CHAPTER 1 ATOMS: THE QUANTUM WORLD General chemistry 1 1 MSc. Nguyen Van Kiet 1 CONTENTS INVESTIGATING ATOMS 1.1 The Nuclear Model of the Atom 1.2 The Characteristics of Electromagnetic Radiation 1.3 Atomic Spectra QUANTUM THEORY 1.4 Radiation, Quanta, and Photons 1.5 The Wave-Particle Duality of Matter 1.6 The Uncertainty Principle 1.7 Wavefunctions and Energy Levels General chemistry 1 2 MSc. Nguyen Van Kiet 2 1.1 The Nuclear Model of the Atom ➢ Discovering the electron J. Thomson (English physicist, 1856- VIDEO 1 VIDEO 2 1949) in 1897 discovers the electron and determines the charge to mass ratio (e/me) as “cathode rays”.
In 1906 he wins the Nobel Prize. General chemistry 1 3 MSc. Nguyen Van Kiet 3 1.1 The Nuclear Model of the Atom Thomson found that the particles that compose the cathode ray have the following properties: – They travel in straight lines – They are independent of the composition of the material from which they originate (the cathode) – They carry a negative electrical charge Thomson was able to measure the value of e/me, the ratio of the magnitude of the electron’s charge e to its mass me. General chemistry 1 4 MSc.
Nguyen Van Kiet 4 1.1 The Nuclear Model of the Atom American physicist Robert Millikan (1868–1953), performed his now famous oil drop experiment in which he deduced the charge of a single electron. Fundamental charge, the smallest increment of charge e = 1.602×10-19 C From the value of e/me measured by Thomson (1.109×10-31 kg VIDEO 3 General chemistry 1 5 MSc. Nguyen Van Kiet 5 1.1 The Nuclear Model of the Atom J. Thomson had discovered the electron, a negatively charged, low mass particle present within all atoms.
The atoms overall have zero charge (neutral); By the beginning of the twentieth century scientists knew that each atom must contain enough positive charge to cancel the negative charge; General chemistry 1 6 MSc. Nguyen Van Kiet 6 1.1 The Nuclear Model of the Atom J. Thomson proposed that the negatively charged electrons were small particles held within a positively charged sphere. Plum-Pudding model The electrons suspended in it like raisins in pudding → this model was overthrown in 1908 by another experimental observation General chemistry 1 7 MSc.
Nguyen Van Kiet 7 1.1 The Nuclear Model of the Atom Ernest Rutherford (1871-1937) and his students in England studied α emission from newly-discovered radioactive elements. a piece of platinum foil only a few atoms thick VIDEO 4 Experiment by Geiger and Marsden Nucleus occupy a small volume at the center of the atom Nucleus contains particles called proton (+e) and neutron General chemistry 1 8 MSc. Nguyen Van Kiet 8 1.1 The Nuclear Model of the Atom Building on this idea, he proposed the nuclear theory of the atom, with three basic parts: 1. Most of the atom’s mass and all of its positive charge are contained in a small core called a nucleus.
Most of the volume of the atom is empty space, throughout which tiny, negatively charged electrons are dispersed. There are as many negatively charged electrons outside the nucleus as there are positively charged particles (named protons) within the nucleus, so that the atom is electrically neutral. General chemistry 1 9 MSc. Nguyen Van Kiet 9 1.1 The Nuclear Model of the Atom 2 amu 4 amu Unbalanced Rutherford Model Later work by Rutherford and one of his students, British scientist James Chadwick (1891–1974), demonstrated that the previously unaccounted for mass was due to neutrons, neutral particles within the nucleus.
General chemistry 1 10 MSc. Nguyen Van Kiet10 1.1 The Nuclear Model of the Atom ➢ Discovering the Neutron Gamma rays VIDEO 4 Although Rutherford’s model was highly successful, scientists realized that it was incomplete. General chemistry 1 11 MSc. Nguyen Van Kiet11 1.1 The Nuclear Model of the Atom The Neutrons ❑ The mass of a neutron is similar to that of a proton; ❑ However, a neutron has no electrical charge – The helium atom is four times as massive as the hydrogen atom because – Hydrogen, on the other hand, contains only one proton and no neutrons.
