IB Physics Power Points Topic 07 and 13 Atomic and Nuclearwww.pedagogics.ca Physics Atomic Structure
Part 1Development of an Atomic ModelDemocritus to Niels Bohr
Early ideas about atomsDemocritus (~ 400 BC) proposedthat matter was composed ofindivisible particles he called“atomos”.This idea was largely ignored untilJohn Dalton (1766-1844) proposedhis atomic theory as a model formatterWhile this theory was not completelycorrect, it revolutionized howchemists looked at matter andbrought about chemistry as weknow it today instead of alchemy
Dalton’s Atomic Theory - Summary 1. matter is composed, indivisible particles (atoms) 2. all atoms of a particular element are identical 3. different elements have different atoms 4. atoms combine in certain whole- number ratios 5. In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements.
Dalton’s theory states 1. matter is composed of indivisible particles called atoms. (Dalton visualized atoms as dense spheres – not unlike billiard balls) However, the discovery of the electron by J.J. Thomson in the late 1800’s introduced the idea that there might be something smaller than atoms.
To includeelectrons, scientists hadto modify the Daltonatomic model. The newvision was a positivesphere that had thenewly discoverednegative electronsembedded in it. This wascalled the plumpudding model.
In the early 1900’s, Ernest Rutherford testedthe plum pudding model with his famousgold foil experiment. a More great info @ http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
Predicted results ofgold foil experiment ifthe Plum Puddingmodel had beencorrect.This is NOT what wasobserved.
What was observed? To explain the results, Rutherford proposed a new model that included a positive center for the atom called the nucleus.
Adding the evidence from emission spectraColored lights do not emit all the wavelengths of the visible light spectrum. For example, a red light emits mostly wavelengths from the red end of the spectrum. An energized gas sample will emit light of specific wavelengths characteristic of the gas. This is called a line spectrum
The Bohr model of the atom• developed using information from hydrogen emission spectrum studies. • a central dense positive nucleus composed of protons and neutrons. • negative electrons orbit the nucleus like planets around the Sun (but not flat like the solar system) • mostly empty space. Nucleus is 10-5 times smaller than atom.
Orbiting electrons occupy discrete energy levels!Electrons can only “jump” between energy levels ifthey absorb or emit a specific amount of energy. Atoms have quantized energy states!
The line spectrum of hydrogen as a direct resultof energized electrons releasing a specificamount of energy by emitting a photon of lightof a certain wavelength.The different lines in the hydrogen spectrumwhere evidence for a number of different energylevels.
lower energy higher energylonger wavelength shorter wavelength Visible spectrum for hydrogen atom convergence
Limitations of the Rutherford BohrNuclear ModelWhy do electrons orbit the nucleus? i.e. model doesnot explain why the electrons orbit rather than spiraltowards the nucleus.Why is there one mass of positive charge in thenucleus? i.e. or if there are multiple positive particleswhy does the nucleus not fly apart?
Part 2 Atomic Structure – The BasicsNuclide A specific atomic structure as determined by the number of protons and neutrons in the nucleusNucleon A nuclear particle (protons and neutrons)Mass The total number of nucleons (protons +Number neutrons)(A)Atomic The number of protons in the nucleusNumber (Z)Isotope Two atoms of the same element (same Z) but with different masses (A). The mass difference between isotopes is due to varying numbers of neutrons.
Atomic NotationShow the name of the element, and the massnumber in hyphen notation sodium-23Show the mass number and atomic number in A-Znotation (atomic number often omitted) mass number A 23 atomic number Z 11 Na