2HOMEWORK Read your textbook pages 74 to 78 and answer q 1 to 6 Start a timeline in your notebook of the discoveries that lead to the present theory of the atom. Include the date and name of the scientist mentioned in the book and the contribution.
3HISTORYDemocritus 400 BC suggested that matter is made up of indivisible particles called atoms.
4The Law of Conservation of Mass Established in 1789 by French Chemist Antoine LavoisierStates that mass is neither created nor destroyed in any chemical reaction.Or more simply, the mass of substances produced (products) by a chemical reaction is always equal to the mass of the reacting substances (reactants).
5LAW OF DEFINITE PROPORTIONS Every compound is a combination of a fixed proportion by mass between the elements that compose the sample.Water is always 1 g of H and 8 g of OCO2 is always 12 g of C and 32 of O
6LAW OF MULTIPLE PROPORTIONS When the same elements combine to form different compounds, they do so in mass ratios that can be expressed by small whole numbers.CO carbon monoxide 12 g C 16 g OCO2 carbon dioxide g C 32 g O
7First Atomic Theory Dalton (1766-1844) All elements are composed of indivisible atoms.Atom of the same elements are identical.Atoms of different elements differ in their physical and chemical properties.Atoms of different elements combine to form compounds in simple whole number ratios.Chemical reactions occur when atoms are separated, joined or rearranged, but they can not change.
8Experiments to determine what an atom was J. J. Thomson (1897)- used Cathode ray tubes
11- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end.
12Thomson’s ExperimentVoltage sourceBy adding an electric field
13Thomson’s Experiment Voltage source + - By adding an electric field, he found that the moving pieces were negative
14OBJECTIVE : WHAT IS INSIDE THE ATOM? Experiments that lead to find what is inside the atomRutherford gold foil experiment –Millikan
15Thomsom’s conclusions Thomson named the cathodic rays electrons and concluded that they must be a part of atoms of all elements.
16The Atom, circa 1900: “Plum pudding” model, put forward by Thompson. Positive sphere of matter with negative electrons imbedded in it.Figure 2.9
17The Gold foil experiment In 1911 Ernest Rutherford designed an experiment using alpha particles and thin gold foil - “gold foil experiment”. Rutherford was very surprised by his finding and his conclusions led to which became known as the “planetary” model of the atom.
18Rutherford’s Experiment Used uranium to produce alpha particles.Aimed alpha particles at gold foil by drilling hole in lead block.Since the mass is evenly distributed in gold atoms alpha particles should go straight through.Used gold foil because it could be made atoms thin.
25The Nuclear AtomSince some particles were deflected at large angles, Thompson’s model could not be correct.Figure 2.11
26ObservationsMost of the alpha particles pass straight through the gold foil.Some of the alpha particles get deflected by very small amounts.A very few get deflected greatly.Even fewer get bounced of the foil and back to the left.
27Conclusions The atom is 99.99% empty space. The nucleus contains a positive charge and most of the mass of the atom. The nucleus is approximately 100,000 times smaller than the atom.
28Vocabulary Deflected: curved or bent downward. To Scatter: to separate and drive off in various directions; to disperseTo Bounce: To rebound, move up and down repeatedly.
29Other Subatomic Particles Protons were discovered by Rutherford in 1919.Neutrons were discovered by James Chadwick in 1932.
30The Bohr ModelNiels Bohr proposed a model of the atom based on the solar system.It was wrong but some of his ideas we still used. It was one step in the direction of the last model for the atom.First let’s take a look at what is inside the atom.
31Subatomic ParticlesProtons and electrons are the only particles that have a charge.Protons and neutrons have essentially the same mass. They are located inside the nucleus.The mass of an electron is so small we ignore it.Table 2.1
32Atomic Number (Z)The number of protons in the nucleus. It identifies the element. In the neutral atom# protons = # electrons
33Atomic NumberAll atoms of the same element have the same number of protons:The atomic number (Z)
34Mass Number (A) * It is not in the periodic table but is the atomic mass rounded off Protons plus neutrons in the atomA= # protons + # neutrons*In the table we can find the Atomic Mass.Number of Neutrons =Mass number-Atomic number
35ISOTOPIC SYMBOL Atomic number (Z) = number of protons in the nucleus. Mass number (A) = total number of nucleons in the nucleus (i.e., protons and neutrons).By convention, for element X, we write ZAX
36Isotopes, Atomic Numbers, and Mass Numbers Atomic number (Z)= number of protons in the nucleus.Mass number (A)= total number of nucleons in the nucleus (i.e., protons and neutrons).By convention, for element X, we write ZAX.Isotopes have the same Z but different A.We find Z on the periodic table.
