2Sizing up the AtomElements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that elementIf you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm longDespite their small size, individual atoms are observable with instruments such as scanning tunneling (electron) microscopes
3An STM image of nickel atoms placed on a copper surface. Source: IBM Research
5Image of a ring of cobalt atoms placed on a copper surface. Source: IBM Research
6Atom- smallest particle making up elementsOne teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water!
7Think about the technological advances of the past 100 years! They have been nothingshort of miraculous!Radios CalculatorsTelevisions ComputersAutomobiles Cell phonesJet airplanes IpodsPlastic VelcroRefrigerators Internet (thanks, Al Gore)Penicillin CD’s & DVD’sInsulin and, of course -Electric guitars Sliced Bread!
8Development of Atomic Theory This explosion of technology occurredonce we had a better understandingof the atom and how it behaves!
9Where did it all begin?The word “atom” comes from the Greek word “atomos” which means indivisible.The idea that all matter is made up of atoms was first proposed by the Greek philosopher Democritus in the 5th century B.C.
10Then came the idea of “The 4 Basic Elements” Earth, Air, Fire, & Water After that came Alchemy.The change to “real” Chemistry didn’t occur until the first true element was discovered! (1774)The first element discovered was
11Karl Scheele (1771) (German) first to prepare and describe oxygen The discovery of oxygen is attributed to 3 scientists (working independently)Karl Scheele (1771) (German)first to prepare and describe oxygenJoseph Priestley (1774) (British)isolated oxygen gas from mercuric oxide.observed accelerated burningAntoine Lavoisier (1784) (French)made accurate measurements and interpreted Priestley’s resultsDiscovery of Oxygen gas is attributed to three personsKarl Scheele who first prepare and discribed oxygenJoseph Priestly isolated oxygen gas from HgO. Collected the gas and observed that accelerated burning and increased activity of mice.Connected oxygen with combustion and plant and animal metabolismAlso prepared many gaseshydrogen chloride, ammonia, sulfur dioxideFounder of the ACSAntoine Lavoiser who made accurate measurements and interpreted Priestly’s results.He determined that mass of oxygen release from HgO is equal to mass loss from HgO.And correctly calculated the change in mass of oxygen durning a combustion reactions.(1784 wrote the Law of Conservation of Mass)
12Carl Wilhelm Scheele beat Priestley to the discovery but published afterwards. Too bad! – So sad!
13Priestley Medal Priestley gets the main credit for discovering oxygen! Source: Roald Hoffman, Cornell University
142HgO(s) → 2Hg(l) + O2 (g)Priestley produced a gas (oxygen) by using sunlight to heat mercuric oxide kept in a closed container. The oxygen forced some of the mercury out of the jar as it was produced, increasing the volume about five times.
16Priestley: Additional Scientific Contributions Discovered the interconnection between photosynthesis and respirationDiscovered carbonated waterDiscovered that India rubber removed graphite pencil marks - the first rubber eraserNow we can make mistakes!!
17Lavoisier: the Founder of Modern Chemistry Lavoisier continued the investigations of PriestlyQuantitative experiments led to:Law of Conservation of Matter.He systematized the language of chemistry, its nomenclature and rhetoric.He was beheaded during the Reign of Terror for his role as a tax “farmer” prior to the Revolution (Priestley escaped to America!)Antoine-Laurent Lavoisier
182Hg(l) + O2 (g) → 2HgO(s)Lavoisier heated a measured amount of mercury to form the red mercuric oxide. He measured the amount of oxygen removed from the jar and the amount of red oxide formed. When the reaction was reversed, he found the original amounts of mercury and oxygen.
19Properties of Oxygen Colorless P Odorless P P P P C C Tasteless Gas at room temperatureSlightly soluble in waterInflammable (does NOT burn)Only part of air that supports combustionPP P P P C CPhysical Property or Chemical Property?
20These properties of oxygen were later used to determine the properties of othersubstances.By the late 18th century, scientists finally cameto the conclusion that Oxygen was truly anelement (can’t be broken down into simplerforms without losing its properties)Scientists began to search for & test othernew elements.
21Sometimes, when they tried to react substances together, nothing happened!Substances that DO NOT react are InertThey found that most materials will react to form new substances. These elements are said to be chemically active (reactive)Oxygen is very reactive, so is hydrogen which we will look at next!OxygenhydrogeninertIncreasing chemical reactivity
22Discovery of Henry Cavendish (1766) Reacted various metals with acids producing a salt and hydrogen gasAcid + metal → hydrogen gas + saltZinc + sulfuric acid → Hydrogen + zinc sulfateZn(s) + H2SO4(aq) → H2 (g) + ZnSO4 (aq)While testing the properties of Hydrogen hefound that water is a compound(1731 – 1810)WordEquationChemicalequation
24Antoine LavoisierNamed Priestly’s newly discovered gas - “oxygen” - meaning “acid former”Named Cavendish’s new gas “hydrogen” -meaning “water former”
25Dalton’s Atomic Theory John Dalton ( )While his theory was not completely correct, it revolutionized how chemists looked at matter and brought about chemistry as we know it today (instead of alchemy)So, it’s an important landmark inthe history of science.
