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A TOMIC T HEORY The history and structure of the atom
A TOMS Atoms are the fundamental building block of ordinary matter. They are smallest identifiable unit of an element. They are so small you can’t even see them with a microscope.
T HE G REEKS Democritus Proposed elements consist of tiny solid particles that cannot be subdivided.
T HE H ISTORY OF THE A TOM Dalton’s Atomic Theory (1808) Each element is composed of tiny, indestructible particles called atoms. All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements. Atoms combine in simple, whole-number ratios for form compounds. Atoms of one element cannot change into atoms of another element. In a chemical reaction, atoms change the way that they are bound together with other atoms to form a new substance.
T HE H ISTORY OF THE A TOM Scientists set out to prove Dalton’s theory Late 1800’s J.J. Thompson’s Cathode Ray Experiment discovered the electron. The cathode ray tube is a tube with partially vacuumed air. In the tube there are two plates one with a positive charge the other with a negative charge When high voltage wires are attached an electrical charge is produced. Rays are produced and can be seen when they hit a fluorescent material at the end of the tube.
What Thompson found The streams of light traveled in straight lines They were independent of the cathode plate material used They carried a negative electrical charge. These negatively charged particles he called Electrons. The mass of these particles was much less than that of Hydrogen the lightest element. This meant that there were smaller particles than the atom.
MovieMovie Click on movie link for cathode ray tube.
T HE H ISTORY OF THE A TOM Millikan’s Oil Drop Experiment (1909) Found the Charge of the electron Fine droplets of oil were sprayed into a drum with two electrical plates The drops were allowed to fall through a small hole into the lower portion of the drum The air was then bombarded with energy producing electrons The negatively charged drops interacted with the negatively charged plate at the bottom of the drum As the charge on the plate was varied the drops slowed, stopped, or reversed. By measuring the voltage on the plate when the drops were stopped the charge of each electron would be found.
Rutherford’s Gold Foil Experiment (1909) Rutherford shot alpha particles at a sheet of gold foil thinking that the particles would just past through. He found that some of the particles were deflected. This lead him to propose that the majority of the mass of the atom and the positive charge of the atom were all in the center of the atom NOT spread throughout. There were large amounts of empty space in the atom.
T HE H ISTORY OF THE A TOM Nuclear Theory of the Atom Most of the atom’s mass and all of its positive charge are contained in a small core called the nucleus. Most of the volume of the atom is empty space, throughout which tiny, negatively charged electrons are dispersed. There are as many negatively charged electrons outside the nucleus as there are positively charged particles (named protons) within the nucleus, so that the atom is electrically neutral.
T HE H ISTORY OF THE A TOM There was still unaccounted mass from the atom presented in Rutherford’s findings. This mass was explained by James Chadwick as neutrons. Neutrons are electrically neutral particles that also reside in the nucleus.
B EYOND THE S UBATOMIC P ARTICLES Scientists next question was what makes up protons, neutrons, and electrons? The answer- Quarks! Found by smashing protons together to break them apart. Using a TEVATRON Six different types.
A TOMIC NUMBER The number of Protons in an element. Find which number changes by one as you move left to right on the periodic table. Do any elements have the same atomic number? If the atomic number ( # of protons )changes then element changes.
Therefore the atomic mass will be different but the atomic number will be the same ( same element)
B UILD AN ATOM P HET SIMULATION Go to link under chapter 4 in Phy Sci resources
E LECTRON C LOUD M ODEL Electrons move in a less predictable way Electron cloud is area around nucleus where electrons are moving Atomic Orbitals – space where an electron is most likely to be found
4.3 M ODERN A TOMIC T HEORY Bohr’s Model of the Atom Better description of electrons Electrons orbit around nucleus in energy levels like planets 1 st Level = holds up to 2 electrons 2 nd Level = holds up to 8 electrons 3 rd level = holds up to 18 electrons Electrons can move to different energy levels if atom gains or loses energy
H OW TO DRAW ELECTRONS IN THE FIRST THREE ENERGY LEVELS 18 total we are not counting 1,2nd 8 total in 2nd 2 total
E LECTRONS CAN MOVE Electrons can move when they gain or lose energy. Therefore we use the electron cloud model to show where electrons maybe located.
E LECTRON C LOUD M ODEL Electrons move in a less predictable way Electron cloud is area around nucleus where electrons are moving Atomic Orbitals – space where an electron is most likely to be found Practice sheet for filling orbits