Atomic Theory – Bohr & Chadwick

Let’s talk about Niels Bohr The Bohr model of the atom Recall: The Rutherford model positive nucleus negative electrons orbiting the nucleus

Niels Bohr (1922) Predicted that electrons are located in different “energy levels”

Features of Bohr’s model: electrons don’t just orbit randomly they are restrict to fixed orbits each orbit represents a different level of energy energy is quantized – i.e., electrons at each energy level possess a specific amount of energy Based his model on the line spectrum of hydrogen

The Line spectrum of Hydrogen Where do these lines come from??

Hydrogen’s electron absorbs EM energy Jumps to a higher energy level The electron is in an “excited state” unstable

As the electron falls back down to its normal stable state (“ground state”), it emits the energy it absorbed. Emitted energy is visible as its line spectrum.

Observations Multiple lines but only one electron The electron can absorb more than one amount of energy Spectrum is not continuous The electron can absorb only specific amounts of energy

Since each energy level is fixed, only certain amounts of energy can be absorbed or emitted. Link here: http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/free/0072482621/59229/Bohr_Nav.swf::The%20Bohr%20Atom

Bohr’s Model of the Atom

Bohr’s Model of the Atom Electrons can move to higher levels if energy is absorbed by the electron – the electron is temporarily unstable. Unstable electrons fall back down to their “normal” or lower level and release the stored energy as light of a specific wavelength (color).

Bohr model Limitations Was not useful for predicting the line spectra of other elements BUT was a good starting point for modern atomic theory

Our classmates are going to present information about electrons/energy levels and more and that is how we will learn more on this timeline of the atomic theory.

James Chadwick (1932) Discovered Neutrons Neutral particles located in the nucleus Bombarded different materials (paraffin, helium, etc.) with high energy gamma photons. Noticed that a particle with no charge was ejected

Unit 2 - Lecture 1: Structure of the Atom James Chadwick – The Neutron In the nucleus with the protons are particles of similar mass but no electrical charge called neutrons. Electrons (-) are now outside the nucleus in quantized energy states called orbitals. (From Niels Bohr and quantum mechanics) The positively charged particles in the nucleus are protons. + n n Structure of the atom after Chadwick (1932)

What we now consider facts about the atom As provided by MANY experiments and consistent results…

Unit 2 - Lecture 1: Structure of the Atom proton (+) neutron electrons - responsible for the volume and size of the atom, negatively charged 10-10 m nucleus - responsible for the mass of the atom, positively charged 10-14 m

Fill in your “Evolution of Atomic Theory” Timeline