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Atomic Theory Defining the Atom • The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”) – He believed that atoms were indivisible and indestructible – His ideas did agree with later scientific theory, but did not explain chemical behavior, and was not based on the scientific method – but just philosophy Dalton’s Atomic Theory (experiment based!) John Dalton (1766 – 1844) 1) All elements are composed of tiny indivisible particles called atoms 2) Atoms of the same element are identical. Atoms of any one element are different from those of any other element. 3) Atoms of different elements combine in simple whole-number ratios to form chemical compounds 4) In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element. Dalton’s Atomic Theory (1808) 1. Elements are composed of extremely small particles called atoms. All atoms of a given element are identical, having the same size, mass and chemical properties. The atoms of one element are different from the atoms of all other elements. 2. Compounds are composed of atoms of more than one element. The relative number of atoms of each element in a given compound is always the same. 3. Chemical reactions only involve the rearrangement of atoms. Atoms are not created or destroyed in chemical reactions. 2.1 2 2.1 16 X + 8Y 8 X2Y 2.1 Structure of the Nuclear Atom Dalton Wasn’t Exactly Correct….. • One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles: –Electrons, protons, and neutrons are examples of these fundamental particles –There are many other types of particles, but we will study these three Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron J.J. Thomson, measured mass/charge of e(1906 Nobel Prize in Physics) 2.2 Cathode Ray Tube 2.2 Thomson’s Atomic Model J. J. Thomson Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. Modern Cathode Ray Tubes Television Computer Monitor Cathode ray tubes pass electricity (electrons) through a gas that is contained at a very low pressure. Mass of the Electron Mass of the electron is 9.11 x 10-28 g The oil drop apparatus 1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge Measured mass of e(1923 Nobel Prize in Physics) e- charge = -1.60 x 10-19 C Thomson’s charge/mass of e- = -1.76 x 108 C/g e- mass = 9.10 x 10-28 g 2.2 (Uranium compound) 2.2 2.2 (1908 Nobel Prize in Chemistry) particle velocity ~ 1.4 x 107 m/s (~5% speed of light) 1. atoms positive charge is concentrated in the nucleus 2. proton (p) has opposite (+) charge of electron (-) 3. mass of p is 1840 x mass of e- (1.67 x 10-24 g) 2.2 Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Conclusions: a) The nucleus is small b) The nucleus is dense c) The nucleus is positively charged Rutherford’s Model of the Atom atomic radius ~ 100 pm = 1 x 10-10 m nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m “If the atom is the Houston Astrodome, then the nucleus is a marble on the 50-yard line.” 2.2 The Rutherford Atomic Model • Based on his experimental evidence: – The atom is mostly empty space – All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus” – The nucleus is composed of protons and neutrons (they make the nucleus!) – The electrons distributed around the nucleus, and occupy most of the volume – His model was called a “nuclear model” Chadwick’s Experiment (1932) H atoms - 1 p; He atoms - 2 p mass He/mass H should = 2 measured mass He/mass H = 4 · In 1932 James proved the existence of neutral particles in an atom · James said that the neutrons were just about the same weight as protons · He discovered this by using alpha rays, which are charged, and therefore repelled by considerable electrical forces present in the nuclei of heavier atoms · Chadwick led the way to the starting of penetrating and splitting the nuclei of atoms. · Also led the way to the fission of uranium 235, which eventually created the atomic bomb HOMEWORK 1. Describe JJ Thompson’s CRT (Cathode Ray Tube) experiment & how it showed that atoms contain particles he called “electrons.” 2. Describe JJ’s model of the atom. 3. Explain Rutherford’s scattering experiment and what it helped to prove. Also, how did it disprove Thompson’s model? 4. Describe Rutherford’s atomic model. 5. What led to Chadwick’s discovery. mass p+ = mass no = 1840 x mass e- Mass (amu) 0 1 1 2.2 Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei Mass Number A ZX Atomic Number 1 1H 235 92 2 1H U Element Symbol (D) 238 92 3 1H (T) U 2.3 2.3 Do You Understand Isotopes? How many protons, neutrons, and electrons are 14 in 6 C ? 6 protons, 8 (14 - 6) neutrons, 6 electrons How many protons, neutrons, and electrons are 11 in 6 C ? 