1. Arrangement of Electrons in Atoms
Chapter 4
“Energy travels through space as waves, but can be thought of as a stream of particles .“
- Albert Einstein

2. Development of New Atomic Model
Scientist Review
Rutherford – Gold Foil Experiment
Nucleus – hard, dense, small center of atom containing protons & neutrons
Most of the atom is empty space in which the electrons move randomly
Did not explain how the electrons filled the empty space around the nucleus.
Bohr – Solar System Model
Electrons orbit the nucleus maintaining a fixed amount of energy, at constant speeds
Energy levels are the circular paths that the electrons travel.
Energy levels are arranged like a ladder. Rungs are not evenly spaced…they get closer together the farther from the ground, nucleus.
Electrons at higher energy levels have greater energy
The amount of energy required by an electron to move from one level to another is a quantum of energy.
A quantum leap is the abrupt change in level by the electron.
Schrodinger – Quantum Mechanical Model
Also called the Electron Cloud Model
The electrons travel randomly with no definite path.
Electrons are particles that act like waves.
The model is based on the mathematical 90% probability of finding an electron within a certain volume of space, called an orbital.
Heisenberg Uncertainty Principle – it is impossible to determine both the position and velocity of an electron simultaneously.
Development of New Atomic Model

3. Wave-Particle Duality of Motion
Wave Description of Light
Visible light is a kind of electromagnetic radiation, which is a form of energy that exhibits wavelike behavior as it travels through space.
Together, all forms of electromagnetic radiation form the electromagnetic spectrum.
Consists of radiation over a broad range of wavelengths
Radio waves
Radar waves
Microwaves
Infrared waves
Visible light waves
Ultraviolet waves
X-rays
Gamma rays
Wave-Particle Duality of Motion

4. Wave-Particle Duality of Motion
Wave Description of Light
Amplitude
The height of the wave from origin to crest
Wavelength (λ)
The distance between waves crests.
Units are the same as length, cm.
Frequency (v)
The number of wave cycles that pass a given point per unit of time.
Units of frequency are cycles/second or hertz.
Wavelength and frequency are inversely proportional, when one goes up the other goes down.
The speed of all forms of electromagnetic radiation in a vacuum is constant at 3.0 x 108 m/s
Formula c = λ · v
We will do a bunch of calculations for frequency and wavelength at the end of class today!!
Wave-Particle Duality of Motion

5. Wave-Particle Duality of Motion
Particle Description of Light
When certain frequencies of light strike a metal, electrons are emitted.
The photoelectric effect refers to the emission of electrons from a metal when light shines on the metal. Electrons are ejected by metals when light shines on them – light waves act like particles
The energy of the light has to be a certain minimum. Light can be absorbed or emitted by matter.
Calculators that require light energy as it’s power source.
Planck, German Physicist, proposed radiant energy is transferred in units (or quanta) of energy called photons.
He wasn’t expected to come up with this – people thought that energy was emitted in waves.
Like an hour-glass – small increments
Wave-Particle Duality of Motion

6. Photoelectric Effect absorption spectrum energy p+ no e-
ground state e-
When a specific or quantized amount of energy is exposed to the atom, the electron jumps from its “ground” or original state to an “excited” state

7. travels at the speed of light
Photoelectric Effect
energy photon p+ no e-
excited state e-
travels at the speed of light
(3.00 x 108 m/s)
emission spectrum!
When the “excited” electron returns to lower energy levels, it releases energy in the form of light

8. Wave-Particle Duality of Motion
Particle Description of Light
A quantum of energy is the minimum quantity of energy that can be lost or gained by an atom.
Planck proposed the following relationship between quantum energy and the frequency of radiation
E = hv
E is the energy, in Joules
v is the frequency, in s-1
h is the constant, 6.626x10-34 J·s
We will do a bunch of calculations at the end of class on this!!!
Planck’s constant shows mathematically that the electromagnetic energy is directly proportional to frequency.
In 1905, Einstein expanded Planck’s theory by introducing the radical idea that electromagnetic radiation has a dual wave-particle nature.
Light exhibits both wavelike properties, but it can also be thought of as a stream of particles.
Each particle carries a quantum or energy.
These particles, called photons, is a particle of electromagnetic radiation having zero mass and carrying a quantum of energy.
Frequency is the energy per photon
Intensity is the number of photons
Wave-Particle Duality of Motion

9. Wave-Particle Duality of Motion
Electrons exist in very specific energy states.
The lowest energy sate of an atom is its ground state.
A state in which an atom has a higher potential energy than it has in its ground state is an excited state.
There are many possible excited states, each with a unique energy, but only one ground state energy for atoms of a given element.
When an excited atom returns to its ground state or lower energy excited state, it gives off the energy it gained in the form of electromagnetic radiation.
Every element emits a unique atomic spectrum, the pattern of frequencies obtained by passing energy emitted by atoms of an element in the gas phase through a prism, producing an emission line spectrum.
Scientists expected to observe the emission of a continues range of frequencies of electromagnetic radiation, or the continuous spectrum.
Attempts to explain emission spectrum led to an entirely new atomic theory called the quantum theory.
Wave-Particle Duality of Motion

10. Bohr Model Niels Bohr e- p+ no
Electrons circle around the nucleus on their energy level
Energy levels

11. The Bohr Model of the Atom
Electron Orbits, or Energy Levels
Electrons can circle the nucleus only in allowed paths or orbits
The energy of the electron is greater when it is in orbits farther from the nucleus
The atom achieves the ground state when atoms occupy the closest possible positions around the nucleus
Electromagnetic radiation is emitted when electrons move closer to the nucleus.

12. The Bohr Atomic Model

13. Energy transitions Energies of atoms are fixed and definite quantities
Energy transitions occur in jumps of discrete amounts of energy
Electrons only lose energy when they move to a lower energy state

14. Shortcomings of the Bohr Model
Doesn't work for atoms larger than hydrogen
(more than one electron)
Doesn't explain chemical behavior