37Mass NumberThe mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom.
38Mass Number (A) * It is not in the periodic table Protons plus neutrons in the atomA= # protons + # neutrons*In the table we can find the Atomic Mass.Number of Neutrons =Mass number-Atomic number
39Isotopesatoms with same number of protons and different number of neutrons. Same atomic number different mass number.SAME ELEMENT with different # of neutrons!
40Isotopes:Atoms of the same element with different masses.Isotopes have different numbers of neutrons.116C126C136C146C40
41Atomic Mass Unit(amu) is one-twelfth the mass of a carbon-12 atom.
42Atomic Massis the weighted average of the mass of all the isotopes of one element.It takes into consideration the mass of the different isotopes of the element and their natural abundance
48Energy levelis the region around the nucleus where the electron is moving.As the electrons are further away from the nucleus they have more energy. The closest orbits or energy levels to the nucleus have low energy.
49He named the orbits with letters beginning with k,l,m,n… The first energy level could hold 2 electrons.Second energy level could hold up to 8 electrons
50Electron Configurations Indicates how the electrons are located in the atom.Niels Bohr proposed that electrons are located in energy levels at different distances from the nucleus.total of electrons=133 energy levels
51HELIUM ATOM + - + - Shell proton neutron electron What do these particles consist of?
52ATOMIC STRUCTUREParticleChargeMassproton+ ve charge1neutronNo charge1electron-ve chargenil
53number of electrons = number of protons ATOMIC STRUCTURE4HeMASS NUMBERthe number of protons and neutrons in an atom2Atomic numberthe number of protons in an atomnumber of electrons = number of protons
54ATOMIC STRUCTUREElectrons are arranged in Energy Levels or Shells around the nucleus of an atom.first shell a maximum of 2 electronssecond shell a maximum of 8 electronsthird shell a maximum of 8 electrons
55SUMMARY The Atomic Number of an atom = number of protons in the nucleus.The Atomic Mass of an atom = number ofProtons + Neutrons in the nucleus.The number of Protons = Number of Electrons.Electrons orbit the nucleus in shells.Each shell can only carry a set number of electrons.
56A closer look at the Bohr model How did Bohr came to the conclusion that the electron moved in orbits?TEXTBOOK PAGES 90 TO 94
57Bohr Model1 An electron in a permitted orbit has a specific energy an is in an “allowed” energy state. It will not spiral into the nucleus2 Energy is emitted or absorbed by the electron only as the electron changes from one allowed state to other
58Energy levels or shells 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.
59The Nature of EnergyA mystery involved the emission spectra observed from energy emitted by atoms and molecules.When gases at low pressure were placed in a tube and were subjected to high voltage, light of different colors appeared
60Line Spectra and the Bohr Model Continuous SpectraRadiation composed of only one wavelength is called monochromatic.Radiation that spans a whole array of different wavelengths is called continuous.White light can be separated into a continuous spectrum of colors.Note that there are no dark spots on the continuous spectrum that would correspond to different lines.
62Line SpectraIf high voltage is applied to atoms in gas phase at low pressure light is emitted from the gas.If the light is analyzed the spectrum obtained is not continuous.SPECTROSCOPE
63Line Spectra.When the light from a discharge tube is analyzed only some bright lines appeared.
64WavesWaves carry energy and have cycles. Each cycle of a wave begins at the origin and ends in the origin.The wavelength (l lambda) of a wave is a measurement of how long it is. It is measured in length units.The longer the wavelength, the smaller amount of energy that is associated with.* Short wavelength, high energy.* Long wavelength, low energy.64
67How is Light Energy related to Color? White light is made up of all colors of the spectrum (ROYGBIB)When passed through a prism a continuous spectrum (rainbow effect) is obtained67
68Nature of lightLight is a form of electromagnetic wave. Each color of the visible light is associated with a different wavelength.R (700 nm)OYGBIV (400nm)
69Line SpectraIf high voltage is applied to atoms in gas phase at low pressure light is emitted from the gas.If the light is analyzed with an SPECTROSCOPE the spectrum obtained is not continuous.