6 protons, 5 (11 - 6) neutrons, 6 electrons 2.3 Ions: Atoms with different number of protons (p+) and electrons (e-) cation – ion with a positive charge If a neutral atom loses one or more electrons it becomes a cation. Na 11 protons 11 electrons Na+ 11 protons 10 electrons anion – ion with a negative charge If a neutral atom gains one or more electrons it becomes an anion. Cl 17 protons 17 electrons Cl- 17 protons 18 electrons Do You Understand Ions? How many protons and electrons are in 27 3+ 13 Al ? 13 protons, 10 (13 – 3) electrons How many protons and electrons are in 78 234 Se ? 34 protons, 36 (34 + 2) electrons 2.5 Practice: 1) Write the elemental symbol if 10 p+, 11no, 10e21Ne 2) Write the elemental symbol if 20p+, 20no, 18e40Ca2+ 2.5 Noble Gas Halogen Group Alkali Metal Alkaline Earth Metal Period 2.4 Chemistry In Action Natural abundance of elements in Earth’s crust Natural abundance of elements in human body 2.4 A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds H2 H2O NH3 CH4 A diatomic molecule contains only two atoms H2, N2, O2, Br2, HCl, CO A polyatomic molecule contains more than two atoms O3, H2O, NH3, CH4 2.5 An ion is an atom, or group of atoms, that has a net positive or negative charge. cation – ion with a positive charge If a neutral atom loses one or more electrons it becomes a cation. Na 11 protons 11 electrons Na+ 11 protons 10 electrons anion – ion with a negative charge If a neutral atom gains one or more electrons it becomes an anion. Cl 17 protons 17 electrons Cl- 17 protons 18 electrons 2.5 A monatomic ion contains only one atom Na+, Cl-, Ca2+, O2-, Al3+, N3- A polyatomic ion contains more than one atom OH-, CN-, NH4+, NO3- 2.5 2.5 2.6 A molecular formula shows the exact number of atoms of each element in the smallest unit of a substance An empirical formula shows the simplest whole-number ratio of the atoms in a substance molecular empirical H2O H2O C6H12O6 CH2O O3 O N2H4 NH2 2.6 ionic compounds consist of a combination of cations and an anions • the formula is always the same as the empirical formula • the sum of the charges on the cation(s) and anion(s) in each formula unit must equal zero The ionic compound NaCl 2.6 Formula of Ionic Compounds 2 x +3 = +6 3 x -2 = -6 Al2O3 Al3+ 1 x +2 = +2 Ca2+ 1 x +2 = +2 Na+ O22 x -1 = -2 CaBr2 Br1 x -2 = -2 Na2CO3 CO322.6 2.6 2.7 Polyatomic Ions +1 -1 -2 -3 Ammonium NH4 Nitrate NO3 Carbonate CO3 Phosphate PO4 Acetate C2H3O2 Sulfate SO4 Phosphite PO3 Cyanide CN Sulfite SO3 Hydroxide OH Dichromate Cr2O7 Permanganate MnO4 Chlorate ClO3 Bicarbonate HCO3 Nitrite NO2 WRITING FORMULAE (DIATOMIC & POLYATOMIC) Polyatomic Mixed 1) Potassium Nitrate 2) Sodium Sulfate 3) Potassium Dichromate 4) Ammonium Phosphate 5) Copper I Carbonate 6) Iron (III) Cyanide 7) Silver Sulfite 8) Tin (II) Nitrite 9) Calcium Hydroxide 10) Boron Acetate 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. Sodium Sulfide Sodium Bicarbonate Potassium Oxide Potassium Permanganate Magnesium Chloride Calcium Chlorate Iron (II) Phosphate Copper Nitride Ammonium Phosphide Aluminum Acetate Chemical Nomenclature • Ionic Compounds – often a metal + nonmetal – anion (nonmetal), add “ide” to element name BaCl2 barium chloride K2O potassium oxide Mg(OH)2 magnesium hydroxide KNO3 potassium nitrate 2.7 • Transition metal ionic compounds – indicate charge on metal with Roman numerals FeCl2 2 Cl- -2 so Fe is +2 iron(II) chloride FeCl3 3 Cl- -3 so Fe is +3 iron(III) chloride Cr2S3 3 S-2 -6 so Cr is +3 (6/2) chromium(III) sulfide 2.7 Hydrates: – ionic compounds with Water Molecules contained in their crystalline structure. CuCl2 · 2 H2O Copper Chloride Dihydrate Fe(SO3) · 5 H2O Iron (II) Sulfate Pentahydrate Cu(NO3)2 · 4 H2O Copper Nitrate Tetrahydrate 2.7 Hydrates: – USE PREFIXES TO DENOTE THE # OF WATER MOLECULES 1 = mono 6 = hexa 2 = di 7 = hepta 3 = tri 8 = octa 4 = tetra 9 = nona 5 = penta 10 = deca • Molecular compounds • nonmetals or nonmetals + metalloids • common names • H2O, NH3, CH4, C60 • element further left in periodic table is 1st • element closest to bottom of group is 1st • if more than one compound can be formed from the same elements, use prefixes to indicate number of each kind of atom • last element ends in ide 2.7 Molecular Compounds HI hydrogen iodide NF3 nitrogen trifluoride SO2 sulfur dioxide N2Cl4 dinitrogen tetrachloride NO2 nitrogen dioxide N2O dinitrogen monoxide TOXIC! Laughing Gas 2.7 2.7 An acid can be defined as a substance that yields hydrogen ions (H+) when dissolved in water. HCl •Pure substance, hydrogen chloride •Dissolved in water (H+ Cl-), hydrochloric acid An oxoacid is an acid that contains hydrogen, oxygen, and another element. HNO3 nitric acid H2CO3 carbonic acid H2SO4 sulfuric acid HNO3 2.7 2.7 2.7 2.7 A base can be defined as a substance that yields hydroxide ions (OH-) when dissolved in water. NaOH sodium hydroxide KOH potassium hydroxide Ba(OH)2 barium hydroxide 2.7 2.7