26Dalton’s Modern Atomic Theory (experiment based!) All elements are composed of tiny indivisible particles called atomsAtoms of the same element are identical. Atoms of any one element are different from those of any other element.Atoms of different elements combine in simple whole-number ratios to form chemical compoundsIn chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.
27Law of Definite Proportions Each compound has a specific ratio of elements by mass.Ex: Water is always 8 grams of oxygen for each gram of hydrogen.
28Discovery of the Electron Began with the invention of the Crooke’s Tube(cathode ray tube) c. 1875
29- + Cathode Ray Tube gas Voltage source Electric current sent through gases sealed in tube at low pressureAnode- positive electrodeCathode- negative electrodeMetal Disks - electrodes
30Modern Cathode Ray Tubes TelevisionComputer MonitorCathode ray tubes pass electricity through a gas that is contained at a very low pressure.
31In 1897, J.J. Thomson used acathode ray tube to study gases.
32- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end – so the ‘beam’ was called a “Cathode Ray”
33Thomson’s Experiment Voltage source - Thomson found that cathode rays were deflected from a negatively-charged plate.
34Thomson’s Experiment Voltage source + Does light bend like this? and that cathode rays were attracted to plates with a positive chargeDoes light bend like this?
35Light doesn’t ‘bend’ so the cathode ray must be made of particles rather thanLight!Since they are attracted to a positiveplate & repelled by a negative one theparticles aren’t neutral –What charge must they have?That’s right! NEGATIVE!!Thomson called these negative particles –ELECTRONS
36Mass of the ElectronMass of the electron is9.11 x gThe oil drop apparatus1916 – Robert Millikan determined the mass of the electron: 1/1840 the mass of a hydrogen atom; and, has one unit of negative charge
38Conclusions from the Study of the Electron: Cathode rays have identical properties regardless of the element used to produce them. Therefore, all elements must contain identically charged electrons.Atoms are neutral, so there must be a positive substance in the atom to balance the negative charge of the electronsElectrons have so little mass that atoms must contain other particles that account for most of their mass
39Thomson’s Atomic Model J. J. ThomsonThomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.
40Plum-Pudding ModelZumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56
41Henri Becquerel discovered radioactivity In 1903, An important discovery leading to further understandings of atomic structure happened by accident.Henri Becquerel discovered radioactivityRadioactivity is the spontaneous emissionof energy from an object1903: Shared a Nobel Prize with Pierre andMarie Curie for discovering radioactivity.
42The Nobel Prize in Chemistry 1908 Ernest Rutherford ( )The Nobel Prize in Chemistry 1908Studied under J. J. Thomson
433 Types of Radiation discovered by Ernest Rutherford Alpha (ά) – a positively charged helium nucleus 42 He+2Beta (β) – fast-moving electrons eGamma (γ) – like high-energyx-rays
44Ernest Rutherford’s Gold Foil Experiment - 1911 Shot alpha particles at a thin sheet of gold foilParticles that hit on a detecting screen (film) were recorded
46He Expected:The alpha particles to pass through the foil without changing direction very much.Because…The positive charges were spread out evenly (according to Thomson’s atomic theory). Alone they were not enough to stop the alpha particles.
50Rutherford’s Observations “Like howitzer shells bouncing off of tissue paper!”Most of the particles went straight through the foil (what he expected)A few particles were slightly deflectedStill fewer actually bounced back towards the source!Astonishing!!!Rutherford said it was like firing a Howitzer shell at a piece of tissue paper & having it bounce back & hit you!
52Rutherford’s Conclusions +Since most of the particles went through the foil - atoms are mostly empty space.Because a few + particles were deflected they must have come close to a positively charged core.Since a very few particles were deflected straight back, the positively-charged core must be very dense.This small dense positive area is the nucleus.
53The Rutherford Atomic Model Based on his experimental evidence:The atom is mostly empty spaceAll the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus”The electrons are distributed around the nucleus, and occupy most of the volumeHis model was called a “nuclear model”
54Discovery of ProtonsEugen Goldstein in 1886 observed particles with a positive charge passing through a perforated cathode.
55In 1920, Rutherford studied these particles & called them protons.They have a charge of positive 1and a mass of 1.7 x grams.This is not a ‘handy’ number to workwith so we use a mass of 1 amu.Amu stands for “atomic mass unit”
56Discovery of the Neutron Rutherford predicted the existence of the neutron in 1920.Twelve years later, his assistant found it!1932 – James Chadwick confirmedthe existence of the “neutron”– a particle with no charge,but a mass nearly equal to a proton(1 amu).So now we have a more complete picture of an atom!
