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The structure of the Atom Chemistry chapter 4 What makes up Matter? In ancient times, people sought to organize the world around the fundamental elements, such as earth, air, fire, and water Democritus (460–370 B.C.) • The first person to propose the idea that matter was not infinitely divisible, but made up of individual particles called atomos. Aristotle (484–322 B.C.) Disagreed with Democritus because he did not believe empty space could exist. Aristotle’s views went unchallenged for 2,000 years until science developed methods to test the validity of his ideas. Alchemy (next 2000 years) • Mixture of science and mysticism. • Lab procedures were developed, but alchemists did not perform controlled experiments like true scientists. Daltons Atomic Theory 1808 All matter is composed of extremely small particles called atoms Atoms cannot be subdivided, created, or destroyed Atoms of a given element are identical in size, mass, and other properties; Atoms of different elements differ in size, mass, and other properties Daltons Atomic Theory 1808 In chemical reactions, atoms are combined, separated, or rearranged Compounds Atoms of different elements combine in simple whole-number ratios to form chemical compounds Compounds are formed when atoms of elements combine Molecules are composed of atoms in definite proportions Law of Constant Composition Billiard Ball Model atom is a uniform, solid sphere Discovery of the Electron Sir William Crookes (1879) Used evacuated tubes containing gas at low pressure Gasses encounter electricity in a sealed environment • When an electric charge is applied, a ray of radiation travels from the cathode to the anode, called a cathode ray • Cathode rays are a stream of particles carrying a negative charge Discovery of the Electron Crookes discovered cathode rays had the following properties: Travel in straight lines from the cathode Cause glass to fluoresce Are deflected by electric fields and magnets to suggest a negative charge Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube (CRT) to deduce the presence of a negatively charged particle. Used a fluorescent screen in CRT to measure deflection of beam Found that all particles in the beam had same charge and mass Proved that the beam, using magnets, was negatively charged particles called Electrons Problem with Negative model? J.J. Thomson realized problem Using CRT again with hydrogen in tube Found 2nd beam of particles traveling toward cathode. Theorized beam was made of Protons Modified Daltons atom model Plum-pudding model Atom is sphere with small electrons embedded in a positively charged mass Now model can be neutral, negatively or positively charged atom An ion is an atom that has either lost or gained electrons Cation – Positive ion Anion – Negative ion Charge and Mass of Atom Millikan (1909 - 1920) Conducted his “oil drop” experiment which allowed him to measure the charge on an electron Difference in mass between Proton and Electron? Proton approximately 2000x more than electron Why important? 1. Mass of proton is much much greater than an electron 2. The mass of an atom is know, If the proton accounts for ½ the mass, what is the remaining “stuff”?? Rutherford Credited with discovering that most of the atoms is made up of “empty space”. In 1909 he conducted the “gold foil” experiment. Gold Foil thickness: 0.00006 cm • By aiming the particles at a thin sheet of gold foil, Rutherford expected the paths of the alpha particles to be only slightly altered by a collision with an electron. Rutherford Gold Foil Experiment http://micro.magnet.fsu.edu/electromag/java/rutherford/ Rutherford He established that the nucleus was: Very dense Very small and positively charged Also assumed that the electrons were located outside the nucleus Rutherford Proposed Planetary Model or Nuclear Model Predicted the existence of the neutron in 1920 James Chadwick discovered the neutron in 1930’s. He found these uncharged particles with essentially the same mass of the proton Rutherford-Bohr Model Niels Bohr (1922) Proposed improvements to Rutherford Atomic Model. For this reason the planetary model of the atoms is sometimes called the Rutherford-Bohr model Bohr added the idea of fixed orbits, or energy levels for the electrons traveling around the nucleus His atomic model has atoms built up of successive orbital shells of electrons Bohr Model or Orbital Model Electron-Cloud Model The charge-cloud model, also called the quantum-mechanical, does not attempt to describe the path of each electron in a fixed point. Computers can calculate the points in space that an electron has the highest probability of occupying. • Scientists have determined that protons and neutrons are composed of subatomic particles called quarks. Chemistry Vocabulary Atoms – Basic building block