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12/17/11 Dalton’s Atomic Theory (1808) • All matter is made of atoms. Atoms are indivisible and indestructible*. Lecture 1 The structure of atom • All atoms of a given element are identical in mass* and properties* • Compounds are formed by a combination of two or more different kinds of atoms. • A chemical reaction is a rearrangement of atoms. *not exactly! But pretty good for 1808 Atom Electron • nucleus very small and dense, contains most of the mass, electrons circle nucleus, they have very little mass but take up most of the space, electrons 1/100,000 the diameter of the nucleus. The equivalent of a taw (2 cm ) on in the middle of the stadium compared to the whole of Giant Stadium at the Meadowlands… • negatively charged subatomic particle located outside the nucleus. Proton • positively charged subatomic particle located in the nucleus. Neutron • neutral subatomic particle located in the nucleus. • Um… by the way, how do we know this? 1 12/17/11 (1908 Nobel Prize in Chemistry) α particle velocity ~ 1.4 x 107 m/s (~5% speed of light) 1. atoms positive charge is concentrated in the nucleus 2. proton (p) has opposite (+) charge of electron (-) 3. mass of p is 1840 x mass of e- (1.67 x 10-24 g) 2.2 2.2 Subatomic Particles (Table 2.1) Rutherford’s Model of the Atom atomic radius ~ 100 pm = 1 to 2 x 10-10 m nuclear radius ~ 1 to 20 x 10-15 m mass p = mass n = 1840 x mass e2.2 Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei Mass Number A ZX Atomic Number 1 1H 235 92 2 1H U Element Symbol (D) 238 92 3 1H (T) U 2.3 2.3 2 12/17/11 A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds Do You Understand Isotopes? 14 6 How many protons, neutrons, and electrons are in C? H2 6 protons, 8 (14 - 6) neutrons, 6 electrons H2O NH3 CH4 A diatomic molecule contains only two atoms 11 6 How many protons, neutrons, and electrons are in C? 6 protons, 5 (11 - 6) neutrons, 6 electrons H2, N2, O2, Br2, HCl, CO A polyatomic molecule contains more than two atoms O3, H2O, NH3, CH4 2.3 An ion is an atom, or group of atoms, that has a net positive or negative charge. A monatomic ion contains only one atom cation – ion with a positive charge If a neutral atom loses one or more electrons it becomes a cation. Na 11 protons 11 electrons 2.5 Na+, Cl-, Ca2+, O2-, Al3+, N3- 11 protons 10 electrons Na+ A polyatomic ion contains more than one atom anion – ion with a negative charge If a neutral atom gains one or more electrons it becomes an anion. Cl 17 protons 17 electrons OH-, CN-, NH4+, NO3- 17 protons 18 electrons Cl- 2.5 Do You Understand Ions? How many protons and electrons are in 2.5 The black body radiation "Blackbody radiation" or "cavity radiation" refers to an object or system which absorbs all radiation incident upon it and re-radiates energy which is characteristic of this radiating system only, not dependent upon the type of radiation which is incident upon it. 27 3+ ? 13 Al 13 protons, 10 (13 – 3) electrons How many protons and electrons are in 78 Se 2- ? 34 The black body radiation curve (Fig1) shows that the black body does radiate energy at every wavelength. 34 protons, 36 (34 + 2) electrons 2.5 Tro, Chemistry: A Molecular Approach 18 3 12/17/11 The Photoelectric Effect The black body radiation • it was observed that many metals emit electrons when a Wien's displacement law light shines on their surface this is called the Photoelectric Effect • classic wave theory attributed this effect to the light energy being transferred to the electron • according to this theory, if the wavelength of light is Stefan–Boltzmann law made shorter, or the light waves intensity made brighter, more electrons should be ejected remember: the energy of a wave is directly proportional to its amplitude and its frequency if a dim light was used there would be a lag time before electrons were emitted to give the electrons time to absorb enough energy Tro, Chemistry: A Molecular Approach 19 The Photoelectric Effect Tro, Chemistry: A Molecular Approach 20 The Photoelectric Effect The Problem • in experiments with the photoelectric effect, it was observed that there was a maximum wavelength for electrons to be emitted called the threshold frequency regardless of the intensity • it was also observed that high frequency light with a dim source caused electron emission without any lag time Tro, Chemistry: A Molecular Approach 21 Einstein’s Explanation 22 Ejected Electrons • Einstein