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Click the left mouse button advance these slides as needed Slide 7 are notes that need to be copied onto the back of the worksheet. Chemistry Vocabulary Review 1. ? – the smallest particle of an element that has all the chemical properties of that element. Atoms are made of protons, neutrons and electrons. Ne H Ne Ne Ne Neon Gas H H H H H HH Ne Ne HH H H Hydrogen Gas HH 2. ? – two or more atoms bonded together. 3. ? – a pure substance made of only one type of atom H H 4. ? – a molecule made of more than one type of atom. It has different kinds of atoms bonded together. Ammonia H NH H 5. ? – a substance made of more than one type of molecule, many different molecules not bonded together, and can be separated physically. For example, a mixture of salt and pepper can be separated by adding water. The salt will dissolve into the water but not the pepper. A strainer or filter can keep the pepper on top while the saltwater goes through. Heat can then be used to boil off the water leaving the salt in the pan. It takes too long to use tweezers to separate salt and pepper grains. A Planetary Model of the Atom / The Bohr model of the Atom Do you like my atom? Before Mr. Bohr’s theory, people didn’t know how electrons moved around the nucleus. They thought they all just orbited in one big cloud like this. This is the symbol for atomic energy In 1913 Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The Bohr Model is probably familiar as the "planetary model" of the atom. In the Bohr Model the neutrons and protons occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun (but the orbits of electrons are not always in the same plane like the planets in the Solar System). The above image is not to scale. In a real atom the radius of the nucleus is about 100,000 times smaller than the radius of the entire atom. Electrons are point particles essentially without a volume. The electrons take up insignificant amount of space. Bohr Model for the Hydrogen Atom The kinetic energy at n=2 would be greater than n=1. Or else the electron would spiral closer to the nucleus. This is because v2 would provides a nice application of classical orbital mechanics half as much when n=2 introduces us to some useful concepts inbe electricity V = velocity vector of the electron to n=1. Assuming you provides something fun to discuss for thecompared last few lectures of the term that we will not need to be tested upon. The fundamental idea of the Bohr model (and models other of the time) was that the electron would orbit the much heavier spinning around the nucleus. nucleus in essentially circular orbits. This sounds very much likeit the motion of with planets aboutyour the sun, for hand example, but the pushed out force of gravity is much too weak to account for the energies involved in the atomic spectra and for energies involved in atomic physics. Instead, the binding force is supplied by the electromagnetic force, whichto we study detail next term. along This force perpendicular itsin more velocity operates between charged objects, and, like the gravitational force, is an inverse square law force: the radius. The force of attraction where Fel is the electrical force between two objects having charges q1 and q2 separated by a distance r. The constant k plays the role of the universal gravitational constant, and we need not worry too much about it right now. In a hydrogenic atom bechargegreater is independent hydrogen, He+, Li++, etc. - those with one would electron, the nuclear is +Ze, where Z is(it the atomic number (1 for H, 2 for He, 3 for Li, etc.) and e is the 'fundamental charge, 1.6x10^-19 Coulombs. The charge of the electron is -e, so the force on the of velocity) than the centripetal electron in a hydrogenic atom is = Force of This looks very much like the gravitation formula, with a slightly different interpretation variables involved. In any case, force (depends onof thevelocity). attain a circular orbit, in the usual way we need equate this central force with the centripetal acceleration. Schematically, the electrostatic attraction toclassical orbit of a hydrogenic atom looks like: The discovery of the nuclear atom, first in the Rutherford laboratory and soon