70Line SpectraThis type of spectra is used to identify atoms of different elements and were used to explain the new model of the atom.
71Bright line spectrumWhen an atom ABSORBS energy, the electrons JUMP (LEAP) to a higher energy level. They are in the EXCITED STATE.When the electrons return to the normal level (GROUND STATE) , they emit energy as LIGHT. The light emitted produce the bright line spectrum that is characteristic of each element.
72Electrons Surround the Nucleus Electrons surround the nucleus and travel at the speed of lightThey are found in only certain allowed energy levels or orbitalsElectrons are at the lowest energy level – the ground state.72
73Different Energy States are Possible When the electrons in an atom become excited by absorbing energy from the surroundings, they jump to new higher energy levels.The excited state is less stable than the lower energy state therefore the electron falls back or returns to the lower energy ground state.73
74Return to Ground StateWhen electrons fall back a wave with a specific amount of energy is emitted – called a photon. If we see color then the emission is in the visible rangeWe perceive this as unique colors associated with particular elements74
75Ground state configuration vs excited state configuration 11Na This is the ground state.If atom is excited some possible electron configuration would beor or 1-6-3The number of electrons must add to the atomic number but the order is not the one that appears in the periodic table.
77The 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.
79The modern model of the atom is called the quantum model or the wave-mechanical model.
80By 1900 scientist were studying energy and waves. It was proposed that energy was made of tiny packets called quanta, these packets acted like particles.That implied that light could behave as a wave and also as a particle.
81Quantum – singular – package or bundle or energy Quantum – singular – package or bundle or energy. If the energy is light is a photon.Quanta is the plural of quantum.
82Duality wave -particle Louis De Broglie discovered that the electron also behaves sometimes as a particle and sometimes as a wave
83OrbitalsIn the wave-mechanical model the electrons are found not in fixed orbits around the nucleus but in ORBITALS.ORBITALS are regions in which an electron of a particular amount of energy is most likely to be found.
84ProbabilityThe wave mechanical model describes the atom using PROBABILITY of finding electrons at different distances from the nucleus.
85Quantum theoryIn 1926 Erwin Schrodinger used the quantum theory to write an equation describing the location and energy of the electron in the atom of hydrogen.
86December 20 Quiz – DO NOW For an atom of C-13 1. Name of element 2. Number of proton3. Number of electron4. Number of neutron5. Electronic configuration6. Isotopic representation7. One possible excited state
875th Solvay Conference of Electrons and Photons - 1927
88The quantum model principles Electrons are found in orbitals.There are different energy levels. Electrons can gain or lose fixed amounts of energy (quanta) to move to different energy levels.When the electrons occupy the lowest available orbitals the atom is in the ground state.If the atom absorbs a quantum of energy the electrons jump to a higher energy level. This UNSTABLE CONDITION is called EXCITED STATE.When the electrons return to the ground state they emit the same amount of energy they absorbed as LIGHT of different colors.The emitted light can be analyzed with a spectroscope and the resulting spectrum can be used to identify the atoms.
89Energy levels 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.
90Energy levels or shells 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.
91Modern Atomic Theory The atom is mostly empty space. Two regions Nucleus- protons and neutrons.Electron cloud- region where you might find an electron.
92SET 1 & 2 ATOMIC SET ANSWERS 1 -12 - 23 - 44 – 25 – 36 – 47 – 48 - 49 –10 –11 –12 –13 do not do29-330-DND
93What happens when the number of protons and the number of electrons in an atom are not equal? IONS
95Atoms are neutral and contain same number of electrons than protons Atoms are neutral and contain same number of electrons than protons. In a chemical reactions atoms can lose, gain or share electrons.When an atom loses an electron it becomes a positive ion. When it gains an electron it becomes a negative ion.
96The nucleus always remains the same in a chemical reaction. The number of protons and neutrons never change.
97IonA charged particleWhen an atom loses or gains electrons it becomes an ION.Remember protons can not be touched!If an atom loses electron the ion will be positive (more protons than electrons)If an atom gains electrons it will became negative (more electrons than protons)
98Metals lose electrons and become positive ions (caTions) Non metals gain electrons and become negative ions ( aNions)
99Greek X Dalton Thomson Rutherford Bohr Wave Indivisible Electron NucleusOrbitElectron CloudGreekXDaltonThomsonRutherfordBohrWave