Light, Electromagnetic Spectrum, & Atomic Spectra

Energy Absorbed = Energy Released = Energy of Light Produced Movement of Electrons e e e* An electron absorbs energy (in photons or quanta) to jump to a higher energy level known as the EXCITED STATE An electron emits energy (in photons or quanta) to fall back to original energy level known as the GROUND STATE Light is produced when electron EMITS energy and falls back to its ground state. Energy Absorbed = Energy Released = Energy of Light Produced

Then what was wrong with Bohr’s Model? Bohr’s Model DID help explain how electrons moved around the nucleus. Bohr’s Model DID NOT account for the behavior of larger atoms with more than one electron. Bohr ONLY looked at the HYDROGEN atom.

The Bohr Hydrogen Model Reference Table How to Read the Diagram Shows the movement of electrons around the nucleus (solid black dot in center). Electrons are represented by little black dots on the rings. Arrows from the electrons show the electrons are JUMPING DOWN (losing energy). Light is only given off when electrons move DOWN in energy levels.

The Bohr Hydrogen Model Reference Table How to Read the Diagram Each electron transition corresponds to a specific wavelength. These electrons only show wavelengths in the IR, Visible, and UV spectrum. Wavelengths from the Visible spectrum correspond to a specific color. (Make sure to change UNITS!)

Example Problem: When an electron falls from n=6 to n=2, what wavelength of light is emitted? Wavelength = 410 nm What is the region of the electromagnetic spectrum that corresponds to this wavelength? Visible Light

Example Problem: When an electron falls from n=6 to n=2, what color of light will be emitted? Already know: wavelength is 410 nm CHANGE UNITS! 410 nm = 4.1 x 10-7 m So what area does this number lie in? Between 4.2 x 10-7 m & 4.0 x 10-7 m Therefore, the color given off is VIOLET

Emission Spectra Emission Spectrum – the spectrum of light released from excited atoms of an element. No 2 elements have the same emission spectrum The farther the electron falls, the greater the amount of energy released. Ground State – the lowest energy level that an electron can occupy.

EMISSION SPECTRA (CONT’D) The frequency (color) of light emitted corresponds to the amount of energy released. (Argon) Atomic emission spectra provides the evidence that there are energy levels.

Electromagnetic Spectrum Electromagnetic radiation - energy waves that have both electrical and magnetic properties. Can travel through empty space at c = 3 x 108 m/s. c = λν E = hν E = energy = J h = Planck’s constant = 6.626 x 10-34 J/sec c = speed of light = 3x108 m/s λ = wavelength = m v = frequency = Hz / s-1 <-same thing

λ = 38.46 m = radiowaves c = λν c = 3x108 m/s What is the wavelength of an EM wave that has a frequency of 7.8 x 106 Hz? What type of wave is it? c = λν c = 3x108 m/s v = 7.8x106 Hz or 7.8x106 s-1 λ = ? 3x108 m/s = (7.8x106 s-1)(λ) λ = 38.46 m = radiowaves

(1)What is the frequency of a wave that has a wavelength of 4x10-7 m (1)What is the frequency of a wave that has a wavelength of 4x10-7 m? (2)What color is this wave? (3)How much energy is produced? (1) c = λν c = 3x108 m/s v = ? λ = 4x10-7 m 3x108 m/s = (v)(4x10-7 m) v= 7.5x1014 s-1 (2) violet (3) E=hv E= ? h= 6.6 x 10-34 J/sec v= 7.5x1014 s-1 E= (6.6 x 10-34 J/sec)(7.5x1014 s-1) E= 4.95x10-19 J

Electromagnetic Spectrum Low Energy, Low Frequency, Long Wavelength 1. Radio Waves (AM, FM, TV) 2. Microwaves (cell phones, microwave ovens) 3. Infrared (given off as heat) 4. Visible Light (ROYGBIV: Red is the lowest energy, violet has the highest) 5. Ultraviolet (causes sunburns) 6. X-rays (can pass through body, except bones) 7. Gamma Rays (can pass through 3 meters of concrete) High Energy, High Frequency, Short Wavelength

LIGHT – QUICK REVIEW Electrons absorb energy as they jump from a lower energy level to a higher energy level Electrons release energy in the form of light when they lose energy and fall back down to a lower energy level The lowest possible energy level for an electron is its ground state. Electrons at higher energy levels than ground state are said to be excited. The light emitted by an electron moving from a higher to a lower energy level has a frequency directly proportional to the energy change of the electrons.

LIGHT The Quantum Mechanical Model was developed out of the study of light. Light travels in waves. What are the parts of a wave? Wavelength Amplitude Crest Trough

Wavelength Verses Frequency Wavelength (λ) – distance between the crests of two different waves. Frequency (ν) – the number of wave cycles to pass a given point per unit of time. Measured in cycles per second (Hertz) Wavelength & Frequency are INVERSELY PROPORTIONAL (i.e. when one goes up, the other goes down)

Wavelength Verses Frequency High frequency, small wavelength Low frequency, large wavelength

Reading Electromagnetic Spectrum This spectrum only shows WAVELENGTH. Remember: Long Wavelength = Low Frequency Short Wavelength = High Frequency In terms of the Visible Spectrum: VIOLET = High Frequency RED = Low Frequency