after confirmed by other groups, spawned a completely new series of theories to explain the discrete lines in the atomic spectra. Initially, the most successful of these was by Niels Bohr, who was working in the Rutherford laboratory. Bohr devised a model that combined the ideas of Einstein, Planck, and Rutherford with classical mechanics of orbiting systems that successfully explained the spectra of hydrogenic spectra, i.e., the spectra of atoms with one electron. The Bohr model was the first to propose 'quantized states' for atomic systems, a radical idea at the time. It was soon replaced by 'real' quantum mechanics, but it is still useful to examine the Bohr model here, since it The Bohr model of a Hydrogen Atom Centripetal Force -keeps the electron away from the nucleus If you push an electron from n=3 to n=2 then the electron would have too much velocity to maintain the orbit at n=2 and the centripetal force would be greater and it would Bohr solved these problems by wishing them away in a way that would also predict the spectral properties of hydrogenic atoms. spin farther from the nucleus back His postulates were that into n=3. the electron could only move in certain non-radiating states, which he called stationary states. This terminology has been The force balance condition requires that There are two apparent flaws with such a model. Firstly, there is no obvious quantization. That is, the final equation does not quantize r, since we can find the electromagnetic analog a Kepplerian orbits for any r, given the appropriate value of v (and thus T). A related problem that does not appear (at least not in the same way) in the gravitational case is that the acceleration of the electron requires that it radiate energy away in the form of light. The mathematics behind this are pretty complex and you won't see it for a couple of years, but the problem can be easily appreciated. An antenna radiates radio waves by accelerating charge (i.e., electrons) back and forth along its length. A circular orbit, in this sense, is just an antenna. This emission of radiation is a dissipative process, as far as mechanical energy is concerned, so classically the electron would spiral into the nucleus in a very short time. 1 proton in the nucleus Z=1 e is the fundamental charge 1.6x10^-19 Coulombs kept in the 'real' quantum theory. the electrons could move from one stationary state to another only by making discrete jumps. The radiation emitted is not related to the electron's motion in either stable orbit, but rather is related by Planck's relationship to the change in energy in going from one orbit to another: This means that energy is released as an electron spins closer to the That is, one of the Planck/Einstein photons is emitted when an electron moves from a high energy stationary state to a low energy stationary state. nucleus. The energy is released in the angular momentum of the electron in the stationary states was quantized to be an integral number of Planck's the form oforbit photons of light. constant divided by 2*pi. Since the angular momentum of a circular is mvr, it follows that the Bohr quantization condition requires that where n is an integer between 1 and infinity. The constant h-bar, defined as h divided by 2*pi, has come to be a more useful constant than h itself. It is also easier to remember, since its numerical value is very nearly 1x10^-34 J-sec. Thus the angular momentum of a Bohr orbit is quantized to be an integral number of units of h-bar. Note that the Planck relation between a photon's energy and the frequency of a light wave is equivalent to Thus the use of h-bar rather than h really amounts to finding the angular frequency omega more useful that the cyclic frequency nu. How did Bohr arrive at this particular quantization condition? Why does n start at 1 and not zero? Obviously, he guessed. He certainly tried many others, but this is the one that worked. At the time, there was no real justification. Force of electrostatic attraction = Centripetal Force keeps the electron away from the nucleus Radii of Bohr Orbits To apply this quantization condition to determine the radii of electronic orbits in atoms, first solve the force balance equation for v^2: and then square the quantization