Unit 3: Atomic Theory Section A.2 – A.3 In which you will learn about: Rutherford’s gold foil experiment Rutherford’s model of the atom The electromagnetic spectrum Calculations with light Light acting as a wave

A.2 The Nucleus In 1911, Ernest Rutherford (1871-1937) began to study how positively charged alpha particles (radioactive helium nuclei) interacted with solid matter. With a small group of scientists (most notably, Hans Geiger, of the Geiger Counter fame), Rutherford conducted an experiment to see if alpha particles would be deflected as they passed through a thin gold foil (like aluminum foil).

Rutherford’s Gold Foil Experiment A narrow beam of alpha particles was aimed at a thin sheet of gold foil A zinc sulfide-coated screen surrounding the gold foil produced a flash of light when struck by an alpha particle (radioactive materials expose photographic film) By noting where the flashes occurred, the scientists could determine if the atoms in the gold foil deflected the alpha particles

Rutherford’s Prediction Rutherford was aware of Thomson’s plum pudding model of the atom He expected the paths of the massive and fast-moving alpha particles to be only slightly altered by a collision with an electron Because the positive charge within the gold atoms was thought to be uniformly distributed, he thought it would not alter the alpha particles, either.

Gold Foil Experiment Results

Rutherford’s Results & Conclusions Most of the particles went straight through the gold foil Conclusion: The atom is made up of mostly empty space Several particles were deflected straight back toward the source! Conclusion: There is a massive, densely packed area within an atom (this is the discovery of the nucleus) Rutherford likened this surprising result to firing a large artillery shell at a sheet of paper and the shell coming back at the cannon! A few particles were deflected at large angles. Conclusion: The nucleus must be positive because the positive alpha particles are being deflected from a positive center (like charges repel). NOTE: Neutrons were not discovered until 1932! Rutherford discovered the nucleus – not protons and neutrons!

Rutherford’s Model of the Atom: Planetary Model (w/o orbits)

A.3 The Wave Nature of Light In the early 1900s, scientists began to unravel the puzzle of chemical behavior. They observed that certain elements emitted visible light when heated in a flame. Analysis of the emitted light revealed that an element’s chemical behavior is related to the arrangement of the electrons in its atoms. To understand this relationship and the nature of atomic structure, it will be helpful to first understand the nature of light.

Electromagnetic (EM) Radiation Electromagnetic radiation = a form of energy that exhibits wavelike behavior as it travels through space

Characteristics of Waves All waves can be described by several characteristics: Wavelength (λ – Greek letter lambda) = the shortest distance between equivalent points on a continuous wave. For example, crest to crest or trough to trough. Usually expressed in meters, centimeters, or nanometers (1 nm = 10-9 m).

Characteristics of Waves Cont’d Frequency (ν – Greek letter nu) = the number of waves that pass a given point per second One Hertz (Hz), the SI unit of frequency, equals one waver per second (1 Hz = 1 wave/sec) In calculations, frequency is expressed as 1/s with waves being understood). EX: 652 Hz = 652 waves/second = 652/s = 652 s-1

Characteristics of Waves Cont’d II Amplitude = the wave’s height from the origin to a crest, or from the origin to a trough Wavelength and frequency do NOT affect the amplitude of a wave

Electromagnetic Wave Relationship All EM waves, including visible light, travels at a speed (c) of 3.00 x 108 m/s in a vacuum. c = λν Wavelength and frequency are inversely related; in other words, as one quantity increases, the other decreases.

EXAMPLE PROBLEM – If you don’t know, GUESS! Microwaves are used to cook food and transmit information. What is the wavelength of a microwave that has a frequency of 3.44 x 109 Hz? GIVEN: ν = 3.44 x 109 Hz c = 3.00 x 108 m/s UNKNOWN: λ = ?

Example Cont’d EQUATION: c = λν SUBSTITUTE: 3.00 x 108 m/s = λ (3.44 x 109 1/s) SOLVE: λ = 8.77 x 10-2 m = 8.77 cm Note that Hz = 1/s, and m/s/1/s = m (It’s easier to see if you write it out as fractions). GIVEN, UNKNOWN, EQUATION, SUBSTITUTE, SOLVE!

A.2 HOMEWORK QUESTIONS 1) Briefly explain how Rutherford discovered the nucleus IN YOUR OWN WORDS. 2) What caused the deflection of the alpha particles in Rutherford’s gold foil experiment? 3) What are the strengths and weaknesses of Rutherford’s nuclear model of the atom (brainstorm your own ideas – this one wasn’t said in the powerpoint!)

A.3 HOMEWORK QUESTIONS 4) Objects get their color from reflecting only certain wavelengths when hit with white light. Light reflected from a green leaf is found to have a wavelength of 4.90 x 10-7 m. What is the frequency of the light? 5) X-rays can penetrate body tissues and are widely used to diagnose and treat disorders of internal body structures. What is the frequency of an X-ray with a wavelength of 1.15 x 10-10 m? 6) After careful analysis, an electromagnetic wave is found to have a frequency of 7.8 x 106 Hz. What is the speed of the wave?

CHALLENGE While an FM radio station broadcasts at a frequency of 94.7 MHz, an AM station broadcasts at a frequency of 820 kHz. What are the wavelengths of the two broadcasts?