Transcript Chapter 7
Chapter 7: Quantum theory of the atom Chemistry 1061: Principles of Chemistry I Andy Aspaas, Instructor Atomic emission and line spectra • When different compounds are burned, they give off surprisingly different colors of light – It can be used to identify certain compounds • If the emitted light is sent through a prism so the colors are separated, only certain discrete colors of light are given off (atomic line spectrum) • The color of light can be related to the amount of energy that light contains The wave nature of light • Electromagnetic radiation: energy that is in the form of a wave, (visible light, x-rays, radio waves, etc) • Wavelength, : distance between any two adjacent identical points of a wave – Visible light, wavelength measured in nm – Radio waves can be measured in m • Frequency, (nu): number of wavelengths that pass a fixed point in one unit of time (usu. 1 second) Electromagnetic spectrum Frequency and wavelength • All electromagnetic waves travel at the speed of light, c = 3.00 x 108 m/s • c = , if is in m, and is in sec-1 – Visible light wavelengths are always given in nm, between 400 and 800 nm – Frequency is usually given in sec-1, or Hz The particle nature of light • While light has wave-like properties, it also has particle-like properties • Photon: discreet particles of energy which make up light (or any electromagnetic radiation) • The energy of one photon of light is related to the frequency of that light E = h • (where h is Planck’s constant, 6.63x10-34 J·s) • This relates the wave-like and particle-like properties of light More about atomic line spectra • Heated solid metals emit light of all wavelengths, or a continuous spectrum – Would form a rainbow if sent through a prism • Heated gases emit light of only particular wavelengths, or a line spectrum – Would form only lines of particular colors if sent through a prism – These lines are associated with energy level transitions Energy levels • Electrons can have only specific energy values in an atom (energy levels) – Energy levels are quantized (only specific allowed values) • When an electron absorbs energy from the environment, it can be promoted to a higher energy level • In order for it to return to a lower level, energy must be released in the form of a single photon – Depending on which levels this transition involves, the photon will have a different amount of energy H atom energy level calculations • Energy levels are numbered with integers starting with 1, symbol is n n = 1, 2, 3, … • The energy of a particular level is given by E = -(RH) / (n2) where RH = 2.179 x 10-18 J • The energy of a photon given off can be calculated by subtracting the lower energy level from the higher energy level (energy of a photon is positive) Quantum mechanics • Just like light can be wave-like and particle-like, so can electrons • The most accurate description of an electron’s behavior is using a wave-like interpretation, this is known as quantum mechanics • An electron can be described by a wavefunction – an equation for the wave that represents an electron • Only the probability of an electron appearing in a certain place can be calculated – Heisenberg uncertainty principle says the more precisely you know the position of a small particle, the less precisely you know its momentum Atomic orbitals • The 3-dimensional space in which there is a high probability of finding an electron in an atom is referred to as an atomic orbital • Can be described by three quantum numbers – Principal quantum number, n: refers to the energy of an electron, it also associates with the size of an orbital (n = 1, 2, 3, 4,…) Atomic orbitals • Angular momentum quantum number, l: indicates shape of orbital (l = 0, 1, 2, 3, …. n-1) – Usually shown by letters: s, p, d, f, and g • Magnetic quantum number, ml: Distinguishes orbitals of same shape but different position (ml = integers from –l to +l) • Spin quantum number, ms: indicates which of 2 possible spin states an electron is in, equal to either -1/2 or +1/2 Permissible atomic orbitals for n = 1, 2, 3 n l ml Notation # orbitals 1 0 0 1s 1 2 0 0 2s 1 2 1 -1, 0, +1 2p 3 3 0 0 3s 1 3 1 -1, 0, +1 3p 3 3 2 -2, -1, 0, +1, +2 3d 5 Atomic orbital shapes