condition and again solve for v^2: and set these two equal: where ao is called the first Bohr radius: Numerically, ao is equal to 0.0529 nm = 0.529 Angstroms. We see that the angular momentum quantization postulate Bohr's model explained how the the colors of light were emitted from hydrogen gas when it was put in a glass tube containing a strong electric field. (cathode tube) This is known as the emission spectra of hydrogen. He promptly won the Nobel prize. The Bohr view of emission and energy is shown schematically in the figure below. Electrons fall from higher energy orbits to lower ones; resulting in the emission of photons of energy of different light wave frequencies. Bohr Atom Pictures for Elements 1 - 13 NAME: _____________ NAME: _____________ Atomic Number: 1 Atomic Mass: 1.0079 Atomic Number: 2 Atomic Mass: 4.003 NAME: _____________ Atomic Number: 3 Atomic Mass: 6.941 protons: _____ neutrons: _____ electrons: _____ protons: _____ neutrons: _____ electrons: _____ protons: _____ neutrons: _____ electrons: _____ NAME: _______________ Atomic Number: 8 Atomic Mass: 16.00 protons: _____ neutrons: _____ electrons: _____ NAME: _______________ Atomic Number: 9 Atomic Mass: 19 protons: _____ neutrons: _____ electrons: _____ NAME: _____________ Atomic Number: 4 Atomic Mass: 9.012 protons: _____ neutrons: _____ electrons: _____ NAME: _____________ per_____ NAME: _____________ NAME: _____________ Atomic Number: 5 Atomic Mass: 10.81 Atomic Number: 6 Atomic Mass: 12.01 protons: _____ neutrons: _____ electrons: _____ protons: _____ neutrons: _____ electrons: _____ Atomic Number: 10 Atomic Mass: 20.18 NAME: _______________ Atomic Number: 11 Atomic Mass: 23 NAME: _______________ Atomic Number: 12 Atomic Mass: 24.31 protons: _____ neutrons: _____ electrons: _____ protons: _____ neutrons: _____ electrons: _____ protons: _____ neutrons: _____ electrons: _____ NAME: _______________ NAME: _____________ Atomic Number: 7 Atomic Mass: 14.01 protons: _____ neutrons: _____ electrons: _____ NAME: _______________ Atomic Number: 13 Atomic Mass: 26.98 protons: _____ neutrons: _____ electrons: _____ Copy this page on back of the worksheet How to read the Periodic Table: 5 B Boron 10.81 Atomic Number This is the number of protons = 5 This is also the number of electrons = 5 (only for neutral atoms) Atomic Symbol name Atomic Mass - Atomic Number = 11 - 5 = 6 neutrons NAME: _B,_ Boron ______ round off 10.81 to 11 Atomic Number: 5 Atomic Mass: 10.81 protons: __5___ neutrons: __6___ electrons: __5___ Draw only up to 8 electrons in the second orbital. 3 in this case = total Draw only up to 2 electrons in5the Draw 6 protons neutronsasashollow solid circles first orbital. Draw 5 circles Also copy this page on back of the worksheet The outermost orbital is called the valence Atoms touch each other at the valence and make chemical bonds. Draw the parts of a Hydrogen atom on your worksheet template. Do these steps on the front of the worksheet NAME: _ H ,_ Hydrogen Atomic Number: 1 = number of protons Atomic Mass: 1.0079 = 1 rounded off 1 1 -1 protons: _____ 0 0 calculate the neutrons by subtracting the protons neutrons: _____ 1 the number of electrons is the same as protons electrons: _____ for a neutral atom. x Draw one electron in the first orbital Draw zero neutrons in the nucleus Draw one proton in the nucleus NAME: _ He ,_ Helium Atomic Number: 2 = Atomic Mass: 4.003 number of protons = 4 rounded off 4 2 -2 protons: _____ 2 2 calculate the neutrons by subtracting the protons neutrons: _____ 2 the number of electrons is the same as protons electrons: _____ for a neutral atom. x Draw 2 neutrons in the nucleus Draw 2 protons in the nucleus x Draw 2 electrons in the first orbital NAME: _ Li ,_ Lithium Atomic Number: 3 = Atomic Mass: 6.941 number of protons = 7 rounded off 7 3 -3 protons: _____ 4 4 calculate the neutrons by subtracting the protons neutrons: _____ 3 the number of electrons is the same as protons electrons: _____ for a neutral atom. x Draw 1 electrons in the second orbital x Draw 4 neutrons in the nucleus Draw 3 protons in the nucleus x Draw 2 electrons in the first orbital NAME: _ BE, Beryllium Atomic Number: 4 = Atomic Mass: 9.012 number of protons = 9 rounded off 9 4 -4 protons: _____ 5 5 calculate