# Chapter 3 1 The Atom w Summarize the

• Slides: 43

Chapter 3 -1: The Atom w Summarize the five essential points of Dalton’s atomic theory w Explain the relationship between Dalton’s Atomic Theory and the laws of conservation of mass and definite composition w Explain the law of multiple proportions.

Early Ideas of the Atom Democritus w Greek Philosopher w 460 -370 B. C. w Stated – Matter could be divided into smaller & smaller particles until it could no longer be divided. w Called these Particles – Atomos (indivisible) w Not based on any physical evidence, only thought

Dalton’s Atomic Theory Late 1700’s – John Dalton – School Teacher(england) Dalton’s Atomic Theory – Summarized 1. All matter is composed of extremely small particles called atoms. 2. Atoms of a given element are identical in size, mass, and other properties. (Isotopes - atoms same element with different mass. ) 3. Atoms cannot be subdivide, created, or destroyed. 4. Atoms of different elements can combine in simple, wholenumber ratios to form chemical compounds. 5. In chemical reactions, atoms are combined, separated, or rearranged.

Today’s Definition of an Atom w Atoms are the smallest particle of matter that retains its unique properties. w We use a scanning tunneling microscope to see the surface of atoms.

Law of Conservation Atoms and the conservation of mass If atoms are indivisible w mass must be conserved A + B AB + 1 a. u. + 3 a. u. 4 a. u. Law of Definite Composition Law of Constant Composition A compound always contains the same elements in the same proportions by mass. The mass ratio of A to B will always be the same. In this case 1: 3 or 25% to 75%

Lavoisier and Proust w Antoine Lavoisier (1743 - w 1794) w w Father of Modern Chemistry w Redefined the compound n 2 or more elements Proust (1754 -1826) Discovered that in the formation of water – the proportions of H and O were constant. n 8: 1 Hydrogen to Oxygen w Performed Experiments w Law of Definite Proportions to test ideas. n Elements combine in w Proved the law of definite mass ratios to conservation form compounds

3. 2 Discovering Atomic Structure w Particle arrangement of the atom. n n n Protons (+) Electrons (-) Neutrons (0) w Michael Faraday n Made the connection between the atom and electricity.

J. J Thomson w Used a Cathode Ray Tube (CRT) n Discovered electrons Charge and mass l Electric current passed through low pressure gas in a glass tube. l Plum-Pudding model or Chocolate Chip Cookie Dough l

J. J Thomson w CRT with an electric field. ®Moved away from neg. charged cathode proving the particles were negatively charged and determined the mass to be 1/1800 of a hydrogen atom.

Robert Millikan w Millikan’s Oil-Drop Experiment Proved the mass and charge of all electrons are identical. n Used an electric plate created a resistant force acting on falling particles n Voltage Controlled n By varying the charge on different drops, he noticed that the charge was always a multiple of -1. 6 x 10 -19 C. n

Ernest Rutherford w Used the Gold Foil Experiment n Marsden and Gieger proved the existence of the nucleus w Bombarded thin sheets of metal with (+) charged particles. w Results of the Particle deflections n Most traveled through n Some Deflected in Both Directions n Very Few Deflected Back

Rutherford Concluded Nuclear Model w Nucleus n n Central part of the atom. Very small and very massive = Very dense. l n All the mass of an atom. Positively charged.

Atoms are Empty Space w C: Documents and SettingsmurbanDesktopspace within atoms

Henry Becquerel w Radioactivity n n n Spontaneous emission of radiation from an element. Caused by unstable atoms, which release subatomic particles. Alpha, Beta and Gamma radiations. Discovered the radioactivity of elements, like uranium, using unexposed photo film. Rutherford’s gold foil exp

Summary of the Atom w Nucleus n Protons l l n Positively charged particle found in the nucleus Gives mass to the atom. Neutrons l l Neutrally charged particle found in the nucleus. Gives mass to the atom and acts as “nuclear glue” w Electrons n Negatively charged particles that give the atom its size.

