Atomic Models. 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
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The smallest particle of a substance (elements and compounds) having specific chemical and physical properties.
J.J. Thompson circa ~ 1904
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 ________________________________.
Circa ~ 1911
1 atomic diameter ~ 100,000 nuclear diameters
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.
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.
Neal Bohr circa~1913
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 ]
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
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 )