 # Bohr vs. Electron Cloud.

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Bohr vs. Electron Cloud

Where are we so far? Students can:
Find the atomic number, the number of protons, neutrons, electrons, and atomic mass of an atom Find the number of valance electrons Draw a Bohr Model of the atom Draw a Lewis Dot Diagram of an atom Purpose – to find the number of bonding sites for atoms

Bohr vs. Electron Cloud Models
Date Bohr vs. Electron Cloud Models Objective: Students will take notes and draw diagrams to explain the advantages and disadvantages of Bohr’s model and the Electron Cloud model of the atom. Page #

Stable Atoms (Magic Number 8!)
Atoms with a full outer shell (8 e-) are stable They don’t react or bond with any other element Ex: He = 2 valence electrons in the first shell Ne = 8 valence electrons in the second shell Ar = 8 valence electrons in the third shell These are the Noble Gasses!

Niels Bohr’s Model (1913) Electrons orbit the nucleus in circular paths of fixed energy (energy levels).

The energy levels are like the rungs of a ladder but are not equally spaced.
Energy levels. The energy levels in an atom are similar to the rungs of a ladder, but they get closer together as they get farther from the nucleus.

Excited State and Ground State
Ground state: the lowest possible energy level an electron can be at. Excited state: an energy level higher than the ground state.

Niels Bohr’s Atom Cont’d
Electrons can jump from energy level to energy level. Electrons ABSORB light energy when they jump from a lower energy level to a higher energy level. Electrons EMIT light energy when they jump from a higher energy level to a lower energy level.

(Draw this) E=hn

Bohr Model for Hydrogen

Hydrogen Emission Spectrum
Violet Blue Red Balmer Series

EVIDENCE: The Bohr model explained the emission spectrum of the hydrogen atom ….. but was not able to explain the spectra of other elements.

Successes and Problems of the Bohr Model
Introduced connection between spectra and electron structure Concept of ‘allowed orbits’ or orbitals is developed further with new knowledge Was an important contribution to the model of the atom - worthy of the Nobel prize Couldn’t explain why orbits were allowed Only successful agreement with experiment was with the H atom…not with any other elements

The All New, Better Electron Cloud Model (Quantum Mechanical Model)
A quantum of energy is the amount of energy required to move an electron from one energy level to another.

Electron Cloud Model cont.
Electrons (e-) are located in specific energy levels. There is no exact path for electrons around the nucleus… instead… The model estimates the probability of finding an electron in a certain position… called an orbital or electron cloud

Atomic Orbital / Electron Cloud
A region in space in which there is high probability of finding an electron.

Atomic Orbital / Electron Cloud
(Draw this) The higher the electron density, the higher the probability that an electron may be found in that region. Higher Probability for e-

Bohr vs. Electron Cloud Models
Date Bohr vs. Electron Cloud Models Objective: Students will take notes and draw diagrams to explain the advantages and disadvantages of Bohr’s model and the Electron Cloud model of the atom. Page #

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