Nuclear Chemistry

The Birth of an Idea Democritus, 400 B.C. coined the term “atom” If you divide matter into smaller and smaller pieces, eventually you will end up with tiny, indestructible pieces called atoms His ideas were rejected in favor of Aristotle’s Aristotle 380 B.C.- all substances are made up of 4 elements Fire- hot Air- light Earth- cool, heavy Water- wet All substances a blend of these 4 elements

Dalton’s Atomic Theory John Dalton, 1800- English teacher who did science on the side. Recorded his ideas in 3 points Dalton’s Atomic Theory Dalton proposed that all matter is made up of tiny particles, which are molecules or atoms Molecules can be broken down into atoms by chemical processes Atoms cannot be broken down by chemical or physical processes

Dalton’s Atomic Theory Law of Definite Composition: the % by mass of an element is always the same Ex: the mass ratio of carbon to oxygen in Carbon Dioxide (CO2) is always the same… 1 carbon to 2 oxygen atoms Law of Conservation of Mass: In chemical reactions, mass is conserved and is not created nor destroyed Dalton proposed the creation of methane (CH4) by substituting 4 hydrogen atoms for the 2 oxygen atoms in carbon dioxide

Dalton’s Atomic Theory An element is composed of tiny, indestructible, indivisible particles called atoms All atoms of the same element are identical, and have the same properties Atoms of different elements combine to form compounds Compounds contain atoms in small whole number ratios Atoms can combine in more than one ratio to form different compounds, or simply, chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed

How small is an atom? An atom is the smallest particle of an element that retains the properties of that element. They are teeny tiny You would need to line up 100,000,000 Cu atoms to measure 1cm A penny that is made of pure Cu would contain 2.4 x 10^22 Cu atoms Atoms can be seen with a Scanning Tunneling Microscope (STM)

Structure of the Atom J.J. Thomson,1900- an English physicist proved that atoms had pieces called electrons (e-) Famous Cathode Ray Tube experiment Hooked up electrodes to a high-voltage source, created an anode (positively charged) and cathode (negatively charged) A glowing beam flowed from the negative disk to the positive disk, called the cathode ray The glowing beam was made up of negative particles (opposites attract)

Thomson’s Plum Pudding Model Atoms are breakable e- are negative, so Must be a positive charge to balance the e- 2 e- repel each other In the Plum Pudding Model: e- are suspended in a positively charged electric field Lots of empty space separates the e-

After Thomson… Milliken (1900, American scientist) determined that the charge of an e- is -1; the mass is 9.11x10-28g E. Goldstein discovered that a proton has a positive charge and is 1840 times heavier than the e- Proton’s charge is +1; the mass is 1.67x10-24g James Chadwick confirmed that the neutron has no charge, but the same mass as a proton Neutron’s charge is +1; the mass is 1.67x10-24g

Subatomic Particle Review Symbol Relative Charge Relative Mass (mass of P+=1) Actual Mass (g) Electron e- 1- 1/1840 9.11x10-28 Proton p+ 1+ 1 1.67x10-24 Neutron N0

Rutherford and Radioactivity There are 3 types of radiation: Alpha Particles (α) composed of positively charged helium nuclei Beta Particles (β) composed of negatively charged e- Gamma Particles (ϒ) composed of high energy radiation Size: α> β>ϒ

Rutherford’s Gold Foil Experiment Ernest Rutherford- 1910, English physicist who believed in the plum pudding model Used radioactivity and shot the positively charged alpha particles at a gold foil which was a few atoms thick

Rutherford’s Gold Foil Experiment Realized that the atom is largely empty space, so most of the α particles passed through the foil There is a dense, positive area at the center of the atom (named the nucleus) The α particles that deflected and bounced backwards did so after nearing or hitting the nucleus Rutherford predicted that because of the density of the nucleus, it must contain neutral particles in addition to protons 30 years later, Chadwick discovered neutrons

Bohr Model Niels Bohr, 1915- proposed Planetary Model Scientists realized that the attraction between e- and p+ should make the atom unstable Bohr proposed that e- occupy stable, fixed orbits around the nucleus with special quantized locations In the Bohr model, the e- can change orbits when it absorbs or gives off a photon of a specific color of light

Quantum Theory Modern quantum theories lead to stable locations of e-, which are not exact orbits, but are characterized by specific quantum numbers e- may be found in clouds of probability, but the exact location of an e- can’t be determined Every orbital has a different shape, and no 2 e- can be in the same orbital unless they have opposite spins (more on this later)

Review Questions Aristotle Suggested that matter existed through a combination of? Fire, Air, Earth, Water What were the 2 characteristics of the Dalton Model of the atom that were later found to be untrue? Atoms are indestructible/indivisible particles All atoms of the same element are identical What was most characteristic of Thomson’s Model of the atom, that is most unlike what is accepted as true today? Plum pudding model, no nucleus, + charges mixed with e- How did Rutherford’s experiments reveal that an atom consists of a dense, positively-charged nucleus, with a large e- cloud around it? Small ratio of α particles were deflected, and fewer bounced back after striking nucleus

Review Questions Why was the Bohr Model an improvement over the Rutherford Planetary model? Bohr proposed a model in which the e- would occupy stable, fixed orbits How did the Bohr Model explain the emission and absorption of light? e- can change orbits, accompanied by the absorption or emission of a photon of a specific color of light The Quantum Theory Model of the atom also describes Quantum levels, similar to Bohr. How did the new model differ? In quantum theory, the e- shells are not fixed orbits, but clouds of probability