Atomic Theory Lecture 1 PDF

PL1001 Pharmaceutical Chemistry ATOMIC THEORY Lecture 1 Dr Alberto Di Salvo Recommended reading Chemistry and Chemical Reactivity by John C. Kotz; Paul M. Treichel; John Townsend. Available in the library in paperback (7th edition, 2 copies and 8th edition, 2 copies) and...

PL1001 Pharmaceutical Chemistry ATOMIC THEORY Lecture 1 Dr Alberto Di Salvo Recommended reading Chemistry and Chemical Reactivity by John C. Kotz; Paul M. Treichel; John Townsend. Available in the library in paperback (7th edition, 2 copies and 8th edition, 2 copies) and as an e-book. All general chemistry textbooks have a section on atomic theory. What comes to mind when we think about an atom? Flaw: an orbit is defined as a curved path followed by a planet (a very large body) in space. Recently actual “picture” of atomic orbitals obtained with a Field Emission Electron Microscope s orbital on the left hand side and p orbital on the right hand side first ever image of atomic orbitals Adapted from I. M. Mikhailovskij et al, Physical Review B 2009, 80, 165404 Atomic theory timeline A B C D E A – Greek (400 BC) and Dalton’s (1803) model B – Thomson’s model (1904) C – Rutherford’s model (1911) D – Bohr’s model (1913) E – Modern atomic model (quantum mechanics) Greek model The universe is composed of two elements: the atoms and the void in which they exist and move. Democritus Dalton’s model 1) All matter is made of atoms. Atoms are indivisible and indestructible. 2) All atoms of a given element are identical in mass and properties. 3) Compounds are formed by a combination of two or more different kinds of atoms. 4) A chemical reaction is a rearrangement of John Dalton atoms. Thomson’s model Discovered the first sub- atomic particle – the corpuscle (electron). Joseph John Thomson Atoms are uniform spheres of positively charged matter (dough) in which electrons (resins) are embedded. Rutherford’s model Discovered the second sub-atomic particle – the nucleus. Ernest Rutherford Most of the atomic mass is concentrated in the positively charged nucleus. The negatively charged electrons, smaller in size, orbit around the nucleus like in a small scale cosmic model. Bohr’s model 1) Electrons rotate around the nucleus in orbits that have a set size and energy. 2) The energy of the orbit is related to its size. The lowest energy is found in the smallest orbit. 3) Radiation is absorbed or emitted when an electron moves from one orbit to another. Niels Bohr Modern atomic model Quantum mechanics (wave-particle) theory. Erwin Schrödinger Werner Heisenberg 1) Bohr’s orbits (energy levels or shells) are quantised and take the name of orbitals (valid solutions of Schrödinger’s equation). 2) Orbitals are regions (of space) where electrons are likely to be found (Heisenberg’s Uncertainty Principle).  Conceptual contributions timeline Electron Indivisible Electron Nucleus Orbit Cloud Greek X Dalton X Thomson X Rutherford X X Bohr X X X Modern X X X Electron vs Golf ball The wavelength of a moving body is inversely proportional to its momentum. momentum = mass (kg) x velocity (m/s) Louis de Broglie Property Electron Golf ball h Mass (kg) 9.11x10-31 0.01 𝜆= 𝑚𝑣 Velocity (m/s) 100 100 Momentum (kg m/s) 9.11x10-29 1 Wavelength (m) 7.27x10-6 6.626x10-34 h= Planck’s constant Large λ Small λ (6.626 x 10-34 J s) Mechanics theory Quantum Classical The motion of large bodies is better described by classical mechanics, while the motion of particles is better described by quantum mechanics. Electromagnetic radiation Electromagnetic radiation Waves have a frequency Use the Greek letter “nu”, , for frequency, and units are “cycles per second” or Hertz (s-1). Waves have a wavelength “lambda” -  All radiation:  = c c = velocity of light = 3.00 x 108 m/s Long wavelength = small frequency Short wavelength = high frequency Electromagnetic radiation – worked example Red light has  = 700 nm. Calculate the frequency. All radiation:  = c Where C = 3.00 x 108 m/s (velocity of light) 1. Convert the wavelength into meters 700 −7 λ= 9 =7.00 𝑥 10 𝑚 1 𝑥 10 2. Calculate the frequency 8 3.00 𝑥 10 14 − 1 ν= =4.29 𝑥 10 𝑠 7.00 𝑥 10− 7 Energy of a radiation Energy of radiation is proportional to frequency E E == hh ·· Max Planck h = Planck’s constant = 6.626 x 10-34 J·s Light Light with with large large  (small (small )) has has aa small small E. E. Light Light with with aa short short  (large (large )) has has aa large large E. E. E λ ↓ ↓ ↑ ↑ ↑ ↓ Electromagnetic radiation Short wavelength high frequency high energy Long wavelength low frequency low energy Electromagnetic Spectrum Energy quantisation E E == hh ·· The relationship between the temperature of a black body and the intensity of energy it emits as radiation follows a stepwise relationship. Each of these steps is called quantum. Photoelectric effect E E == nh nh The outcome of this experiment demonstrates the particle nature of light. Photoelectric effect E E == nh nh Classical theory stated that E of ejected electron should steadily increase with an increase in light intensity (not observed!) No e- is ejected until light of a certain minimum E is used. Albert Einstein Number of e- ejected (n) depends on light intensity. Photoelectric effect Experimental observations can be explained on the basis that light consists of particles called PHOTONS of discrete (quantised) energy. PROBLEM: PROBLEM: CalculateCalculate thethe energy energy of of 11 mole mole of of photons photons of of red red light. light.  == 700 700 nm nm == 4.29 x 10 14 sec-1 14 4.29 x 10 sec-1 Energy of Radiation – worked example Energy of 1 mole of photons of red light. E = h· = (6.63 x 10-34 J·s)(4.29 x 1014 s-1) = 2.85 x 10-19 J per photon E per mol = (energy of a photon x n photons in a mole) (2.85 x 10-19 J/ph)(6.02 x 1023 ph/mol) = 172 kJ/mol This is in the range of energies that can break chemical bonds.