 F

12 Using Molar Masses The mole is the SI unit for amount, and allows us to do comparisons. But there are no balances that weigh in moles! We will use the molar masses to convert between moles and grams. A reaction requires mol of Magnesium turnings. How much magnesium, in g, is that? Step 1: Do you have a formula? Step 2: Determine the molar mass. Step 3: Use the molar mass to convert g mol mol x g 1 mol = 11.5 g CHEM& 141 F08 24 Using Molar Masses How many atoms of sodium are in 13.0 g of sodium metal? For an experiment that I am performing, I need to add 12.5 mmol of iron (II) sulfate. How many grams must I weigh out? CHEM& 141 F

13 Practice Problems What is the mass of 4.91 x platinum atoms? How many atoms of copper are in a 133 kg pure copper statue? CHEM& 141 F08 26 Practice Problems Determine the number of sulfur atoms in 1.59 mmol of carbon disulfide. Determine the number of H atoms are in 9.88 mol of NH 3. CHEM& 141 F