Atomic Models

1. Atomic Models

2. Atomic Models • What does the Atom Look Like? • Since the micro fine structure of the atom can not be viewed directly, deductive process must be used to discern its structure. • The language of the atom includes • Electrostatic behavior; i.e., positive/negative interaction • Responses to Heat, Light and Electricity • Observed Wave-Partical Duality of the Electron

3. Atomic Models • The history of atomic structure dates back as far as Democrites (600 BC) who concluded from a purely philosophical point of view that if matter were to be subdivided continusously, one would eventually come to an ‘ulitmately small’ particle That could not be subdivided any longer. This he called the ‘atomosparticle’ or indivisible particle. • Such is the origin of the term ‘atom’.

4. Atomic Models • The modern atomic model has evolved from application of the scientific method to experimental results associated with the study of matter. • The Modern Definition of the Atom The smallest particle of a substance (elements and compounds) having specific chemical and physical properties.

5. Atomic Models • Dalton’s Atomic Theory • Although Dalton did not define the structure of the atom, he did establish conclusions based upon the scientific method. • Dalton’s Postulates: • All matter is composed of particles – atoms, elements and molecular compounds, • Particles of matter interact by attractive and repulsive forces, • Particles of matter are in constant motion demonstrating translation, rotation and vibrational modes, • The velocity of particles of matter are directly proportional to their kinetic energy content, and • Collisions between particles of matter are considered to be perfectly eleastic;i.e., sum of all energy change in any collision between particles equals zero.

6. Atomic Models • Historically, four atomic models (structure discriptions) have evolved: • Thompson ‘Raisin Pudding’ of ‘Plum Pudding’ Model • Rutherford ‘Shell’ or ‘Nuclear’ Model • Bohr ‘Concentric Ring’ or ‘Planetary’ Model • Schrodinger ‘Quantum’ Model

7. Atomic Models • The Thompson ‘Raisin Pudding’ or ‘Plum Pudding’ Model • Based upon ‘opposites attract’ concept • This model promotes a spherical electronic cloud of positive charge embedded with negatively charged particles, or electrons. J.J. Thompson circa ~ 1904

8. Atomic Models • At the time of Thompson’s Experiments, science was aware of radioactivity which consists of ‘cathode rays’ having negative charges. • Thompson is given credit for discovery of the electron in 1894 using cathode ray tubes. • Such is observed using cathode ray tubes.

9. Atomic Models Development of Atomic Theory: Part 1 (Democritus to Thomson) -Who was Democritus? -What did he conclude when he thought about cutting a piece of matter in half over and over? -What does the Greek word atomos” mean? -What is our modern definition of the atom? -In the late 1700’s a British chemist and school teacher, John Dalton, brought back Democritus’s idea of the atom. What were the postulates of Dalton’s Atomic Theory? -In 1897 a British scientist named Thomson discovered that atoms were not indivisible but were made up of smaller particles. What particle did Thomson discover and what electrical charge does it have? The particle Thomson discovered was the ____________. It has a ___________charge. -Draw a diagram that shows the “Cathode Ray Experiment” that Thomson used to prove that atoms included smaller particles. -Thomson’s model was called the ________________________________.

10. Atomic Models • The Rutherford ‘Shell’ Model of the Atom • Based upon conclusions derived from the classic ‘Gold Foil’ Experiment. Earnest Rutherford Circa ~ 1911

11. Atomic Models • The Rutherford ‘Shell’ Model of the Atom • Rutherford visualized the occurrences as follows:

12. Atomic Models • Conclusions of Rutherford Model • The atom consists of two parts, a dense indivisible nucleus and a diffuse electron cloud of negatively charged particles. • The atom is mostly space: • Average Nuclear Diameter ~ 10-13 cm. • Average Atomic Diameter ~ 10-8 cm. • The nuclear mass comprised ~99.973 wt% of the atom with the electronic cloud ~0.027 wt%. • The average atomic volume is ~ 1000 million million nuclear volumes. That is: 1 atomic diameter ~ 100,000 nuclear diameters