General chemistry 1 12 MSc. Nguyen Van Kiet12 1.1 The Nuclear Model of the Atom • All atoms are composed of the same subatomic particles: Protons; Neutrons; Electrons • The charge of the proton and the electron are equal in magnitude but opposite in sign. The neutron has no charge. General chemistry 1 13 MSc.
Nguyen Van Kiet13 1.1 The Nuclear Model of the Atom The number of protons in the nucleus is different for each element and is called the atomic number, Z, of the element General chemistry 1 14 MSc. Nguyen Van Kiet14 1.1 The Nuclear Model of the Atom Some Questions Posed by the Nuclear Model: 1. How are the electrons arranged around the nucleus? 2. Why is the nuclear atom stable? (classical physics predicts instability) 3.
What holds the protons together in the nucleus? → Atomic spectroscopy (involving the absorption by, or emission of electromagnetic radiation from atoms) provided many of the clues needed to answer questions 1 and 2. General chemistry 1 15 MSc. Nguyen Van Kiet15 1.2 Electromagnetic Radiation Spectroscopy – the analysis of the light emitted or absorbed by substances Light is a form of electromagnetic radiation, which is the periodic variation of an electric field (and a perpendicular magnetic field) Amplitude: the height of the wave above the center line Intensity: the square of the amplitude wavelength (brightness) (the Greek): peak-to-peak distance wavelength × frequency = speed of light .9979 x 108 ms-1 General chemistry 1 16 MSc. Nguyen Van Kiet16 1.2 Electromagnetic Radiation The color of visible light depends on its frequency and wavelength; long-wavelength radiation has a lower frequency than short-wavelength radiation.
infrared > 800 nm: the radiation of heat visible light = 700 nm (red light) to 400 nm (violet light) ultraviolet < 400 nm responsible for sunburn General chemistry 1 17 MSc. Nguyen Van Kiet17 1.2 Electromagnetic Radiation General chemistry 1 18 MSc. Nguyen Van Kiet18 Types of Radiation General chemistry 1 19 MSc. Nguyen Van Kiet19 Examples 1.
Calculate the wavelengths of the light from traffic signals as they change. Assume that the lights emit the following frequencies: green, 5.998 x 108 m/s Solution: Green light = 5.21 x 10-7 m = 521 nm Similarly, yellow light is 582 nm and red light is 702 nm wavelength General chemistry 1 20 MSc. Nguyen Van Kiet20 1.3 Atomic Spectra Using the properties of electromagnetic radiation to investigate atomic structure. continuous spectrum White light passed through a prism Discharge lamp of hydrogen (emission spectrum) discrete energy spectral lines levels General chemistry 1 21 MSc.
Nguyen Van Kiet21 1.3 Atomic Spectra Johann Rydberg’s general empirical equation R (Rydberg constant) = 3.29×1015 Hz an empirical constant n1 = 1 (Lyman series), ultraviolet region n1 = 2 (Balmer series), visible region n1 = 3 (Paschen series), infrared region For instance, n1 = 2 and n2 = 3, = 6.57×10-7 m General chemistry 1 22 MSc. Nguyen Van Kiet22 Examples 2. Calculate the wavelength of the radiation emitted by a hydrogen atom for n1 = 2 and n2 = 4. Identify the spectral line Solution 1 _ 1 3 = R = R 22 42 16 c 16c 16 x 2.86 x 10-7 m or 486 nm It is the second (green/blue) line in the spectrum.
General chemistry 1 23 MSc. Nguyen Van Kiet23 1.3 Atomic Spectra Absorption Spectra When white light passes through a gas (hydrogen), radiation is absorbed by the atoms at wavelengths that correspond to particular excitation energies. The result is an atomic absorption spectrum. A series of dark lines (absorption lines) on an otherwise continuous spectrum.
