2 Defining the AtomThe Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”)He believed that atoms were indivisible and indestructibleHis ideas did agree with later scientific theory, but did not explain chemical behavior, and was not based on the scientific method – but just philosophy
3 Dalton’s 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.John Dalton(1766 – 1844)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.
4 Sizing 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
5 Structure of the Nuclear Atom One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles:Electrons, protons, and neutrons are examples of these fundamental particlesThere are many other types of particles, but we will study these three
6 Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron
7 Modern Cathode Ray Tubes TelevisionComputer MonitorCathode ray tubes pass electricity through a gas that is contained at a very low pressure.
8 Mass of the Electron The oil drop apparatus Mass of the electron is9.11 x gThe oil drop apparatus1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge
9 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 electronsElectrons have so little mass that atoms must contain other particles that account for most of the mass
10 Conclusions from the Study of the Electron: Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron)1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton
11 Subatomic Particles Particle Charge Mass (g) Location Electron (e-) -1 9.11 x 10-28Electron cloudProton (p+)+11.67 x 10-24NucleusNeutron(no)
12 Thomson’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.
13 Ernest Rutherford’s Gold Foil Experiment - 1911 Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foilParticles that hit on the detecting screen (film) are recorded
14 Rutherford’s problem: In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?Target #2Target #1
16 Rutherford’s Findings Most of the particles passed right throughA few particles were deflectedVERY FEW were greatly deflected“Like howitzer shells bouncing off of tissue paper!”Conclusions:The nucleus is smallThe nucleus is denseThe nucleus is positively charged
17 The 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 nucleus is composed of protons and neutrons (they make the nucleus!)The electrons distributed around the nucleus, and occupy most of the volumeHis model was called a “nuclear model”
18 Bohr ModelIn 1913, the Danish scientist Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level.
19 Bohr ModelAccording to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus.
21 The Wave ModelToday’s atomic model is based on the principles of wave mechanics.According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun.
22 The Wave ModelIn fact, it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has.According to the modern atomic model, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.
23 Electron Cloud A space in which electrons are likely to be found. Electrons whirl about the nucleus billions of times in one secondThey are not moving around in random patterns.Location of electrons depends upon how much energy the electron has.
24 Electron CloudDepending on their energy they are locked into a certain area in the cloud.Electrons with the lowest energy are found in the energy level closest to the nucleusElectrons with the highest energy are found in the outermost energy levels, farther from the nucleus.
25 Greek X Dalton Thomson Rutherford Bohr Wave Indivisible Electron NucleusOrbitElectron CloudGreekXDaltonThomsonRutherfordBohrWave
27 The Atom An atom consists of: nucleus – contains protons and neutrons electrons in space around the nucleus.The atom is mostly empty spaceElectron cloudNucleus
28 ATOMIC COMPOSITION Protons (p+) Electrons (e-) Neutrons (no) + electrical chargemass = x grelative mass = atomic mass units (amu) but we can round to 1Electrons (e-)negative electrical chargerelative mass = amu but we can round to 0Neutrons (no)no electrical chargemass = amu but we can round to 1
29 Atomic Number, ZAll atoms of the same element have the same number of protons in the nucleus, Z13Atomic numberAlAtom symbol26.981AVERAGE Atomic Mass
30 Atomic NumberAtoms are composed of identical protons, neutrons, and electronsHow then are atoms of one element different from another element?Elements are different because they contain different numbers of PROTONSThe “atomic number” of an element is the number of protons in the nucleus# protons in an atom = # electrons
31 Atomic Number Element # of protons Atomic # (Z) Carbon 6 Phosphorus 15 Gold79
32 Mass Number, AA carbon atom with 6 protons and 6 neutrons is the mass standard = 12 atomic mass unitsMass Number (A) = # protons + # neutronsNOT on the periodic table…(Round the AVERAGE atomic mass on the table)A boron atom can have A = 5 p n = 10 amu
33 Mass NumberMass number is the number of protons and neutrons in the nucleus of an isotope:Mass # = p+ + n0Nuclidep+n0e-Mass #Oxygen -10-3342- 3115188818Arsenic753375Phosphorus161531
34 Complete SymbolsContain the symbol of the element, the mass number and the atomic number.MassnumberXSuperscript →AtomicnumberSubscript →
35 Br Symbols 80 35 Find each of these: number of protons number of neutronsnumber of electronsAtomic numberMass Number80Br35
36 SymbolsIf an element has an atomic number of 34 and a mass number of 78, what is the:number of protonsnumber of neutronsnumber of electronscomplete symbol
37 SymbolsIf an element has 91 protons and 140 neutrons what is theAtomic numberMass numbernumber of electronscomplete symbol
38 SymbolsIf an element has 78 electrons and 117 neutrons what is theAtomic numberMass numbernumber of protonscomplete symbol
40 IsotopesDalton was wrong about all elements of the same type being identicalAtoms of the same element can have different numbers of neutrons.Thus, different mass numbers.These are called isotopes.
41 IsotopesFrederick Soddy ( ) proposed the idea of isotopes in 1912Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.
42 We can also put the mass number after the name of the element: Naming IsotopesWe can also put the mass number after the name of the element:carbon-12carbon-14uranium-235
43 Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons. ProtonsElectronsNeutronsNucleusHydrogen–1(protium)1Hydrogen-2(deuterium)Hydrogen-3(tritium)2
44 Isotopes Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons.
45 Isotopes & Their UsesBone scans with radioactive technetium-99.
46 Isotopes & Their UsesThe tritium content of ground water is used to discover the source of the water, for example, in municipal water or the source of the steam from a volcano.
47 Atomic 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 of that element in nature.We don’t use grams for this mass because the numbers would be too small.
48 Measuring 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 atomic mass, thus we determine the average from percent abundance.
49 To calculate the average: Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results.If not told otherwise, the mass of the isotope is expressed in atomic mass units (amu)
50 Composition of the nucleus Atomic MassesAtomic mass is the average of all the naturally occurring isotopes of that element.IsotopeSymbolComposition of the nucleus% in natureCarbon-1212C6 protons6 neutrons98.89%Carbon-1313C7 neutrons1.11%Carbon-1414C8 neutrons<0.01%Carbon =
51 Average Atomic Mass Avg. Atomic Mass weighted average of all isotopes on the Periodic Tableround to 2 decimal placesAvg.AtomicMass
52 D. 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
53 D. Average Atomic MassEX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.Avg.AtomicMass35.40 amu
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