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Linked 1 Linked 2 What is matter ? To describe the Democritus model of the atom Democritus ((& Leucippus)) ~ 400 BC Pg 87-91 • Early philosophers thought that the material world must be made up of tiny indivisible particles called atomos, (meaning indivisible or uncuttable) that move through space – Later Democritus could not answer questions from Aristotle regarding his notion of empty space and the “atomic” view of matter faded away for centuries Scientists studying gases…you can feel the wind, thus air must be composed of invisible particles of air Linked To describe the Dalton model ofPg38-39 the atom John Dalton - Devised an atomic theory based on the Greek idea of “atomos” Atom-smallest particles of an element that retain the chemical identity of the element Dalton’s Atomic Theory 1) Each element is composed of extremely small indivisible particles called atoms 2) All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements. 3) Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions. 4) Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kinds of atoms. Dalton’s Theory Explained several simple laws of chemical combination known at the time • Law of constant composition (definite proportions)– in a compound the relative numbers and kinds of atoms are constant (pure H2O is 11% H and 89% O by mass) • Law of conservation of matter – matter is neither created nor destroyed total massbefore = total massafter Law of Multiple Proportions if 2 elements A&B combine to form more than one compound, different masses of B combine with the same mass of A in the ratio of small whole numbers There is twice as much oxygen by mass in H2O2 as there is in H2O Pg38-39 A problem with Dalton’s Theory Are atoms “really” the smallest particles we know? Are atoms “really” indivisible? What are atoms made up of? Are atoms of the same elements “really” identical? Cathode Rays an Electrons JToJdescribe Thompson the Thomson model of the atom To state relative charge & mass of electron and proton Cathode Rays Pg 92-94 • Partially evacuated tubes produced radiation under high voltage • Called cathode rays = initiated from cathode side • Rays could not be seen but made materials fluoresce Cathode Rays • Experiments showed cathode rays are deflected by electric or magnetic fields Link • Thomson found that cathode rays are the same regardless of what material the cathode is made of • How do these findings help us answer the question…are cathode rays radiation (energy) or particles? Link 1 Electrons • Cathode rays deflected by “field” = must be electrical particle not pure energy • Deflected TOWARD positive field = must be negative particle • Rays were the same no matter what substance cathode was made of = must be a fundamental particle (all the same) “So how do all these particles that make up matter go together? Pg42 JJ Thomson’s “Plum Pudding” – Atomic Model • Small negatively charged electrons embedded in a positively charged atom • Like seeds (negative electrons) embedded in a watermelon (positive atom) Finding the Center “Ernest Rutherford and Gold Foil” To describe the Rutherford model of the atom To state relative charge & mass of electron, proton and neutron Radioactivity & Rutherford Pg41-42 Revealed three types of radiation: alpha (α) beta (β) gamma (γ) – How does each type respond to electric/magnetic field? And what does that imply about it? Rutherford’s Gold Foil Experiment Pg42 • almost all α particles passed directly through the foil without deflection • a small % slightly deflected, on the order of 1 degree, consistent with Thomson’s plum-pudding model Rutherford’s Gold Foil Experiment Pg42 •BUT eventually … a small amount of scattering was observed at large angles & some particles were even scattered back in the direction from which they had come What implications does this finding have about the atom? Linked Rutherford’s Atom Pg42 • most of the mass of each atom and all of its positive charge reside in a very small, extremely dense region • Rutherford called this region the nucleus LiNkeD Protons LiNkeD Pg42-43 Subsequent experimental studies led to the discovery of positive particles (called protons) in the nucleus. • Protons were discovered in 1919 by Rutherford. • HOWEVER, when the nuclear mass was computed based on the number of protons, the mass value was much less than the actual mass… • What does this then imply about the atom??? The Neutron Pg42-43 …and subsequent experimental studies led to neutral particles (called neutrons) in the nucleus. • Neutrons