the neutrons by subtracting the protons neutrons: _____ 4 the number of electrons is the same as protons electrons: _____ for a neutral atom. x Draw 2 electrons in the second orbital x Draw 5 neutrons in the nucleus Draw 4 protons in the nucleus x x Draw 2 electrons in the first orbital NAME: _ B, Boron Atomic Number: 5 = Atomic Mass: 10.81 number of protons = 11 rounded off 11 5 -5 protons: _____ 6 6 calculate the neutrons by subtracting the protons neutrons: _____ 5 the number of electrons is the same as protons electrons: _____ for a neutral atom. x Draw 3 electrons in the second orbital x Draw 6 neutrons in the nucleus Draw 5 protons in the nucleus x x x Draw 2 electrons in the first orbital NAME: _ C, Carbon Atomic Number: 6 = Atomic Mass: 12.01 number of protons = 12 rounded off 12 6 -6 protons: _____ 6 6 calculate the neutrons by subtracting the protons neutrons: _____ 6 the number of electrons is the same as protons electrons: _____ for a neutral atom. Draw 4 electrons in the second orbital x x x x x x Draw 6 neutrons in the nucleus Draw 6 protons in the nucleus Draw 2 electrons in the first orbital NAME: _ N, Nitrogen Atomic Number: 7 = Atomic Mass: 14.01 number of protons = 14 rounded off 14 7 -7 protons: _____ 7 7 calculate the neutrons by subtracting the protons neutrons: _____ 7 the number of electrons is the same as protons electrons: _____ for a neutral atom. Draw 5 electrons in the second orbital x x x x x x x Draw 7 neutrons in the nucleus Draw 7 protons in the nucleus Draw 2 electrons in the first orbital NAME: _ O, Oxygen Atomic Number: 8 = Atomic Mass: 16.00 number of protons = 16 rounded off 16 8 -8 protons: _____ 8 8 calculate the neutrons by subtracting the protons neutrons: _____ 8 the number of electrons is the same as protons electrons: _____ for a neutral atom. Draw 6 electrons in the second orbital l l l l l l l l Draw 8 neutrons in the nucleus Draw 8 protons in the nucleus Draw 2 electrons in the first orbital NAME: _ F, Fluorine = number of protons = 19 rounded off 19 9 -9 protons: _____ 10 10 calculate the neutrons by subtracting the protons neutrons: _____ 9 the number of electrons is the same as protons electrons: _____ for a neutral atom. Atomic Number: 9 Atomic Mass: 19 Draw 7 electrons in the second orbital l l l l l l l l l Draw 10 neutrons in the nucleus Draw 9 protons in the nucleus Draw 2 electrons in the first orbital NAME: _ Ne, Neon Atomic Number: 10 = Atomic Mass: 20.18 number of protons = 20 rounded off 20 10 -10 protons: _____ 10 10 calculate the neutrons by subtracting the protons neutrons: _____ 10 the number of electrons is the same as protons electrons: _____ for a neutral atom. Draw 8 electrons in the second orbital l l l l l l l l l l Draw 10 neutrons in the nucleus Draw 10 protons in the nucleus Draw 2 electrons in the first orbital NAME: _ Na, Sodium Atomic Number: 11 = Atomic Mass: 23 number of protons = 23 rounded off 23 11 -11 protons: _____ 12 12 calculate the neutrons by subtracting the protons neutrons: _____ 11 the number of electrons is the same as protons electrons: _____ for a neutral atom. Draw 8 electrons in the second orbital l l l l l l l l l ll Draw 12 neutrons in the nucleus Draw 11 protons in the nucleus Draw 2 electrons in the first orbital Draw 1 electron in the third orbital NAME: _ Mg, Magnesium Atomic Number: 12 = Atomic Mass: 24.31 number of protons = 24 rounded off 24 12 -12 protons: _____ 12 12 calculate the neutrons by subtracting the protons neutrons: _____ 12 the number of electrons is the same as protons electrons: _____ for a neutral atom. l Draw 8 electrons in the second orbital l l l l l l l l l ll Draw 13 neutrons in the nucleus Draw 12 protons in the nucleus Draw 2 electrons in the first orbital Draw 2 electrons in the third orbital NAME: _ Al, Aluminum Atomic Number: 13 = Atomic Mass: 26.98 number of protons = 27 rounded off 27 13 -13 protons: _____ 14 14 calculate the neutrons by subtracting the protons neutrons: _____ 13 the number of electrons is the same as protons electrons: _____ for a neutral atom. l Draw 8 electrons in the second orbital l l l l l l l Draw 2 electrons in the first orbital l l ll Draw 14 neutrons in the nucleus Draw 13 protons in the nucleus l Draw 3 electrons in the third orbital