Models, Waves, and Light

Models of the Atom Many different models: – Dalton-billiard ball model (1803) – Thompson – plum-pudding model (1897) – Rutherford – Nuclear model of the atom (1911) – Bohr – uses quantized energy of the atom (1913) – Quantum Mechanical Model of the Atom (1926)

Each new model contributed to the model we use today. Even our current model, does not give us an exact model of how electrons interact.

Quantum Mechanical Model of the Atom Quantum Mechanical Model is the current description of electrons in atoms -does not describe the electron’s path around the nucleus Quantum Mechanical Model is based on several ideas -Schrodinger wave equation (1926) treats electrons as waves. -Heisenberg uncertainty principle (1927) states that it is impossible to know both the velocity and position of a particle at the same time.

When a current is passed through a vacuum tube of gas at low pressure, a set of frequencies of the electromagnetic waves are emitted by atoms of the element Used to determine which elements are present in a sample Used to determine which elements are present in a star Each element has a unique spectrum Only certain colors are emitted meaning only certain frequencies of light are emitted Atomic Emission Spectrum

A spectroscope that has a diffraction grating is needed to see the atomic emission spectra, which acts similar to a prism, separating different frequencies of light

Explanation of Atomic Spectra Step 1 Step 2 Electrons start at their lowest energy level (ground state) When an electron absorbs energy it moves to a higher energy level (excited state) When an e- drops back down to a lower energy level it gives off a quantum of energy called a “photon” Only certan atomic spectra are possible and emitted

Photons Behaves like a particle Behaves like a wave

Electromagnetic Spectrum Electromagnetic spectrum is the range of all energies emitted from photons acting like waves. If it is not in the visible light range, it may be giving off other forms of electromagnetic radiation like radio, microwaves, infrared, ultra violet, x-rays, or gamma rays. Used to determine which elements are present in a star (because stars are gases)

Electromagnetic Spectrum with Visible Light Spectrum

How do Neon Signs work? They have “excited” gases in them.

Characteristics of a Wave Amplitude (Wavelength) Wavelength (lambda) – shortest distance between equivalent points on a continuous wave [Unit = meters] Frequency (nu) – the number of waves that pass a given point per second [Unit = 1/second = s -1 = Hertz (Hz)] Crest – Highest point of a wave Trough – Lowest point of a wave Amplitude (a)– height from its origin to its crest (highest point) or trough (lowest point)

Wavelength and Frequency Wavelength ( ) and frequency ( ) are related As wavelength goes up, frequency goes down As wavelength goes down, frequency goes up This relationship is inversely proportiona l

Wavelength and Frequency cont. c = Speed of light (c) = 3 x 10 8 m/s c wavelengthfrequency Speed of light c = = c /

Practice 1: Calculate the wavelength ( ) of yellow light if its frequency ( ) is 5.10 x Hz. **Hz = 1/s c = c ÷ = 3 x 10 8 m/s ÷ 5.10 x Hz = 5.88 x m

Practice 2 What is the frequency ( ) of radiation with a wavelength ( ) of 5.00 x m? What region of the electromagnetic spectrum is this radiation? c = c ÷ = 3 x 10 8 m/s ÷ 5.00 x m = 6.00 x /s ultraviolet region (just barely)

How Much Energy Does a Wave Have? Energy of a wave can be calculated Use the formula E= h  E= Energy, = frequency h = Planck’s constant = x Joule. Sec Joule is a unit for energy (J) Energy and frequency are directly proportional, as frequency increases, energy increases E h Planck’s constant frequency Energy

Practice 3 Remember that energy of a photon given off by an electron is E =h  How much energy does a wave have with a frequency of 2.0 x 10 8 s -1 ? ( h = x J  s) E =h  E = (6.626 x Joule  s)(2.0 x 10 8 s -1 ) E = 1.3 x Joule

Visible Light, Frequency, and Energy Red: longest wavelength ( ), smallest frequency ( ) Red: frequency smallest ( ), least amount of energy (E) Violet: smallest wavelength ( ), largest frequency ( ) Violet: frequency largest ( ), greatest amount of energy (E)