of matter smallest unit of an element that retains properties of that element Atom consists of 3 particles Protons: positive charge (+) Neutrons: neutral charge Electrons: negative charge (-) Atom divided into 2 parts 1) Nucleus – center of atom, contains protons and neutrons nucleus has positive charge 2) Electron cloud – region around nucleus that contains electrons probable location where electrons will be located this outer region is much larger than nucleus diameter of nucleus = 1/100,000 of electron cloud Electron cloud has several layers – like an onion Electrons near the nucleus have low energy Electrons further away have higher energy Chemistry Vocabulary Symbol Each element is identified by a symbol and Atomic Number Atomic number Atomic number is unique for each element # of protons in nucleus = atomic number number of protons for an element NEVER changes # of electrons = atomic number As long as atom is neutral Each element has a mass number sum of the number of protons and neutrons in an atom Periodic table lists the Average mass number Mass number = # protons + # neutrons 8 O Oxygen 15.99 Name Chemistry Vocabulary Examples of number of protons, electrons: Oxygen 8 protons, 8 electrons Gold 79 protons, 79 electrons Difference between neutral atoms and ions: Neutral Atoms Same number of electrons & protons This what you read off the periodic table Ions Atoms that have either lost or gained electrons Positive ions are cations Negative ions are anions Ca Ca O +2 -2 Chemistry Vocabulary Two atoms of the same element can have differing numbers of neutrons. These are called isotopes Different atomic masses Hydrogen has three isotopes: Hydrogen with 1 proton and 0 neutrons Deuterium with 1 proton and 1 neutron Tritium with 1 proton and 2 neutrons Remember!! Periodic table lists the Average mass number Chemistry Vocabulary Isotopes written 2 different ways 1. Chlorine – 35 means it has mass of 35 How many neutrons does it have? From periodic table atomic number = 17 protons # neutrons = 35 (mass #) – 17 (atomic #) # neutrons = 18 2. 35 17 Cl EXAMPLE: How many protons, neutrons and electrons are found in an atom of 133 55 Cs Atomic number = protons and electrons There are 55 protons and 55 electrons Mass number = sum of protons and neutrons 133 – 55 = 78 There are 78 neutrons Radioactive decay Radionuclides undergo radioactive decay, meaning they eject small nuclear fragments and sometimes high energy electromagnetic radiation as well This is called Radioactivity About 50 of the approximately 350 naturally occurring isotopes are radioactive Few exist in nature—most have already decayed to stable forms Naturally occurring radiation consists principally of alpha, beta, and gamma radiation The goal for radioactive decay is for unstable nuclides to give off radiation to become more stable Nuclear reactions can change one element into another element Alpha Radiation • Alpha Radiation is made up of positively charged particles called alpha particles Each alpha particle contains two protons and two neutrons and has a 2+ charge. 4 Rapidly moving helium ions (no electrons) 2 He α alpha particle radioactive isotope neutron proton Alpha Radioactive Decay (cont.) • The figure shown below is a nuclear equation showing the radioactive decay of radium-226 to radon-222. • The mass is conserved in nuclear equations. Beta Decay • Beta radiation is radiation that has a negative charge and emits beta particles • Each beta particle is an electron with a 1– charge. 0 1 e Gamma Radiation • Gamma Rays are high-energy radiation with no mass and are neutral. γ Pure energy; called a ray rather than a particle Form of electromagnetic radiation Gamma rays account for most of the energy lost during radioactive decay. 241 96 Cm 237 94 Pu He 4 2 Review Conclusions from the Study of the Electron Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons Electrons have so little mass that atoms must contain other particles that account for most of the mass Modern Atomic Theory Several changes have been made to Dalton’s theory. Dalton said: Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties Modern theory states: Atoms of an element have a characteristic average mass which is unique to that element. Dalton said: Atoms cannot be subdivided, created, or destroyed Modern theory states: Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions! One atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom •One amu is nearly, but not exactly, equal to one proton and one neutron •Carbon 12 is assigned an atomic mass of 12.00 g •12.00 is one atomic mass unit The number of protons and neutrons in an atom is its mass number. • Atomic numbers are whole numbers • Mass numbers are whole numbers • The atomic mass is not a whole number • The atomic mass of an element is the weighted average mass of the isotopes of that element.