proposed that the light energy was delivered to the atoms in packets, called quanta or photons • the energy of a photon of light was directly proportional to its frequency inversely proportional to it wavelength the proportionality constant is called Planck’s Constant, (h) and has the value 6.626 x 10-34 J·s Tro, Chemistry: A Molecular Approach Tro, Chemistry: A Molecular Approach • 1 photon at the threshold frequency has just enough energy for an electron to escape the atom binding energy, φ • for higher frequencies, the electron absorbs more energy than is necessary to escape • this excess energy becomes kinetic energy of the ejected electron Kinetic Energy = Ephoton – Ebinding KE = hν - φ 23 Tro, Chemistry: A Molecular Approach 24 4 12/17/11 Spectra • when atoms or molecules absorb energy, that energy is Emission Spectra often released as light energy fireworks, neon lights, etc. • when that light is passed through a prism, a pattern is seen that is unique to that type of atom or molecule – the pattern is called an emission spectrum non-continuous can be used to identify the material flame tests • Rydberg analyzed the spectrum of hydrogen and found that it could be described with an equation that involved an inverse square of integers Tro, Chemistry: A Molecular Approach 25 Tro, Chemistry: A Molecular Approach 26 Examples of Spectra Exciting Gas Atoms to Emit Light with Electrical Energy Oxygen spectrum Hg He H Neon spectrum Tro, Chemistry: A Molecular Approach 27 Identifying Elements with Flame Tests Na Tro, Chemistry: A Molecular Approach K Li Tro, Chemistry: A Molecular Approach 28 Emission vs. Absorption Spectra Spectra of Mercury Ba 29 Tro, Chemistry: A Molecular Approach 30 5 12/17/11 Bohr Model of H Atoms Bohr’s Model • Neils Bohr proposed that the electrons could only have very specific amounts of energy fixed amounts = quantized • the electrons traveled in orbits that were a fixed distance from the nucleus stationary states therefore the energy of the electron was proportional the distance the orbital was from the nucleus • electrons emitted radiation when they “jumped” from an orbit with higher energy down to an orbit with lower energy the distance between the orbits determined the energy of the photon of light produced Tro, Chemistry: A Molecular Approach 31 Tro, Chemistry: A Molecular Approach 32 Electron Diffraction Wave Behavior of Electrons • de Broglie proposed that particles could have wave-like character • because it is so small, the wave character of electrons is significant • electron beams shot at slits show an interference pattern the electron interferes with its own wave • de Broglie predicted that the wavelength of a particle was inversely proportional to its momentum Tro, Chemistry: A Molecular Approach 33 34 Uncertainty Principle Complimentary Properties • when you try to observe the wave nature of the • Heisenberg stated that the product of the uncertainties electron, you cannot observe its particle nature – and visa versa in both the position and speed of a particle was inversely proportional to its mass x = position, Δx = uncertainty in position v = velocity, Δv = uncertainty in velocity m = mass wave nature = interference pattern particle nature = position, which slit it is passing through • the means that the more accurately you know the • the wave and particle nature of nature of the position of a small particle, like an electron, the less you know about its speed electron are complimentary properties as you know more about one you know less about the other Tro, Chemistry: A Molecular Approach Tro, Chemistry: A Molecular Approach however, electrons actually if electrons behave like present an interference particles, there should pattern, demonstrating the only be like two waves bright spots behave on the target and visa-versa 35 Tro, Chemistry: A Molecular Approach 36 6 12/17/11 The Franck-Hertz experiment The Franck-Hertz experiment • The electrons were accelerated by an electric field (Figure 4.) between the cathode (C) and the grid (G) in a tube filled with Hg vapour. After passing through the grid, an opposite field slowed down the electrons and prevented them from reaching the anode (A), unless they gained enough kinetic energy in the previous acceleration. The electrons reaching the anode formed a measurable current, I. Tro, Chemistry: A Molecular Approach • The Hg atoms can not absorb any energy, just well defined values, namely 4.9 eV. This energy excites the ground state electron to the first excited state, e.g. it equals exactly to the energy difference between the E1 and E2 states of the Hg atom. Thus, this experiment gives an excellent proof of the validity of Bohr’s theory. 