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Chapter 3: Atoms – The Building Blocks of Matter Section 2: The Structure of the Atom Lesson Starter • Even though the two shapes look different, the characteristics of the various parts that compose them are the same. • The same is true with the atom. • Though atoms of different elements display different properties, isolated subatomic particles have the same properties. Objectives • Summarize the observed properties of cathode rays that led to the discovery of the electron. • Summarize the experiment carried out by Rutherford and his co-workers the led to the discovery of the nucleus. • List the properties of protons, neutrons, and electrons. • Define atom. The Structure of the Atom • ATOM – the smallest particle of an element that retains the chemical properties of that element. • NUCLEUS – small region located near the center of the atom • PROTON – positively charged particle found in the nucleus The Structure of the Atom • NEUTRON – electrically neutral particle found in the nucleus • ELECTRON – negatively charged particle surrounding the nucleus • SUBATOMIC PARTICLES – protons, neutrons, and electrons Cathode Rays and Electrons • JJ THOMSON -- Experiments in the late 1800s showed that cathode rays were composed of negatively charged particles. • When a CURRENT was passed through a cathode ray tube, the cathode glowed • caused by a stream of particles called a CATHODE RAY • traveled from the CATHODE to the ANODE Cathode Rays and Electrons Cathode Rays and Electrons • An object placed between the cathode and the opposite end of the tube cast a SHADOW on the glass. Cathode Rays and Electrons • A PADDLE WHEEL placed on rails between the electrodes, rolled along the rails from the CATHODE toward the ANODE. • CRT Paddle Wheel Video Cathode Rays and Electrons • Cathode rays were deflected by a MAGNETIC FIELD in the same manner as a wire carrying ELECTRIC CURRENT (which has a NEGATIVE CHARGE) Cathode Rays and Electrons • The rays were deflected AWAY FROM a NEGATIVELY charged object. • These particles were named ELECTRONS. Charge and Mass of the Electron • Millikan’s experiment measured the CHARGE-TOMASS RATIO of an electron. • Robert A. Millikan’s oil drop experiment measured the CHARGE of an ELECTRON. • the mass of an electron is ABOUT 1/2000 the mass of the simplest hydrogen atom • electrons carry a NEGATIVE charge • electrons are present in ATOMS OF ALL ELEMENTS Charge and Mass of the Electron • inferences were made about atomic structure • Because atoms are electrically neutral, they must contain a POSITIVE CHARGE to BALANCE the negative electrons • Because electrons have so much less mass than atoms, atoms must contain other particles that ACCOUNT FOR MOST OF THEIR MASS. Charge and Mass of the Electron • Oil Drop Experiment Video Discovery of the Atomic Nucleus • Gold Foil Experiment Video • GOLD FOIL EXPERIMENT – 1911 – ERNEST RUTHERFORD • a thin piece of gold foil was bombarded with ALPHA PARTICLES • most of the particles were expected to GO STRAIGHT THROUGH the foil and they did Discovery of the Atomic Nucleus • About 1 in 8000 particles that struck the foil were deflected back towards the source • conclusions were drawn about the structure of an atom • particles must have been deflected by a DENSELY PACKED BUNDLE of POSITIVELY charged matter Discovery of the Atomic Nucleus • the VOLUME of a nucleus is SMALL compared to that of the atom itself • like a MARBLE in a FOOTBALL FIELD • Rutherford suggested that the ELECTRONS SURROUND THE NUCLEUS like planets around the sun Composition of the Nucleus • Except for the nucleus of the simplest type of hydrogen atom, ALL ATOMIC NUCLEI are made of PROTONS and NEUTRONS. • A PROTON has a POSITIVE CHARGE EQUAL in magnitude to the NEGATIVE CHARGE of an ELECTRON. Composition of the Nucleus • Atoms are ELECTRICALLY NEUTRAL because they contain EQUAL NUMBERS of protons and electrons. • A neutron is electrically NEUTRAL. Composition of the Nucleus • The nuclei of atoms of DIFFERENT ELEMENTS differ in their number of PROTONS and therefore in the amount of POSITIVE CHARGE they possess. • Thus, the NUMBER OF PROTONS determines that atom’s IDENTITY. Forces in the Nucleus • When TWO PROTONS are EXTREMELY CLOSE to each other, there is a STRONG ATTRACTION between them. • A similar attraction exists when NEUTRONS are very close to each other or when PROTONS AND NEUTRONS are very close together. • The SHORT-RANGE proton-neutron, proton-proton, and neutron-neutron FORCES that hold the nuclear particles together are referred to as NUCLEAR FORCES. The Sizes of Atoms • The RADIUS of an atom is the distance from the CENTER OF THE NUCLEUS to the outer portion of its ELECTRON CLOUD. • Because atomic radii are so small, they are expressed using a unit that is more convenient for the sizes of atoms. • The unit is the PICOMETER, pm.