Have the same frequencies as the lines in the emission spectrum and suggest that an atom can absorb radiation only of those same frequencies. General chemistry 1 24 MSc. Nguyen Van Kiet24 1.3 Atomic Spectra Knowledge Astronomers are able to use absorption spectra to identify elements in the outer layers of stars, because each element has a characteristic absorption spectrum, and the star itself is a good source of nearly white light from below those outer layers. We can begin to understand the presence of spectral lines in an emission spectrum if we suppose that when it is part of a hydrogen atom an electron can exist with only certain energies, and that a line arises from a transition between two of the allowed energies.
The presence of spectral lines of specific frequencies suggests that the energy of an electron in an atom is restricted to a series of discrete values, called energy levels. The difference in the energies of two levels is carried away by the electromagnetic radiation emitted by the atom. General chemistry 1 25 MSc. Nguyen Van Kiet25 1.4 Radiation, Quanta, and Photons QUANTUM THEORY Some problems: 1.
Toward the end of the nineteenth century, scientists became increasingly perplexed as they gathered more information about electromagnetic radiation that could not be explained by classical mechanics, and the lines in the spectrum of hydrogen remained deeply puzzling. Then, from 1900 on, a series of imaginative suggestions was made. By 1927 the puzzles had been resolved, only to be replaced by new and even more intriguing puzzles. two phenomena that classical physics was unable to explain (along with atomic line spectra): black body radiation and the photoelectric effect.
General chemistry 1 26 MSc. Nguyen Van Kiet26 1.4 Radiation, Quanta, and Photons Important clues to the nature of electromagnetic radiation came from observations of objects as they are heated. Qualitative observations: As the object is heated to higher temperatures it glows more brightly, and the color of light it gives off changes from red through orange and yellow toward white. To understand what the color changes mean, scientists had to study the effect quantitatively: They measured the intensity of radiation at each wavelength and repeated the measurements at a variety of different temperatures.
These experiments led to one of the greatest revolutions that has ever occurred in science. General chemistry 1 27 MSc. Nguyen Van Kiet27 1.4 Radiation, Quanta, and Photons Black body radiation is the radiation emitted at different wavelengths by a heated black body, for a series of temperatures. As the temperature rises, the maximum intensity of the radiation emitted occurs at shorter and shorter wavelengths Two empirical laws are associated with it Stefan-Boltzmann law (in 1879): Total intensity = constant × T4 The experimental value of the constant is 5.mm General chemistry 1 28 MSc.
Nguyen Van Kiet28 Examples Astronomers are often very interested in the temperatures o f stars (including the Sun) because that gives a clue to their size, composition, and age. The maximum intensity of solar ra diation occurs at 490. What is the temperature of the surface of t he Sun? 4. In 1965, electromagnetic radiation with a maximum of 1.76K discovered to pervade the universe.
What is the temperature of ‘empty’ space? General chemistry 1 29 MSc. Nguyen Van Kiet29 Black Body Radiation Theories Rayleigh-Jeans theory: – based on classical physics. – assumed the black body atoms behave like mechanical oscillators that absorb and emit energy continuously. 8kBT Radiant energy density = 4 – equation did not agree with experimental data, except at high wavelengths.
Classical physics predicts intense UV or higher energy radiation from hot black bodies! General chemistry 1 30 MSc. Nguyen Van Kiet30 Planck’s Quantum Theory • Max Planck (1900) made the assumption that black body atoms could absorb and emit energy only in multiples of a fundamental quantity (a quantum), whose value is E = h • The constant h as Planck’s constant (= 6.626 x 10-34 Js) • The Planck equation describing the black body radiation profile agreed well with experiment. 1 8hc Radiant energy density = _ hc 5 kB T _ e 1 General chemistry 1 31 MSc. Nguyen Van Kiet31 1.