Friday  March 17, 2017 TODAY is the late/missing/redo work cut off! You don’t need your notebook. Turn in any Snapchat or Instagram sheets Extra credit ones are due no later than Monday. TODAY is the late/missing/redo work cut off!

The Bohr Model of the Atom Niels bohr The Bohr Model of the Atom NOTE: To change the image on this slide, select the picture and delete it. Then click the Pictures icon in the placeholder to insert your own image. Objective SWBAT: Explain how Niels Bohr arranged electrons in an atom. Determine an element’s atomic number, atomic mass, number of protons, number of neutrons and number of electrons by the information given on the periodic table.

Before Bohr… Thomson: “plum-pudding” model – basically, positively charged “goo” sprinkled with negatively charged particles. Discovered the electron Rutherford: Discovered the nucleus Discovered protons “Gold foil” experiment Something in the center of the atom that deflects alpha (low energy) particles Small, positive nucleus at the center of the atom But what were the electrons doing? (Planetary model – orbiting the nucleus?)

Niels Bohr Any charged particle that is moving is going to give off EM radiation (thus, it will continue to lose energy until the atom is basically destroyed) Bohr model was the first of its kind because it was the first model to suggest that the energy of an electron is “quantized” This means that the energy of an electron in an atom is limited to certain discrete values It is the emission spectra of the atoms that led to this model

Emission Spectrum (plural: spectra) How do atoms behave when they absorb energy? When they absorb energy (heat, light, electricity) they often re-emit that energy in the form of light

Here is the light emitted by different elements - Nitrogen, Neon, and Krypton Notice that each element gives off it’s own characteristic color of light BUT WHY??? If we inspect this light more closely we see a very revealing clue that shows how electrons exist within atoms

Review Passing light through a prism will separate the light into its constituent wavelengths. We can observe this from the white light of the sun or a light bulb

Line Spectrum The spectrum of light given off by atoms, the so-called emission spectrum, is composed of specific bright lines with dark spots in between – this is called a line spectrum. Each element has its own unique emission spectrum A specific emission spectrum can be used to identify an element. Astrophysicists are able to identify the elements that make up a star by obtaining the emission spectra from the light given off by the stars. (How cool?!)

How do these emission spectra change the way we think about atomic structure? Bohr proposed a model in which electrons could only exist in certain orbits, in certain fixed distances from the nucleus Also argued that the energy of an electron is limited to certain, discrete values… Location and energy are “quantized”

Imagine a ladder… Just as your feet can only stand on the rungs of a ladder but not in between, electrons can only exist in these orbits but not in between them…

When does an electron absorb or emit energy?? Only when it makes a transition (or jumps) between orbits

Emission lines of the Hydrogen spectrum Notice that the energy of the transition increases, the wavelength of the emitted light decreases…

Other transitions occur…

WHY can electrons only exist in certain fixed orbits?? These orbits are actually the result of electrons behaving as waves Who came up with this idea?? (hint… who came after Bohr in our video?) De Broglie, Heisenberg and Schrodinger **Remember, Bohr’s model is not the model we currently we accept today but still important because it served as an intermediate model between the classical understanding model of the electron and the fully quantum mechanical interpretation

Before we can practice creating Bohr models (Monday)… We need to look at the periodic table of elements (handout). How is it arranged? What does each square include?

Atomic Number Chemical Symbol Chemical Name Atomic Weight (Mass)

Atomic number represents the number of protons or electrons in an atom. Atomic weight represents the number of protons PLUS (+) the number of neutrons. Atomic Number Chemical Symbol Chemical Name Atomic Weight (Mass)

How would you… Figure the number of protons or electrons in an atom? The atomic number tells us how many protons and how many electrons. Figure the number of neutrons in an atom? Subtract the atomic number from the atomic mass.

Let’s practice! Atomic calculations half sheet. Homework?

Emission spectrum (plural: spectra) How do atoms behave when they absorb energy? When they absorb energy (heat, light, electricity) they often re- emit that energy in the form of light Let’s look at Hydrogen, for example…