Slide 1

ATOMIC STRUCTURECh. 4

Sizing up the Atom Elements 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) microscopesAn STM image of nickel atoms placed on a copper surface.Source: IBM Research

Red ridge is a series of Cesium atomsImage of a ring of cobalt atoms placed on a copper surface.Source: IBM Research

Atom

- smallest particle making up elements One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water!Think about the technological advances of the past 100 years! They have been nothingshort of miraculous!RadiosCalculatorsTelevisionsComputersAutomobilesCell phonesJet airplanesIpodsPlasticVelcroRefrigeratorsInternet (thanks, Al Gore)PenicillinCDs & DVDsInsulinand, of course -Electric guitars Sliced Bread!Development of Atomic TheoryThis explosion of technology occurredonce we had a better understanding of the atom and how it behaves!8Where 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.

Then came the idea of The 4 Basic ElementsEarth, Air, Fire, & Water

After that came Alchemy.

The change to real Chemistry didnt occur until the first true element was discovered! (1774)The first element discovered was

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 Priestleys results11Discovery of Oxygen gas is attributed to three personsKarl Scheele who first prepare and discribed oxygen Joseph 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 Priestlys 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)Carl Wilhelm Scheele beat Priestley to the discovery but published afterwards. Too bad! So sad!

Priestley Medal

Source: Roald Hoffman, Cornell University

Priestley gets the main credit for discovering oxygen!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.

2HgO(s) 2Hg(l) + O2 (g)Priestley: Scientific ContributionsDISCOVERY OF 8 GASESOxygenNitrogenCarbon DioxideCarbon MonoxideSulfur DioxideNitrous OxideNitric OxideHydrogen Chloride

Priestley: Additional Scientific ContributionsDiscovered the interconnection between photosynthesis and respirationDiscovered carbonated waterDiscovered that India rubber removed graphite pencil marks - the first rubber eraser

Now we can make mistakes!!

Antoine-Laurent LavoisierLavoisier: the Founder of Modern ChemistryLavoisier continued the investigations of Priestly Quantitative 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!)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.

2Hg(l) + O2 (g) 2HgO(s)Properties of OxygenColorlessOdorlessTastelessGas at room temperatureSlightly soluble in waterInflammable (does NOT burn)Only part of air that supports combustionPhysical Property or Chemical Property?PPPPPCCThese properties of oxygen were later used to determine the properties of other substances.

By the late 18th century, scientists finally came to the conclusion that Oxygen was truly an element (cant be broken down into simpler forms without losing its properties)

Scientists began to search for & test other new elements.

Sometimes, when they tried to react substances together, nothing happened!

Substances that DO NOT react are Inert

They 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!

Increasing chemical reactivityinertOxygenhydrogenDiscovery ofHenry 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 he found that water is a compound

(1731 1810)WordEquationChemicalequationHydrogen + Oxygen Water

2H2 + O2 2H2OAntoine LavoisierNamed Priestlys newly discovered gas - oxygen - meaning acid formerNamed Cavendishs new gas hydrogen -meaning water former

Daltons Atomic TheoryJohn Dalton (1766-1844)

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, its an important landmark in the history of science.

Daltons Modern Atomic Theory (experiment based!)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.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.

Law of Definite ProportionsEach compound has a specific ratio of elements by mass.

Ex: Water is always 8 grams of oxygen for each gram of hydrogen. Discovery of the ElectronBegan with the invention of the Crookes Tube (cathode ray tube) c. 1875

Cathode Ray TubeElectric current sent through gases sealed in tube at low pressureAnode- positive electrodeCathode- negative electrode

Voltage sourceMetal Disks - electrodes-+gas

Modern Cathode Ray Tubes

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.TelevisionComputer MonitorIn 1897, J.J. Thomson used a cathode ray tube to study gases.

Passing an electric current makes a beam appear to move from the negative to the positive end so the beam was called a Cathode RayThomsons ExperimentVoltage source+-32Voltage sourceThomsons ExperimentThomson found that cathode rays were deflected from a negatively-charged plate.-

33Voltage sourceThomsons Experimentand that cathode rays were attracted to plates with a positive chargeDoes light bend like this?+

34Light doesnt bend so the cathode ray must be made of particles rather than Light!Since they are attracted to a positive plate & repelled by a negative one the particles arent neutral What charge must they have? Thats right! NEGATIVE!!Thomson called these negative particles ELECTRONS

Mass of the Electron1916 Robert Millikan determined the mass of the electron: 1/1840 the mass of a hydrogen atom; and, has one unit of negative chargeThe oil drop apparatus

Mass of the electron is 9.11 x 10-28 g

37Conclusions 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 massThomsons Atomic ModelThomson believed that the electrons were like plums embedded in a positively charged pudding, thus it was called the plum pudding model.

J. J. ThomsonPlum-Pudding Model

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 5640

In 1903, An important discovery leading to further understandings of atomic structure happened by accident. Henri Becquerel discovered radioactivityRadioactivity is the spontaneous emission of energy from an object1903: Shared a Nobel Prize with Pierre and Marie Curie for discovering radioactivity. Ernest Rutherford (1871-1937)The Nobel Prize in Chemistry 1908Studied under J. J. Thomson

3 Types of Radiation discovered by Ernest Rutherford

Alpha () a positively charged helium nucleus 42 He+2Beta () fast-moving electrons -eGamma () like high-energy x-rays

Ernest RutherfordsGold Foil Experiment - 1911Shot alpha particles at a thin sheet of gold foil Particles that hit on a detecting screen (film) were recorded

Lead blockPoloniumGold FoilFlourescent Screen45He Expected:The alpha particles to pass through the foil without changing direction very much.BecauseThe positive charges were spread out evenly (according to Thomsons atomic theory). Alone they were not enough to stop the alpha particles.

