Modern Atomic Model

Electron modeling… To understand electrons, scientists began comparing them to something they knew - light.

What they knew about light Light is a wave – similar to water waves Visible light belongs to electromagnetic radiation spectrum Origin - the base line of the energy. Crest - high point on a wave Trough - Low point on a wave Amplitude - distance from origin to crest Wavelength - distance from crest to crest

Behavior of light All forms of EMR have common properties: Amplitude Wavelength Frequency Speed 1- electric & magnetic field oscillating @ right angles to each other 2- gamma rays [10-11], X-rays [10-9], UV [10-8], visible spectrum (ROYGBV) [4.00 x 10-7 – 7.00 x 10-7], IR [10-6], microwave [10-2], TV [100], radio [101] 3 - IR longer than visible spectrum wavelengths, is radiant heat, Microwaves longer still (heat only food by transferring E to moisture in food), Short waves on other side of visible light (UV  sunburn & skin cancer) 4 - When we say light, we are talking about visible light.

Behavior of Light Wavelength (l), frequency (n), and speed (c) are mathematically related c = ln

Behavior of light Frequency and wavelength: Are inversely related As one goes up, the other goes down. Different frequencies of light go with different colors of light. There is a wide variety of frequencies The whole range is called a continuous spectrum

Continuous EMR Spectrum Radiowaves Microwaves Infrared . Ultra-violet X-Rays GammaRays Low Frequency High Frequency Long Wavelength Short Wavelength Visible Light

Wave model had problems At the start of the 20th century, scientists made observations of light that didn’t fit the model…

Wave model problems Black body radiation In 1900, Max Planck heated matter that didn’t burn and studied the radiation emitted. It was predicted that matter could absorb or emit any quantity of energy. His experimental data did not fit that statement.

What happened to the rest of the light? Max Planck What happened to the rest of the light?

E = h Wave model problems Based on Planck’s work, we know the following about light: The energy is in specific amounts called quanta. The light’s energy (E) and frequency (n) are directly related by a constant (h). E = h E = h x nu E is the energy of the photon nu is the frequency h is Planck’s constant and equals 6.626 x 10 -34 Joules × seconds

Wave model problems Planck’s ideas weren’t accepted until some time later when Albert Einstein used Planck’s equation to work on solving the photoelectric effect.

Wave model problems  Photoelectric effect Light shining on certain metals can eject electrons. Simulation Ex. an intense light w/low frequency could shine all day w/o knocking out any electrons The fact that light was able to knock electrons loose wasn’t a problem. What wave theory couldn’t explain was why only certain frequencies of light (or higher) could knock out electrons.

Wave model problems Which has greater energy – red or violet light?

Wave model problems  Photoelectric effect Einstein proposed that light consisted of energy quanta that behaved as particles – not waves. He called them photons instead of quanta. The problem was then solved by the notion that radiation is emitted or absorbed in whole numbers of photons. Electrons need specific energy photon, not several photons of any energy. Higher energy comes from light with higher frequencies [smaller wavelengths]. Higher energy is also associated with damage to organisms [X-rays…]

Wave model problems  Photoelectric effect It was later proven that light could definitely act as a particle. So, we now have light acting as both a wave and a particle. This will be the basis for understanding how e- behave.

Wave model problems  Bright line spectrum Scientists noticed that you could vaporize an element in a flame to produce different flame colors. You can then use a prism to sort the colors to produce a line spectrum (only certain colors are produced).

Prism with white light White light is made up of all the colors of the visible spectrum. Passing it through a prism separates it.

If the light entering the prism is not white… By adding energy to a gas, we can get the gas to give off colored light Passing this light through a prism does something different than white light

Wave model problems  Bright line spectrum Problem: Each element produced a different line spectrum.

These are called line spectra They are unique to each element. These are emission spectra (the light is emitted or given off) Each element gives off its own characteristic colors *Can be used to identify the element *How we know what stars are made of

Rutherford’s Model Doesn’t work to help explain the bright line spectrum – we need something better

Bohr’s Model: An explanation for the observed atomic spectra

Bohr Model for Hydrogen The e- goes around the nucleus only in allowed paths called orbits. The H atom has energy possibilities based on which orbit the e- occupies. The ground state occurs when the e- is in the orbit closest to the nucleus. The orbit containing the e- determines the outer dimensions of the atom. The energy of the e- increases as it moves to orbits that are farther from the nucleus (excited state).

Bohr’s Model simplified } Energy is in Levels Further away from the nucleus means more energy. There is no “in between” energy Fifth Fourth Increasing energy Third Second The different possible orbits in the H atom can be visualized in the following analogy. [ladder analogy – When on a ladder, you have to stand on the rungs. You can’t stand halfway in between the rungs. For each rung, there is a set amount of potential energy. So, the distance between the rungs can be used to explain the gain or loss of E ( = gain,  = loss).] An e- can only be in an orbit, it can’t be between orbits. First Nucleus

Picture the Loss and Gain of Energy for an e- Higher Energy Level or excited state Light Energy G a I n Loss Summary: The electron went from particle to wave and back! *The farther the electrons fall, the more energy released and higher frequency produced *All the electrons can move Lower Energy Level or ground state

Bohr Model - Explains where an electron is most likely to be found The first shell is lowest in energy 2nd level next Electrons occupy shells in order They are numbered: 1, 2, 3, 4, 5, 6 etc Nucleus Then 4th Then 3rd

Energy input will determine which level the electron moves to Jumps and Lines Energy Hydrogen Spectrum 400 450 500 550 600 650 700 410 434 486 656 Energy Energy Nucleus Energy input will determine which level the electron moves to Nucleus

Energy input will determine which level the electron moves to Jumps and Lines Energy Helium Spectrum 400 450 500 550 600 650 700 447 501 587 667 Note that these are different frequencies from H Every element is unique! Energy Energy Nucleus Energy input will determine which level the electron moves to Nucleus Ideas are the same. The second electron effects the atomic structure and therefore the spectrum.

Bohr Model Problems Unfortunately, Bohr’s model only worked for H. So what about the 100+ other elements?

Matter exhibits both wave and particle properties! De Broglie Determined that particles of matter could act as waves. Described the wavelength of moving particles. Conclusion: Matter exhibits both wave and particle properties! For that matter, all matter is able to act as a wave. The problem w/ things bigger than atoms is that the wavelength is too small to be detected.