light and the electron quantized energy. the wave-particle duality light sometimes behaves like a...
TRANSCRIPT
LIGHT AND THE ELECTRON
Quantized Energy
The Wave-Particle Duality
Light sometimes behaves like a wave. At other times, it acts as a particle.
Scientists have found strong evidence that light is both a particle and wave.
Therefore, light is considered to be both.
Light as a Wave
Light, and other types of waves, can be described in terms of wavelength, frequency, and amplitude.
Wavelength () - length of one complete wave
Frequency () - number of waves that pass a point during a certain time period (measured in hertz where 1 Hz = 1 cycle/s.)
Amplitude (A) - distance from the origin to the trough or crest
Low frequency
High frequency
Amplitude
Amplitude
long wavelength l
short wavelength l
Visible light is just one portion of the electromagnetic spectrum.
The Electromagnetic Spectrum
Light as a Particle
The observations of Planck and Einstein contributed to the particle theory of light.
Light is a stream of small particles or packets of energy called photons.
A photon is a massless particle of electromagnetic radiation that contains one quantum of energy.
EXCITED STATE VS. GROUND STATE
Electron States
Niels Bohr (1913): electrons can possess only certain amounts of energy, and can therefore be only certain distances from nucleus.
When all electrons are in the lowest possible energy state, an atom is in the _________________________.
If the “right” amount of energy is absorbed by an electron it can “jump” to a higher energy level. This is an unstable, momentary condition called the __________________________.
Energy Level Concept
e-e- Ground state
Excited state
Electrons can only be atspecific energy levels,NOT between levels.
Color = Energy of Photons
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 329
Energy Level
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 329
A
B
C
D
Ground state
En
erg
y
Fourexcitedstates
An Excited Lithium Atom
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 326
Photon ofred lightemitted
Li atom inlower energy state
Excited Li atom
En
erg
y
Excitation of Hydrogen Atoms
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 328
Return to Ground State
Release of Energy
When an electron falls back to a lower-energy, more stable orbital (it might be the orbital it started out in, but it might not), the atom releases the “right” amount of energy as light.
Any-old-value of energy to be absorbed or released is NOT OK. This explains the lines of color in an emission spectrum.
Atomic Emission Spectra
The atomic emission spectrum of an element is a set of specific wavelengths of light (certain colors) that are emitted by atoms of that element.
Each element’s emission spectra is unique—it something like a fingerprint for an element. An emission spectra can be used to identify an element.
The emission spectrum of an element can be seen be using a prism to separate the light from a glowing sample of the element (such as neon in a neon light).
Emission Spectrum of Hydrogen
1 nm = 1 x 10-9 m = “a billionth of a meter”
410 nm 434 nm 486 nm 656 nm
Continuous and Line Spectra
Why are emission spectra important?
Historically, the emission spectra of the elements were a puzzle for many scientists. Based on classical (older-style) physics, many scientists thought the emissions spectra should be continuous (like a rainbow).
The fact that the elements had line spectra defied the common theory of the time.
This discrepancy eventually led to discovery of energy levels and the development of quantum mechanics.
Lab Questions
1. How can the existence of line spectra help to prove that energy levels in atoms exist?
2. How can the single electron in a hydrogen atom produce all of the lines found in its emission spectrum?
3. How can spectra be used to identify the presence of specific elements in a substance?