46

What he expected47Again, because he thought the mass was evenly distributed in the atom48

What he got49Rutherfords ObservationsMost 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!

Like howitzer shells bouncing off of tissue paper!

50+51Rutherfords ConclusionsSince 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.+52The Rutherford Atomic ModelBased 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 nucleusThe electrons are distributed around the nucleus, and occupy most of the volumeHis model was called a nuclear modelDiscovery of ProtonsEugen Goldstein in 1886 observed particles with a positive charge passing through a perforated cathode.

In 1920, Rutherford studied these particles & called them protons.

They have a charge of positive 1and a mass of 1.7 x 10-24 grams.

This is not a handy number to work with so we use a mass of 1 amu.Amu stands for atomic mass unit1932 James Chadwick confirmed the existence of the neutron a particle with no charge, but a mass nearly equal to a proton(1 amu).

Discovery of the NeutronRutherford predicted the existence of the neutron in 1920. Twelve years later, his assistant found it!So now we have a more complete picture of an atom!Subatomic ParticlesParticleChargeMass (g)LocationElectron (e-)-19.11 x 10-28g(virtually 0)

outside nucleusProton (p+) (H+)+1

1 amu (1.7 x 10-24g)

in nucleusNeutron (no)0 1 amu (1.67 x 10-24g)in nucleus

Elements are the new building blocksHydrogenNitrogen-7Oxygen-8Carbon-6Between 1912 and 1914, the physicist H.G.J. Moseley conducted a series of experiments where he bombarded targets made out of different kinds of metals with cathode rays. Each metal he studied emitted X-rays of a characteristic frequency, almost like a set of "fingerprints". Henry Moseley (1887 1915)

The pattern that emerged when the observed X-rays were organized in order of increasing frequency suggested to Moseley a regular increase in the positive charge on the nuclei of the atoms.

He called this positive nuclear charge-the Atomic Number of the elementAtomic 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 = # electronsHenry Moseley used x-ray spectra& came up with the idea of the Atomic NumberAtomic Number, ZAll atoms of the same element have the same number of protons in the nucleus, Z13Al26.981Atomic numberAtom symbolAVERAGE Atomic MassMass NumberMass number is the number of protons and neutrons in the nucleus of an isotope:Mass # = # protons + # neutrons

Subatomic Particles

Most of the atoms mass.NUCLEUSELECTRONSPROTONSNEUTRONSNEGATIVE CHARGEPOSITIVE CHARGENEUTRAL CHARGEATOMAtomic Numberequals the # of...equal in a neutral atomIsotopesFrederick Soddy (1877-1956) proposed the idea of isotopes in 1912 (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.

IsotopesAtoms 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 n

10B11B

IsotopesRadioisotopes (radioactive isotopes) - unstable isotopes that spontaneously decay emitting radiation

They 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 research Sterilization of meatDisinfestation of grain and spicesIncreasing shelf life (eg, fruits)

Nuclear SymbolsContain the symbol of the element, the mass number and the atomic number (represent isotopes of elements)X MassnumberAtomicnumberSubscript Superscript Element symbolREMEMBER! number of electrons = number of protonsSo all atoms are neutral!70RheniumRe18675Protons: 75Neutrons: 111Electrons: 75Nuclear SymbolsFind each of these: number of protons number of neutrons number of electrons Atomic number Mass NumberBr80 3572Nuclear SymbolsIf an element has an atomic number of 34 and a mass number of 78, what is the: number of protons number of neutrons number of electrons Write the complete symbolSe783434443473Naming IsotopesWe can name isotopes by placing the mass number after the name of the element:carbon-12carbon-14uranium-235Mass numbers74ISOTOPESIsotopep+n0e-Mass #Oxygen - 10 - 3342 - 3115881818Arsenic753375Phosphorus153116IsotopeProtonsElectronsNeutronsNucleusHydrogen1 (protium)

1

1

0Hydrogen-2(deuterium)

1

1

1

Hydrogen-3(tritium)

1

1

2

The element hydrogen has 3 isotopesExamples of Isotopes

77Learning 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 13C14C 6 6 6#p+ _______ _______ _______ #no _______ _______ _______ #e- _______ _______ _______

Answers12C 13C14C 6 6 6#p+ 6 6 6 #no 6 7 8 #e- 6 6 6

Learning CheckAn atom has 14 protons and 20 neutrons.A.Its atomic number is1) 142) 163) 34

B. Its mass number is1) 142) 163) 34

C. The element is1) Si2) Ca3) Se

D.Another isotope of this element is1) 34X 2) 34X 3) 36X 16 14 14

Atomic MassHow 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 dont use grams for this mass because the numbers would be too small 81Measuring 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 elements percent abundance.82To calculate the average atomic mass:Multiply the mass of each isotope by its abundance, then add the results. Abundance may be expressed as a decimal or a %, (Divide by 100 if using %s)

Avg.AtomicMass83IsotopeSymbolComposition of the nucleus% in natureCarbon-12C-126 protons6 neutrons98.89%Carbon-13C-136 protons7 neutrons1.11%Carbon-14C-146 protons8 neutrons