13. Atomic Models Development of Atomic Theory: Part 2 (Rutherford to Modern Theory) In 1909 a former student of Thomson’s, Ernest Rutherford decided to test the idea that electrons are evenly distributed throughout the atom. Draw a diagram that shows Rutherford’s Gold Foil Experiment. What did Rutherford discover about the movement of the particles that he shot at the gold foil? -Most of the particles _________________________________________________________ -But surprisingly a few particles _______________________________________ and some even ________________________________ Due to the results of his experiment Rutherford revised the atomic theory in 1911. Draw a revised model of the atom based upon Rutherford’s conclusions. Rutherford Postulates: Most of the positively charged alpha particles went straight through the gold foil. -Most of the matter of the atom is found in a _________________part of the atom. This is called the ___________ of the atom. It is very tiny and extremely __________. -Some of the positively charged particles were deflected or even bounced back. -Since like charges repel, the nucleus must have a _____________________ charge. -If electrons have a negative charge they could not be in a positively charged nucleus. Electrons must be located ______________________________________. The diameter of the nucleus is ______________ times smaller than the diameter of the entire gold atom which implies that atoms are mostly space with a tiny massive nucleus comprising about _______________ wt% of the mass of the atom.

14. Atomic Models • The Bohr ‘Concentric Ring’ Model • The ‘Concentric Ring’ Model of the atom was proposed by Neal Bohr ~ circa 1913 proposed an atomic model consisting of a positively charged nucleus surrounded by a series of electron energy levels containing electrons orbiting the nucleus much like the planets orbit the sun. Neal Bohr circa~1913

15. Atomic Models • The Bohr ‘Concentric Ring’ Model • The Bohr Concentric Ring Model is based upon conclusions derived from the study of the ‘Bright Line Emission Spectra’ of the Hydrogen atom.

16. Atomic Models • The Bohr ‘Concentric Ring’ Model • The ‘bright lines’ indicate ‘Discrete Energy Transitions’ of electrons moving from one energy level to another; i.e., ‘Quantum Transitions’ or ‘Quantum Leaps’.

17. Atomic Models • The Bohr ‘Concentric Ring’ Model • The Visible Transition Spectrum

18. Atomic Models • Equations for Bohr Model Analysis • Wave Equations c = f•λ => f = c/ λ => λ = c/f ( c = 3 x 108 m/sec ) [ speed of light(c), frequency (f), wavelength (λ) ΔEt = h•f = h•c/λ = Transition Energy [ h = Planck’s Constant = 6.63 x 10-23 J-sec ] • Bohr - Rydberg Equations En = -(A/n2) = Positional Energy ΔEt = -A(1/ni2 – 1/nf2); A = 2.18 x 10-18 joule = Transition Energy 1/ λ = RH (1/ni2 – 1/nf2); RH = 109,678 cm-1 = Reciprocal Wavelength

19. Atomic Models In 1913 Neals Bohr proposed the concentric ring model of the atom. Draw a typical model diagram of the Bohr model. -What did Bohr conclude about the behavior of the electron? -What are the possible energy level transitions of the electron? -Which energy level transitions are considered to be visible spectrum transitions. What was the name given to this series of transitions? -Calculate the energy released for an electron transition form energy level n=3 to n= 2. -What wavelength of electromagnetic radiation does a 3 to 2 transition give off and is it visible to the human eye? -For the hydrogen atom, draw the bright line spectrum and identify the associated wavelengths for each band. Is this an emission spectrum or adsorption spectrum? -What frequency does the blue line of the hydrogen visible EMR spectrum have? ( λblue = 434 nm ). -Use the Rydberg wave equation to calculate the wavelength of an n = 4 to n = 2 transition. Express final answer in nanometers. ( RH = 109,678 cm-1 )

20. Atomic Models • The Schrodinger Modern Quantum Model Under Construction