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Transcript
10/4/2015 Unit 3: Atomic Structure - The Nucleus Lesson 3.2: Defining The Atom 12 Lesson 3.2 Defining the Atom • Define atom. • Distinguish between the subatomic particles in terms of relative charge and mass. atom, including the • Describe the structure of the atom locations of the subatomic particles. model: a visual, verbal, and/or mathematical explanation of data collected from many experiments 13 Lesson 3.2 Defining the Atom •Atom •Cathode ray •Electron •Nucleus •Proton •Neutron An atom is made of a nucleus containing protons and neutrons; electrons move around the nucleus. 14 1 10/4/2015 Sizing up the Atom • Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that element • The smallest particle of an element that retains the properties of the element is called an atom. 15 The Atom • If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long • An instrument called the scanning tunneling microscope (STM) allows individual atoms to be seen. 16 Iron Atoms Seen Through a Scanning Tunneling Microscope 17 2 10/4/2015 • Silicon Atoms : As “felt” by an Atomic Force Microscope 18 Not Really Indivisible Atoms are composed of three major “subatomic” particles (even they are divisible further—but we won’t really deal with that): In the nucleus: Nucleons protons: mass of 1 amu (1.66E-24 g), charge of +1 neutrons: mass of 1 amu, charge of 0 Outside the nucleus: electrons: negligible mass (1/1836 amu), charge of -1 19 Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron • Modern Cathode Ray Tubes Television Computer Monitor 20 3 10/4/2015 Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. 21 The Electron • J.J. Thomson measured the effects of both magnetic and electric fields on the cathode ray to determine the charge-to-mass ratio of a charged particle, then compared it to known values. • The mass of the charged particle was much less than a hydrogen atom, then the lightest known atom. • Thomson received the Nobel Prize in 1906 for identifying the first subatomic particle—the electron 22 Conclusions from the Study of the Electron: a) Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. b) Atoms are neutral neutral, so there must be positive particles in the atom to balance the negative charge of the electrons c) Electrons have so little mass that atoms must contain other particles that account for most of the mass 23 4 10/4/2015 Thomson’s Atomic Model J. J. Thomson Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. 24 Mass of the Electron • In the early 1910s, Robert Millikan used the oil-drop apparatus shown below to determine the charge of an electron. Mass of the electron is 9.11 x 10-28 g http://g.web.umkc.edu/gounevt /Animations/Animations211/Mil likanOilDropExp.swf 25 The Electron (cont.) • Charges change in discrete amounts —1.602 10–19 coulombs, the charge of one electron (now equated to a single unit, 1–). • With the electron’s charge and charge-to-mass ratio known, known Millikan calculated the mass of a single electron. the mass of a hydrogen atom 26 5 10/4/2015 The Nucleus • In 1911, Ernest Rutherford studied how positively charged alpha particles interacted with solid matter. • By aiming the particles at a thin sheet of gold foil foil, Rutherford expected the paths of the alpha particles to be only slightly altered by a collision with an electron. 27 Rutherford’s Gold Foil Experiment • http://www.mhhe.com/physsci/chemistry/ess entialchemistry/flash/ruther14.swf 28 Ernest Rutherford’s Gold Foil Experiment - 1911 Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil Particles that hit on the detecting screen (film) are recorded 29 6 10/4/2015 The Nucleus (cont.) • Rutherford concluded that atoms are mostly empty space. • Almost all of the atom's positive charge and almost all of its mass is contained in a dense and positive region in the center of the atom called the nucleus. • Electrons are held within the atom by their attraction to the positively charged nucleus. 30 The Nucleus (cont.) • The repulsive force between the positively charged nucleus and positive alpha particles caused the deflections. • R Rutherford’s h f d’ model d l of the atom = the nuclear model 31 The Nucleus (cont.) • Rutherford refined the model to include positively charged particles in the nucleus called protons. • James Chadwick received the Nobel Prize in 1935 for discovering the existence of neutrons, neutral particles in the nucleus which accounts for the remainder of an atom’s mass. James Chadwick (1891 ‐ 1974) 32 7 10/4/2015 Protons Discovered by Ernest Rutherford (2nd) Every element has a different number of protons Atomic Number = an element’s element s number of protons The periodic table lists elements by Atomic Number Nuclear charge = # of protons in the nucleus ALWAYS POSITIVE!!! Ernest Rutherford (1871 ‐ 1937) 33 The Nucleus (cont.) • All atoms are made of three fundamental subatomic particles: the electron, the proton, and the neutron. • Atoms are spherically shaped. • Atoms are mostly empty space, and electrons travel around the nucleus held by an attraction to the positively charged nucleus. 34 The Nucleus (cont.) • Scientists have determined that protons and neutrons are composed of subatomic particles called quarks. • Chemical behavior can be explained by considering only an atom's electrons. 35 8