Download Niels bohr

Friday  March 17, 2017
 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!
NIELS BOHR
The Bohr Model of the Atom
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…
1. Figure the number of protons or electrons in an atom?
The atomic number tells us how many protons and how
many electrons.
2. 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 reemit that energy in the form of
light
 Let’s look at Hydrogen, for
example…