were discovered in 1932 by the British scientist James Chadwick (1891– 1972). “Planetary” Model Pg43 • Every atom has an equal number of electrons and protons, and so atoms have no net electrical charge. • The electrons are attracted to the • The vast majority of an protons in the nucleus atom’s volume is the by the force that exists space in which the between particles of electrons reside. LinKed opposite electrical charge. Rutherford’s Paradox • You cannot simply explain electron as “orbiting” the nucleus…because (as per classical physics) the e- will loose energy and be more attracted toward nucleus Pg 228-229 Properties of Particles Link Pg43-44 • charge of electron is −1.602 × 10−19 C and that of a proton is +1.602 × 10 −19 C (charges of particles are expressed as multiples of this charge) • mass of atoms very small so instead of grams we use amu (“atomic mass units”) (1 amu =1.66054 × 10−24 g ) • Atoms are extremely small (SI unit angstrom; Å is used) (1 Å = 0.0000000001m) Atomic Notation To draw a diagram of an atom given its atomic notation To explain & illustrate the concept of isotopes Pg45 Practice Pg45 How many protons , neutrons and electrons are there in each? p+: n0: e-: p+: n0: e-: p+: n0: e-: Another Problem with Dalton’s Theory Pg45-46 • Most elements are uniform mixtures of 2 or more unique substances called isotopes. • Isotopes of an element have very similar chemical properties but their atoms have slightly different masses What about the different atoms, of the same element, Link 1 could make their masses different??? Link 2 Link 3 Atomic Mass Scale Pg46-48 • Masses of atoms are measured in atomic mass units (amu) • 1 amu = 1.66054x10-24g • 1g = 6.02214x1023amu • Defined by assigning a mass of exactly 12amu to an atom of the 12C isotope of carbon. Average Atomic Weight Pg47-48 • Every sample of an element has the same isotopic composition (the average mass per atom is the same from sample to sample) • So…we can use the masses of each isotope and its relative abundance to determine the average atomic weight for the element! Average Atomic Weight Pg47-48 Naturally occurring chlorine is 75.78% 35Cl which has an atomic calculate the37atomic mass of 34.969 amu,To and 24.22% Cl, which has an atomic mass of weight an element 36.966 amu. Calculate thefor average atomic mass (that is, the atomic weight) of chlorine. given the mass and abundance the naturally occurring isotopes. Solution The average atomic mass is found by multiplying the abundance of each isotope by its atomic mass and summing these products. Because 75.78% = 0.7578 and 24.22% = 0.2422 we have Average atomic mass = (0.7578)(34.969 amu) + (0.2422)(36.966 amu) = 26.50 amu + 8.953 amu = 35.45 amu This answer makes sense: The average atomic mass of Cl is between the masses of the two isotopes and is closer to the value of 35Cl which is the more abundant isotope Practice Example: Copper occurs naturally as two isotopes Cu-63 and Cu-65. Given the atomic mass and the % natural abundance of each isotope below, calculate atomic weight of Cu (as on the periodic table). Isotope Cu-63 Cu-65 mass 62.930 amu 64.928 amu natural abundance 69.09% 30.91% Radioactivity – spontaneous emission of radiation (high energy & particles) Pg41-42 To state the properties of alpha, beta and gamma radiation • In a nuclear reaction, protons and neutrons are rearranged • In a chemical reaction, electrons rearranged Often, at least one isotope is unstable – the strong force of the nucleus is overcome by positive-positive repulsion of the+ protons Radioactive Decay - breakdown of atom – Nucleus becomes unstable – Radiation is emitted until nucleus becomes stable – Stable, non-radioactive atom is formed – Can occur naturally, or be caused by bombarding the atom with energy LinKeD 3 Main Types of Radiation Particles Charge Mass Alpha Stopped By +2 4 Paper or light clothing -1 0 0 1 1 2 p+ 2 no no e1 eno p+ no no Heavy Clothing, Sunscreen or Lead (1cm thick) Gamma None None 0 Concrete or Lead (10cm thick) 4 2 2 Beta e 0 0 Penetrating Ability • HALF-LIFE is the is the time that it takes for 1/2 a sample to decompose To relate the amount of • The rate of a nuclear transformation depends only on radioactive sample, or its the “reactant” (original compound) concentration. radioactivity, to a given half-life. • For each duration (half-life), For each duration (halflife), one half of the substance decomposes For example Ra-234 has a half-life of 3.6 days; if you start with 50 grams of Ra-234, how much do you have … After 3.6 days : After 7.2 days : After 10.8 days :