37 Tro, Chemistry: A Molecular Approach orbital n Wave Function, ψ l 3 quantum numbers “address” ml magnetic • calculations show that the size, shape and orientation in space of an orbital are determined be three integer terms in the wave function added to quantize the energy of the electron -l, …, l orientation angular momentum • these integers are called quantum numbers 0, 1, 2, …, (n - 1) shape principal quantum number, n angular momentum quantum number, l magnetic quantum number, ml Tro, Chemistry: A Molecular Approach requires 38 Principal 1, 2, 3, … 39 Principal Quantum Number, n size and energy Principal Energy Levels in Hydrogen • characterizes the energy of the electron in a particular orbital corresponds to Bohr’s energy level • n can be any integer ≥ 1 • the larger the value of n, the more energy the orbital has • energies are defined as being negative an electron would have E = 0 when it just escapes the atom • the larger the value of n, the larger the orbital • as n gets larger, the amount of energy between orbitals gets smaller for an electron in H Tro, Chemistry: A Molecular Approach 41 Tro, Chemistry: A Molecular Approach 42 7 12/17/11 • ψ2 Probability & Radial Distribution Functions Probability Density Function is the probability density the probability of finding an electron at a particular point in space for s orbital maximum at the nucleus? decreases as you move away from the nucleus • the Radial Distribution function represents the total probability at a certain distance from the nucleus maximum at most probable radius • nodes in the functions are where the probability drops to 0 43 Radial Distribution Function Tro, Chemistry: A Molecular Approach 44 The Shapes of Atomic Orbitals • the l quantum number primarily determines the shape of the orbital • l can have integer values from 0 to (n – 1) • each value of l is called by a particular letter that designates the shape of the orbital s orbitals are spherical p orbitals are like two balloons tied at the knots d orbitals are mainly like 4 balloons tied at the knot f orbitals are mainly like 8 balloons tied at the knot Tro, Chemistry: A Molecular Approach 45 Tro, Chemistry: A Molecular Approach 46 2s and 3s l = 0, the s orbital 2s n = 2, l=0 • each principal energy state has 1 s orbital 3s n = 3, l=0 • lowest energy orbital in a principal energy state • spherical • number of nodes = (n – 1) Tro, Chemistry: A Molecular Approach 47 48 8 12/17/11 p orbitals l = 1, p orbitals • each principal energy state above n = 1 has 3 p orbitals ml = -1, 0, +1 • each of the 3 orbitals point along a different axis px, py, pz • 2nd lowest energy orbitals in a principal energy state • two-lobed • node at the nucleus, total of n nodes Tro, Chemistry: A Molecular Approach 49 Tro, Chemistry: A Molecular Approach 50 d orbitals l = 2, d orbitals • each principal energy state above n = 2 has 5 d orbitals ml = -2, -1, 0, +1, +2 • 4 of the 5 orbitals are aligned in a different plane the fifth is aligned with the z axis, dz squared dxy, dyz, dxz, dx squared – y squared • 3rd lowest energy orbitals in a principal energy state • mainly 4-lobed one is two-lobed with a toroid • planar nodes higher principal levels also have spherical nodes Tro, Chemistry: A Molecular Approach 51 Tro, Chemistry: A Molecular Approach 52 f orbitals l = 3, f orbitals • each principal energy state above n = 3 has 7 f orbitals ml = -3, -2, -1, 0, +1, +2, +3 • 4th lowest energy orbitals in a principal energy state • mainly 8-lobed some 2-lobed with a toroid • planar nodes higher principal levels also have spherical nodes Tro, Chemistry: A Molecular Approach 53 Tro, Chemistry: A Molecular Approach 54 9 12/17/11 Stern-Gerlach experiment Particles possess intrinsic angular momentum. Spin angular momentum is quantized (it can only take on discrete values) For the completely filled shells, subshell (4d10) the orbital magnetic momentum is zero; for the 5s orbital ML is also zero. Hypothesis: the argent atom possesses no magnetic momentum >> they move in an inhomogeneous magnetic field along the same path. The electron possesses an intrinsic angular momentum and an intrinsic magnetic momentum: spin S — intrinsic angular momentum Ms — intrinsic magnetic momentum 10