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
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)
Higher Probability for e-
The higher the
electron density,
the higher the
probability that
an electron may
be found in that
region.
http://www.chemeng.uiuc.edu/~alkgrp/mo/gk12/quantum/H_S_orbital.jpg
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 #