BMC II, 2025 Chemistry Lecture Notes PDF

The Atomic Theory, Structure of the Atom Nuclear Arithmetic BMC II, 2025 Endre Kókai  BMC II 2025 3rd Chemistry Lecture The Atomic Theory, Structure of the Atom Nuclear Arithmetic - McMurry pp. 75-86 Elements  canno...

The Atomic Theory, Structure of the Atom Nuclear Arithmetic BMC II, 2025 Endre Kókai  BMC II 2025 3rd Chemistry Lecture The Atomic Theory, Structure of the Atom Nuclear Arithmetic - McMurry pp. 75-86 Elements  cannot be decomposed by CHEMICAL means  „Building blocks” of the universe  118 elements, 90 occur naturally  abbreviated by symbols  History of Atomic Theory Democritus - Ancient Greek philosopher (460-371 BC) - Believed all matter consisted of extremely small particles that could not be divided - Atoms, from Greek word atomos, means „uncut” Aristotle - Believed all matter came from only four elements: earth, air, fire and water  Theory of Matter in Antiquity Can matter be divided into smaller and smaller pieces forever, or is there a limit to the number of times a piece of matter can be divided? Greek philosophers (Aristotle, Plato): Matter is continuous, can be infinitely divided Elements: earth, air, water, fire  Particulate Theory of Matter Leucippus, Democritus 400 BC  Matter can not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained.  The smallest piece of matter was named “atomos,” meaning “not to be cut.” All matter is composed of atoms, which are bits of matter too small to be seen. These atoms are indivisible. There is a void, which is empty space between atoms. Atoms are completely solid. Atoms are homogeneous, with no internal structure. Atoms are different in: their sizes, their shapes, and their weights. This theory was ignored and forgotten for more than 2000 years. Atomic Structure Revolution Robert Boyle (1627-1691) performed detailed experiment with gases; provided evidence for the atomic makeup of matter defined atom as a substance that cannot be chemically broken down further Joseph Priestley (1703-1804) prepared and isolated the gas oxygen 2 HgO → 2 Hg + O2 Law of Mass Conservation Antoine Lavoisier (1743-1794) He demonstrated the role of oxygen in combustion. When combustion is carried out in a closed container, the mass of the products is exactly equals the mass of starting reactants. He concluded that mass is neither created nor destroyed. Law of Mass Conservation 3.25 g + 3.32 g = 6.57 g Hg(NO3)2(aq) + 2KI(aq) HgI2(s) + 2KNO3(aq) 4.55 g + 2.02 g = 6.57 g Law of Mass Conservation Mass is neither created nor destroyed in chemical reactions.  Law of Definite Proportions Different samples of a pure chemical substance always contain the same proportion of elements by mass. –i.e., for H2O, H : O mass ratio = 2:16 or 1 : 8 By mass, water is: 88.8 % oxygen Joseph Proust 11.2 % hydrogen (1754-1826) Elements combine in specific proportions, not in random proportions.  Law of Multiple Proportions nitrogen monoxide: 7 grams nitrogen per 8 grams oxygen nitrogen dioxide: 7 grams nitrogen per 16 grams oxygen N:O mass ratio in NO 7g N / 8 g O = =2 N:O mass ratio in NO2 7g N / 16 g O Law of Multiple Proportions Elements can combine in different ways to form different chemical compounds, with mass ratios that are small, whole-number multiples of each other.  Law of Multiple Proportions nitrogen monoxide: 7 grams nitrogen per 8 grams oxygen nitrogen dioxide: 7 grams nitrogen per 16 grams oxygen N:O mass ratio in NO 7g N / 8 g O = =2 H:O mass ratio in NO2 7g N / 16 g O Timeline of Atomic Theory It took ~2400 years from when it was conceived to the time when experimental evidence proved the existence of atoms.  Dalton’s Atomic Theory Compounds consist of atoms which combine in whole number ratios. John Dalton (1766-1844)  Dalton's Atomic Theory Elements are made of atoms. Each element is characterized by the mass of its atoms. Atoms of the same element have the same mass, but atoms of different elements have different masses. Chemical combination of elements to make different chemical compounds occurs when atoms join together in small whole-number ratios. Chemical reactions only rearrange how atoms are combined in chemical compounds; the atoms themselves don’t change, are neither created, nor destroyed.  