Unit 2: Atomic Structure and Nuclear Chemistry

Chapter 4 Atomic structure

4.1 Defining the atom

Atom Smallest particle of an element that retains the properties of the element

What discoveries lead to the Atomic Theory? Democritus – Father of the Atom 400 B.C. Atomos: Greek for “indivisible” Philosophy – not Science No measured or experimental support

John Dalton – Father of the Modern Atom 1808 All elements are composed of tiny indivisible particles called atoms. Atoms of the same element are identical. Atoms of different elements are different. Atoms combine in simple whole number ratios. Atoms of one element are never changed to atoms of another element during chemical reactions.

Sizing up the Atom What instruments are used to observe individual atoms? Despite their small size, atoms are observable with scanning electron microscopes. Radii fall between 5 x 10-11 m – 2 x 10-10 m 50 – 200 pm (picometer) Example 1 Example 2 http://www.youtube.com/watch?v=fUAFqkS7y9M&feature=related

4.2 structure of the nuclear atom

Subatomic Particles Electrons Protons Neutrons Nucleus

Electrons J.J. Thomson – 1897 Discovered the electron using a cathode ray tube (CRT) Negatively charged particles

Protons E. Goldstein – 1886 Discovered the proton by careful observations of another ray in the Cathode Ray tube Positively charged particles

Neutrons James Chadwick – 1932 Discovers the neutron Neutrons are the “glue” that holds the nucleus of the atom together.

Subatomic Particles Electrons Protons Neutrons Nucleus Atomic Charges: Neutral atom: Equal # of Protons & Electrons Element dependant on # of Protons Number of Electrons can change – creates charged atoms (called IONs)

“Plum Pudding Model” Electrons evenly distributed in an atom filled with uniform positive chare First Atomic Model Example

The Atomic Nucleus Ernest Rutherford – 1911 Using the “Gold Foil” experiment, Rutherford determines: An atom is mostly space. There is a nucleus in the middle of the atom containing the protons The electrons orbit a large distance away from the nucleus. Proposes a “Solar System” or “Orbital” model of the atom Cute Summary of History

Atomic Particle Summary Name Symbol Charge Mass (amu) Electron e- -1 0.000543 Practically 0 proton p+ +1 1 neutron n

4.3 distinguishing among atoms

Atomic Number Number of protons in the nucleus of an atom Each element has a unique atomic number. The number of electrons usually equals the atomic number in a neutral atom, but doesn’t need to.

Mass Number The total number of protons and neutrons in an atom Atoms of the same element can have different mass numbers Mass number is NOT normally found on the periodic table Shorthand notation Example: Gold

Number of Neutrons Number of Neutrons = Mass # - Atomic #

Sample Problem 4.2 How many protons, electrons, and neutrons are in each atom?

Isotopes Atoms with the same number of protons, but different numbers of neutrons Isotopes of an element have the same atomic number, but different mass numbers Nuclear Symbol or Isotopic symbol: shows number of protons & mass number of an atom

Nuclear or Isotopic Symbols Examples Carbon -12 Carbon -14 Nuclear Symbol: Name - Mass # Isotopic Symbols Mass Number Atomic Number

Atomic Mass: A Weighted Average Atomic Mass Unit: (amu) Unit of mass for atomic nuclei 1 amu = 1/12 the mass of Carbon-12 Atomic Mass: Weighted average mass of all the naturally occurring isotopes in a sample of an element

Sample Problem 4.5 Element X has two naturally occurring isotopes. The isotope with a mass of 10.012 amu has a relative abundance of 19.91%. The isotope with a mass of 11.009 amu has a relative abundance of 80.09%. Calculate the atomic mass of element X.

Chapter 25: Nuclear Chemistry Regular Chemistry Nuclear Chemistry Deals with the arrangement of atoms to form compounds Never about the changing of the nucleus Elements NEVER change to other Elements Deals with the decay of unstable isotopes to form a stable nucleus About the changing of the nucleus Elements CAN change to other Elements

25.1 Nuclear Radiation Radiation Radioactive decay: Radioisotope: The penetrating light rays and/or particles emitted by a radioactive source Radioactive decay: The spontaneous emission of radiation by an unstable nucleus Radioisotope: An isotope that has an unstable nucleus and undergoes radioactive decay

High-Energy Radiation Types of Radiation Property Alpha Beta Gamma Composition Alpha Particle (helium nucleus) Beta Particle (electron) High-Energy Radiation Symbol α β γ Charge 2+ 1- Mass (amu) 4 1/1840 Penetrating Power Low Tissue Paper (0.05 mm) Moderate Skin or Foil (4 mm) Very high Lead/Concrete Entire body

Radiation Examples Alpha Radiation Beta Radiation Gamma Radiation Uranium-238 to Thorium-234 Beta Radiation Carbon-14 to Nitrogen-14 Summary: What has really happened? Gamma Radiation Thorium-230 to Radium-226 Thorium-234 to Protactinium-234

25.2 Nuclear Transformations Why does the nucleus stay together? Strong Nuclear Force Short range, attractive force that acts among nuclear particles Neutrons and Protons attract one another! Much stronger than electrical or gravitational force Electromagnetic Force Repulsive force between protons. Tries to pull the nucleus apart. Stable Nucleus Correct balance of Strong Nuclear & Electromagnetic Forces Correct p+ to n ratio MOST ATOMS ARE STABLE!!

Band of Stability 1:1 Ratio Line Band of Stability

Most common transformation particles Radiation Name Symbol & Iso. Notation Charge Mass Alpha particle α, +2 ~4 amu Beta particle β -1 1/1840amu (nothing) Gamma radiation γ “none” Positron β+ +1 Neutron n ~1 amu Proton p+

Nuclear Stabilization Equations Alpha emission: Uranium-238 Beta emission: Carbon-14 Positron emission: Oxygen-15 Neutron emission: Silver-107

Additional Example Beta Absorption of Krypton - 81

Transformation Examples Silicon-27  ______ + β Bismuth-214  _____ + α Aluminum-27 + α  Silicon-30 + ____

25.3 Fission and Fusion Fission: Fusion: Large nucleus breaking down into smaller nuclei Fusion: Smaller nuclei combining to larger nucleus

Youtube: Nuclear Reaction Overview

10/18 Warm-up Watch the following youtube video on carbon dating. Carbon dating video Watch the following youtube videos on the strong and weak nuclear force. Video 1 Video 2