Learning Standards <ul><li>Atomic Structure B road Concept : Atomic models are used to explain atoms and help us understand the interaction of elements and compounds observed on a macroscopic scale. </li></ul>
W HAT IS AN ATO M ? A n atom a fundamental piece of matter. (Matter is anything that can be touched physically.) Everything in the universe (except energy) is made of matter, and, so, everything in the universe is made of atoms. An atom itself is made up of three tiny kinds of particles called subatomic particles: protons, neutrons, and electrons. The protons and the neutrons make up the center of the atom called the nucleus and the electrons fly around above the nucleus in a small cloud. The electrons carry a negative charge and the protons carry a positive charge. In a normal (neutral) atom the number of protons and the number of electrons are equal. Often, but not always, the number of neutrons is the same, too.
SUBPARTICLES IN AN ATOM ELECTRONS PROTONS NEUTRONS
Discovery of Protons <ul><li>Eugene Goldstein noted streams of positively charged particles in cathode rays in 1886 . </li></ul><ul><ul><li>Particles move in opposite direction of cathode rays. </li></ul></ul><ul><ul><li>Called “Canal Rays” because they passed through holes (channels or canals) drilled through the negative electrode . </li></ul></ul>
<ul><li>1.) Canal rays must be positive. </li></ul><ul><ul><li>2.) Goldstein postulated the existence of a positive fundamental particle called the “proton ”. </li></ul></ul>
<ul><li>Passing an electric current makes a beam appear to move from the negative to the positive end. </li></ul>T homson’s Experiment And Discovery of Electrons + - Voltage source
Thomson’s Experiment <ul><li>By adding an electric field he found that the moving pieces were negative. </li></ul>Voltage source + -
<ul><ul><ul><ul><ul><li>The electron was discovered in 1897 by Thomson. He imagined the atom as a “raisin pudding” with electrons stuck in a cake of positive charge. </li></ul></ul></ul></ul></ul>J.J Tomson
J.J. Thomson’s Model of Atom <ul><li>Plum Pudding Model, 1896 </li></ul><ul><li>Thought an atom was like plum pudding </li></ul><ul><ul><li>D ough was cloud </li></ul></ul><ul><ul><li>R aisins were electrons </li></ul></ul><ul><ul><li>D idn’t know about neutrons at this time </li></ul></ul>
Rutherford’s experiment and discovery of nucleus <ul><li>English physicist Ernest Rutherford (1911) </li></ul><ul><li>Shot alpha particles at fluorescent screen. </li></ul><ul><li>When an alpha particle hits a fluorescent screen, it glows. </li></ul>
Lead block Uranium Gold Foil Fluorescent Screen Rutherford's Expriment
What He Expected He Expected T he alpha particles to pass through without changing direction very much.
He thought the mass was evenly distributed in the atom
The Nuclear Atom <ul><li>Since some particles were deflected at large angles, Thomson’s model could not be correct . </li></ul>
<ul><li>Atom is mostly empty space. </li></ul><ul><li>Small dense, positive piece at center is (NUCLEUS) </li></ul>How Rutherford explained results …..
RUTHERFORD’S MODEL OF AN ATOM <ul><li>In 1912, Rutherford discovered the nucleus by doing scattering experiments . He concluded the atom was mostly empty space, with a large dense body at the center, and electrons which orbited the nucleus like planets orbit the Sun. </li></ul>
T he model created by Rutherford had still some serious discordance. According to the classic science, electron moving around the nucleus should emit an electromagnetic wave. Electron should than move not by the circle but helical and finally collide with the nucleus. But atom is stable.
Rutherford also realized that the nucleus must contain both neutral and positively charged particles. The neutron was then discovered in 1932 by Chadwick .
Bohr’s Model of the Atom <ul><li>Similar to Rutherford’s model </li></ul><ul><li>Thought atom was mostly empty space </li></ul><ul><li>Neils Bohr, 1913 </li></ul><ul><ul><li>Nucleus in center is dense, positively charge </li></ul></ul><ul><ul><li>Electrons revolve around the nucleus . </li></ul></ul>
Following Rutherford’s planetary model of the atom, it was realized that the attraction between the electrons and the protons should make the atom unstable Bohr proposed a model in which the electrons would stably occupy fixed orbits, as long as these orbits had special quantized locations
Subatomic Particles Electron Proton Neutron Name Symbol Charge Actual mass (g) e - p + n 0 9.11 x 10 -28 1.67 x 10 -24 1.67 x 10 -24
B o h r ’ s m o d e l <ul><li>Electrons move around the nucleus at stable orbits without emitting radiation. </li></ul><ul><li>Electron in one of these stable orbit has a definite energy. </li></ul><ul><li>Energy is radiated only when electrons make transitions from high energy orbit to a low energy orbit. </li></ul>
In the Bohr model, the electron can change orbits, accompanied by the absorption or emission of a photon of a specific color of light.
Bohr’s Model 1913 – Atomic Model with Fixed Orbits proposed – The Atom has No Charge Number of Protons= Number of Electrons
An easy way to calculate the total number of electrons that can be held by a given energy level is to use the formula 2*n 2 , 98 7 72 6 50 5 32 4 18 3 8 2 2 1 Maximum number of electrons 2*n 2 Principle energy level ( n )
The most electrons possible in the first shell are 2. After the first shell is filled, the second shell starts filling up, according to the number of positive charges in the nucleus. The most allowed in the second shell is 8 electrons. Then the third shell starts to fill.