Problems with Dalton’s Atomic Theory? Matter is composed of indivisible particles. Atoms Can Be Divided, but only in a nuclear reaction All atoms of a particular element are identical. Does Not Account for Isotopes (atoms of the same element but a different mass due to a different number of neutrons)! Different elements have different atoms. YES! Atoms combine in certain whole-number ratios. YES! Called the Law of Definite Proportions In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements. Yes, except for nuclear reactions that can change atoms of one element to a different element The Discovery of Atomic Structure: What is an atom made of? The ancient Greeks were the first to postulate that matter consists of indivisible constituents. Later scientists realized that the atom consisted of charged entities. Cathode Rays and Electrons J. J. Thomson (1856-1940)  A Cathode-Ray tube A cathode ray tube (CRT) is a hollow vessel with an electrode at either end. A high voltage is applied across the electrodes. The voltage causes negative particles to move fom the negative electrode (cathode) to the positive electrode (anode) in straight lines.  Cathode Rays and Electrons Cathode rays undergo deflection: parallel to an applied electrostatic field (toward the positive plate) negatively charged particles (electrons) perpendicular to an applied magnetic field The degree of deflection of the electron beam depends on the strength of the deflecting magnetic and electric field size of the negative charge on the electron mass of the electron  Cathode Rays and Electrons By varying the two opposing forces until they balance each other, Thomson determined the charge to mass ratio of an electron: e m = 1.76  108 C/g  Millikan’s Oil Drop Experiment: the charge and mass of the electron Millikan’s original oil-drop apparatus, circa 1909–1910 https://en.wikipedia.org/wiki/Robert_Andrews_Millikan Robert A. Millikan (1868-1953) Millikan’s Oil Drop Experiment © 2012 Pearson Education, Inc. Chapter 2/24  Millikan’s Oil Drop Experiment The charge of a given droplet was always a small whole- number multiple of 1.60  10-19 C, the charge of an electron. e = 1.60  10-19 C Knowing the charge to mass ratio, 1.76  108 C/g, Millikan calculated the mass of the electron: m = 9.10  10-28 g. With more accurate numbers, we get the mass of the electron to be 9.10939  10-28 g. Thomson’s Atom Model From the separation of radiation we conclude that the atom consists of neutral, positively, and negatively charged entities. Thomson assumed all these charged species were found in a sphere. 1900 J.J. Thomson: Plum Pudding Model Radioactivity  Radioactivity An alpha particle is a helium nucleus (2 protons and 2 neutrons). A beta particle is an electron. A gamma particle is a high-energy photon.  Rutherford’s Scattering Experiment Ernest Rutherford (1871-1937) A source of -particles was placed at the mouth of a circular detector. The -particles were shot through a piece of gold foil. Most of the -particles went straight through the foil without deflection. Some -particles were deflected at high angles. Rutherford’s Scattering Experiment and the Nuclear Atom The Nuclear Atom The relative size of the nucleus in an atom is roughly the same as that of a pea in the middle of this stadium.  1900 J.J. Thomson: 1911 E. Rutherford Plum Pudding Model Nuclear Model Atoms contains The atom has a very dense protons and electrons. nucleus…100,000 times smaller than the whole atom… Negative electrons 10-15 m Positive charges spread 10-10 m over sphere The Modern View of Atomic Structure The atom consists of positive, negative, and neutral entities (protons, electrons, and neutrons). Protons and neutrons are located in the nucleus of the atom, which is small. Most of the mass of the atom is due to the nucleus. There can be a variable number of neutrons for the same number of protons. Isotopes have the same number of protons but different numbers of neutrons. Electrons are located outside of the nucleus. Most of the volume of the atom is due to electrons.  