57Subatomic Particles Particle Charge Mass (g) Location Electron (e-) -1 9.11 x 10-28g(virtually 0)outside nucleusProton (p+)(H+)+11 amu(1.7 x 10-24g)in nucleusNeutron(no)(1.67 x 10-24g)
58Elements are the new building blocks Nitrogen-7HydrogenCarbon-6Oxygen-8
59Henry Moseley(1887 – 1915)Between 1912 and 1914, the physicist H.G.J. Moseleyconducted a series of experiments where he bombarded targetsmade out of different kinds ofmetals with cathode rays. Eachmetal he studied emitted X-raysof a characteristic frequency,almost like a set of "fingerprints".
60The pattern that emerged when the observed X-rays were organized inorder of increasing frequencysuggested to Moseley a regularincrease in the positive charge onthe nuclei of the atoms.He called this positive nuclear charge-the Atomic Number of the element
61Since all atoms are neutral - the # protons in an atom = # electrons Atomic NumberHenry Moseley – used x-ray spectra& came up with the idea of theAtomic NumberElements are different because they contain different numbers of PROTONSThe “atomic number” of an element is the number of protons in the nucleusSince all atoms are neutral - the# protons in an atom = # electrons
62Atomic Number, ZAll atoms of the same element have the same number of protons in the nucleus, Z13Atomic numberAlAtom symbol26.981AVERAGE Atomic Mass
63Mass # = # protons + # neutrons Mass NumberMass number is the number of protonsand neutrons in the nucleus of anisotope:Mass # = # protons + # neutrons
64Subatomic Particles equal in a neutral atom Atomic Number NUCLEUSELECTRONSequal in a neutral atomPROTONSNEUTRONSNEGATIVE CHARGEAtomic Numberequals the # of...POSITIVE CHARGENEUTRAL CHARGEMost of the atom’s mass.
65IsotopesFrederick Soddy ( ) proposed the idea of isotopes in (worked with Rutherford)Isotopes are atoms of the same element having different mass numbers, due to varying numbers of neutrons.Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.
66IsotopesAtoms of the same element (same Z) but different mass number (A).Boron-10 (B-10) has 5 p and 5 n Boron-11 (B-11) has 5 p and 6 n10B11B
68IsotopesRadioisotopes (radioactive isotopes) - unstable isotopes that spontaneously decay emitting radiationThey play an important part in the technologies that provide us with food, water and good health.Radio-carbon dating of fossilsIn medicine, diagnosis, treatment, and researchSterilization of meat Disinfestation of grain and spices Increasing shelf life (eg, fruits)
70Nuclear SymbolsContain the symbol of the element, the mass number and the atomic number (represent isotopes of elements)MassnumberXSuperscript →ElementsymbolAtomicnumberSubscript →REMEMBER! number of electrons = number of protonsSo all atoms are neutral!
78Learning Check – Counting Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.12C C 14C#p+ _______ _______ _______#no _______ _______ _______#e- _______ _______ _______
80Learning Check An atom has 14 protons and 20 neutrons. A. Its atomic number is1) ) ) 34B. Its mass number isC. The element is1) Si 2) Ca 3) SeD. Another isotope of this element is1) 34X 2) 34X 3) 36X
81Atomic Mass How heavy is an atom of oxygen? It depends, because there are different kinds of oxygen atoms.We are more concerned with the average atomic mass.This is based on the abundance (percentage) of each variety (isotope) of that element in nature.We don’t use grams for this mass because the numbers would be too small –
82Measuring Atomic MassInstead of grams, the unit we use is the Atomic Mass Unit (amu)It is defined as one-twelfth the mass of a carbon-12 atom.Carbon-12 chosen because of its isotope purity.Each isotope has its own mass number, so we determine the average atomic mass from the element’s percent abundance.
83To calculate the average atomic mass: Multiply the mass of each isotope by it’s abundance, then add the results.Abundance may be expressed as a decimal or a %, (Divide by 100 if using %’s)Avg.AtomicMass
84Composition of the nucleus Atomic Mass is the weighted average of all the naturally occurring isotopes of an element. on the Periodic TableIsotopeSymbolComposition of the nucleus% in natureCarbon-12C-126 protons6 neutrons98.89%Carbon-13C-137 neutrons1.11%Carbon-14C-148 neutrons<0.01%(98.89 x 12) + (1.11 x 13) + (0.01 x 14)100=
85D. Average Atomic MassEX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O.Avg.AtomicMass16.00amu
86Sub-atomic Particles - Summary Protons p+ - positive charge, in nucleus , mass of 1 amuNeutrons n0 – no charge, in nucleus, mass of 1 amuElectrons - e- negativecharge, orbiting nucleus, “no mass”