Neils Bohr
Atomic Theory
• Rutherford’s model did not explain how electrons were
arranged in the atom
• Bohr experimented on electron arrangement using the
spectra of elements (hydrogen in particular)
• His observations:
– White light (sunlight) consists of a continuous spectrum
(all colours or frequencies of visible light are present)
– When electricity passes through a gaseous element at low
pressure, light of a characteristic colour is given off
(Ne=red)
– If this light passes through a prism (crystal), a series of
coloured lines appear between black spaces
– Called a line spectrum
• Bohr proposed that electrons in atoms can move in
certain number of allowed energy levels
• Electrons can change energy levels by gaining energy
but cannot remain between levels
• When electron falls back to lower level, energy is
released (light)
• Energy given off=difference in distance between the 2
energy levels
• Energy released has specific values that correspond to
a line in the line spectrum
Bohr’s Proposal for Atomic Structure
1. Based on hydrogen atom, electron found in
certain allowed areas.
– These areas have definite fixed level of energy
2. Each energy level corresponds to an orbit (path
around nucleus) where electron can move
– First orbit = smallest radius
– 2nd orbit = larger radius
3. Electron can travel in its allowed orbit
indefinitely
– There is no loss of energy and atom will not collapse
4. Electron can change energy levels ONLY by
gaining a specific amount of energy (quanta)
– Electron must “jump” from one level to the next,
not halfway
– Quantity of energy needed for jump = difference
in energy between energy levels
Ground state
Excited electron
gains energy and
jumps to a higher
Energy level
Excited state
when electron falls back
to original energy level
energy in the form of light
is given off
This corresponds to a
specific line on the
spectrum
Downfalls:
1. Electrons do not move in an orbit at a fixed
distance from nucleus
2. Not all orbitals are circular
Line Spectrum
• Sun’s energy is transmitted to earth through
space by waves of radiation
• Visible light is one form of radiation
• All forms of radiation have their own
wavelength
• X-rays are short, radio waves are long
• Shorter wavelengths = more energy
• Full spectrum is made up of different
wavelengths
• Measured in nanometers
• One wavelength
Visible Light
• only a very narrow band of wavelength in
middle of spectrum can be absorbed by the
human eye
• Visible spectrum is a series of wavelengths
that make the colours red-orange-yellowgreen-blue-indigo-violet
•
•
•
•
Violet=shortest
Red=longest
Visible light = 400-700 nm range
White light=mixtures of all colours in
approximately the same amount
Homework
• Merrill text:
– Supplement notes p.87-91, p93-98
– P.103 #14-15
– Read p.90-1
• Nelson text
– Supplement notes p13-14
– p.16 #1-4, 6, 7
• Prep Line spectrum lab
• Prep for upcoming quiz