Atomic Structure w Development of the atomic Model

3. 3 Modern Atomic Theory Properties of Subatomic Particles Particle Relative Mass Symbols Charge Number Relative Mass Actual Mass Electron e- -1 0 0. 00055 u 9. 11 x 10 -28 g Proton p+ +1 1 1. 00728 u 1. 67 x 10 -24 g Neutron n 0 0 1 1. 00866 u 1. 68 x 10 -24 g

Atomic Mass Units (amu) w the equivalent mass of an atom. n n n Protons = 1 amu Neutrons = 1 amu Electrons = 0 amu

Subatomic calculations w Atomic Number (Z) –the number of protons within an atom. (Atomic # = p+) n n n Every element has its own unique atomic number. p+ = e- , in an atom Mass # = p+ + n 0

Isotope Abbreviations w Atomic Symbol Notation: n X = atomic symbol w. Mass Number form: n name of element – mass#

Isotope Form Practice w Write the element uranium with a mass # of 235 in atomic and mass form: n n Mass # = Atomic form =

Subatomic Particle Practice w How many protons, neutrons and electrons do the following contain? Carbon – 13 n. Carbon lp+ = le - = lno = n Gold p+ = e- = n 0 =

Symbolic Mass Form Atomic Mass p+ # # n 0 Zinc-63 74 106 139 56 25 28 e-

Ions w atoms that have lost or gained one or more electrons. n Octet rule – atoms try to maintain a full outer electron shell. l n 8 for most elements, 2 for hydrogen Ions charge indicates the # of electrons gained or lost. (+) charge = loss of electrons l (-) charge = gain of electrons l n Charge of ion = # protons - # electrons

Ion Practice w Write the symbol for the ion with 9 protons and 10 electrons. n F has 9 protons 9 -10 = -1 l F 1 l

Subatomic Particles of Ions • How many protons and electrons are present in the following? Cl 11) Cl 1 - : 2) Na 1+ : 3) Fe 3+ : Na 1+ Fe 3+

Ion Practice w What is the formula for the ion of Nitrogen, which has 10 electrons?

Isotopes w Atoms of the same element with different mass, due to the number of neutrons. n n n The number of protons determines the identity of an atom !!!!!!! The number of neutrons can be different for the same type of element. Hydrogen

Hydrogen Isotope Names w 3 isotopes of hydrogen n Protium, Deuterium, Tritium, Hydrogen – 1 Hydrogen – 2 Hydrogen – 3

Relative Atomic Masses w Although we know actual masses of atoms it is more practical to use their relative masses w The arbitrary point – Carbon-12 w 1 atomic unit (a. u. ) = Exactly 1/12 of C-12 w All other atoms, weighed based upon Carbon-12

Atomic Mass Average mass of all naturally occurring isotopes of an element. w Calculation Atomic mass = (mass of each isotope)(% abundance)/100 l Add the answers to each of the isotopes to get the average mass. l

Modern Atomic Theory w Determine the atomic mass of Oxygen. Isotope Mass (amu) % abundance Oxygen – 16 15. 994915 99. 762 Oxygen – 17 16. 999131 . 038 Oxygen – 18 17. 999160 . 200

Modern Atomic Theory w Determine the atomic mass of Uranium. Isotope Mass (amu) % abundance Uranium-235 235. 043924 . 720 Uranium - 238. 050784 99. 280 Uranium =

3. 4 Changes in the Nucleus w Nuclear Reactions n n Changes that occur within an atom’s nucleus. Change the composition of the atom’s nucleus by altering the number of protons or neutrons. w Strong Nuclear Forces n Hold the nucleus together, neutrons help increase or add to this nuclear force.

Radioactive Decay w Occurs when an atom emits one of the different forms of radiation. w The original nucleus decomposes, ”decays, ” to form a new nucleus. w The new nucleus will continue to decay until it reaches a stable configuration.

Types of Radioactive Decay Alpha Decay l (Not a serious health hazard) Nucleus ejects a helium nucleus, called an alpha particle, and becomes a smaller nucleus with less positive charge. l. What would alpha decay of U-238 look like?

Alpha Decay

3. 4 Beta Decay 100 times more penetrating than alpha l The effect of turning a neutron in the nucleus into a proton, causing a beta particle (electron) to eject from the atom and the formation of a neutrino. Write the equation for the beta decay of sodium – 24.

Beta Decay

3. 4 Gamma Decay High energy non-particle, with no charge or particle mass called a photon. Caused by the nucleus changing to a lower energy level, releasing photons in the form of x-ray or other high end radiation. l Only stopped by thick concrete or lead. l Extremely dangerous. l

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Fusion vs. Fission w Fusion n n Process of combining nuclei of two atoms. Occurs in the stars. w Fission n n Process of splitting the nucleus of an atom. Nuclear reactors or bombs.

Gamma Decay