Atomic Structure: Protons and Neutrons The charge of the proton is opposite in sign but equal to that of the electron.  Atomic Numbers, Mass Numbers and Isotopes Atomic Number (Z): Number of protons in an atom’s nucleus. Equivalent to the number of electrons around an atom’s nucleus Mass Number (A): The sum of the number of protons and the number of neutrons in an atom’s nucleus Isotope: Atoms with identical atomic numbers but different mass numbers  Isotopes Hydrogen has three isotopes. radioactive Isotopes, Atomic Numbers, and Mass Numbers carbon-12 mass number 12 6 protons 6C 6 electrons 6 neutrons atomic number carbon-14 mass number 14 6 protons 6C 6 electrons 8 neutrons atomic number  Micro World Macro World atoms & molecules grams Atomic mass the mass of an atom in atomic mass units (amu), also called Dalton (Da) By definition: 1 atom 12C “weighs” 12 amu mass of one 126C atom 1 amu = = 1.660 539x10-24 g 12  Atomic Mass 1H = 1.008 amu On this scale 16O = 16.00 amu Mass number = number of protons + number of neutrons Mass (amu) 5.48580x10-4 Protons and neutrons 1.00728 each have a mass of 1.00866 almost exactly 1 amu. The mass of an atom in amu ≈ the atom's mass number. Atomic Masses of Elements  Atomic Masses The mass of 1 atom of carbon-12 is defined to be 12 amu. Atomic Mass: The weighted average of the isotopic masses of the element’s naturally occurring isotopes © 2012 Pearson Education, Inc. Chapter 2/41  Atomic Masses Why is the atomic mass of the element carbon 12.01 amu? carbon-12: 98.89 % natural abundance 12 amu carbon-13: 1.11 % natural abundance 13.0034 amu mass of carbon = (12 amu)(0.9889) + (13.0034 amu)(0.0111) = 11.87 amu + 0.144 amu = 12.01 amu © 2012 Pearson Education, Inc. Chapter 2/42  Atomic Masses 13 Atomic number Al Atom symbol 26.981 AVERAGE Atomic Mass Isotope Half Life Al-26 730000.0 years Al-27 Stable Al-28 2.3 minutes Uses of Radioactive Isotopes (Examples) The tritium content of ground water is used to discover the source of the water, for example, in municipal water or the source of the steam from a volcano. Radioactive Isotopes in Nuclear Medicine (Example) Bone scans with radioactive technetium-99.  Exercises The observation that 15.0 g of hydrogen reacts with 120.0 g of oxygen to form 135.0 g of water is evidence for the law of A) definite proportions. B) energy conservation. C C) mass conservation. D) multiple proportions. The charge-to-mass ratio of an electron was established by A) Millikan's oil drop experiment. B) Rutherford's gold foil experiment. C C) Thomson's cathode ray tube experiment. D) None of these. Element X is toxic to humans in high concentration but is essential to life in low concentrations. Identify element X, whose atoms contain 24 protons, and write the symbol for the isotope of X that has 28 neutrons. According to the periodic table, the element with atomic number 24 is chromium (Cr). The particular isotope of chromium in this instance has a mass number of 24 + 28 = 52 and is written Cobalt has three radioactive isotopes used in medical studies. Z= 27, and atoms of these isotopes have 30, 31, and 33 neutrons, respectively. Give the symbol for each of these isotopes. 57 A1 = 27 + 30 = 57 27 Co 58 A2 = 27 + 31 = 58 27 Co 60 A3 = 27 + 33 = 60 27 Co Which of the following represent isotopes of the same element? Which element? 234 234 235 238 X X X X 92 93 92 92 (a) (b) (c) (d) (a), (c), (d) uranium An atom has 14 protons and 20 neutrons. 1. Its atomic number is (A) 14 (B) 16 (C) 20 (D) 34 2. Its mass number is (A) 14 (B) 16 (C) 20 (D) 34 3. The element is (A) Si (B) Ca (C) Se (D) S 4. Another isotope of this element is (A) 34X (B) 34X (C) 36X (D) 20X 16 14 14 16 Complete the following table. 23 11 12 11 14 6 8 6 37 17 20 18 56 26 30 23 Chlorine has two naturally occurring isotopes: 35Cl (34.969 amu) with 75.77 % abundance and 37Cl (36.966 amu) with 24.23 % abundance. What is the atomic mass of chlorine? Fraction abundance = % abundance/100 Atomic mass of chlorine = 0.7577 x 34.969 amu + 0.2423 x 36.966 amu Atomic mass of chlorine = 35.45 amu Thank You for Your Attention