History of the Atom…Part 2

History of the Atom…Part 2 • Left off with Rutherford’s Gold Foil Experiment… Conclusions from it • Atom is mostly empty space vs. JJ Thomson’s solid plum-pudding model • There wasa very small center that was termed the Nucleus • Since the overall electrical charge of the atom is neutral, a positively charged nucleus must offset the negative charge of the electrons

Where are the Rutherford Electrons? • Rutherford kept up with the recent advances in physics dealing with charged particles and radiation • Rutherford in 1911 concluded that the electrons circled the nucleus like the planets do the Sun • So that solves the issue? • Not really

One problem with this model: how do we explain that the electrons are located outside the nucleus? • Why do they stay there? • If the electrons were stationary, they would be attracted by the positive charge of the nucleus into it • If they were moving than they would be ok; correct?

No, because classical mechanics predicts the electrons would lose energy and spiral inward towards the nucleus • To the rescue, experimentation with hydrogen gas • When energy is added to hydrogen gas, gives off light which hen passed through a prism gives off a distinct emission spectrum

SIDEBAR– Light & Spectra • Light is one form of electromagnetic radiation • There is a continuous spectrum of this energy form; we classify it into different types • Gamma Radiation is the type of electromagnetic radiation with the most energy • Next comes X-ray • Followed by Ultraviolet

Visible Light or White Light is that electromagnetic radiation that our human eyes detect; it is less energetic than ultraviolet • The Visible Light can be separated into different color bands: Their sequence • ROY G BIV is a mnemonic to remember the order from least energetic to most • Above Visible Light in descending order of energy comesInfrared, Microwaves, and Radio waves

Electromagnetic Spectrum

Electromagnetic Waves • Light is an electromagnetic wave because it exhibits the property of diffraction • All waves, regardless of type, has several parts: • Wavelength- • Frequency - • Amplitude -

Crest – top of the wave • Trough – bottom of the wave • Normal – midpoint between crest and trough • Wavelength – the distance from a point on one wave to the corresponding point on an adjacent wave (check next slide carefully) • Frequency – the number of waves that pass a given point in one second; often referred to as the number of cycles

Wave Diagram

Amplitude- the vertical distance from the normal (midpoint) to the top of the wave or to the trough • Represents different properties depending on the type of wave • Sound – Loudness • Oceanic – Height of the wave • Light – Brightness • For electromagnetic radiation we are only concerned with frequency and wavelength

Units of Measurement • Wavelength – meter or nanometer • Nanometer = 1/1 000 000 000 of a meter • Said another way: 1 m = 1 x 109 nm • Frequency – Hertz, abbreviated Hz • Think of 1 Hz = wave/second or cycle/second; some books write this using seconds only; giving you: s-1

How fast is LIGHT? • The speed of light is 3.00 x 108m/s • Symbol is “c” • Related to frequency and wavelength • Their product = speed of light • Formula: • c = frequency x wavelength • Frequency = v = Greek letter nu • Wavelength =λ = Greek letter lambda

Wavelength-Frequency ProblemHINTS • Change your frequency from hertz to waves per second or cycles per second • waves • s • Write the wavelength as meters per wave or meters per cycle • m . • wave

Important relationship • The greater the frequency the greater the energy – check your textbook for the EM Spectrum • If the frequency goes up the wavelength get shorter • If the wavelength get longer the frequency decreases • Remember the product of the frequency and wavelength is a constant- the speed of light

Duality of Light • We already have seen that light behaves as a wave • But it also acts as a PARTICLE • Why? • Because the properties of a wave can not explain many interactions with matter such as blackbody radiation and the photoelectric effect

Both involve the emission of electromagnetic radiation in discrete packets- not as a continuous spectrum • As iron gets hotter and hotter it color changes- black at room temperature, red when heated, and blue at higher temperatures; not a gradual change of the spectral colors • Observed by Max Planck, a German, in the 1900, who further studied this phenomena

Particle Nature of Light • Matter can gain or lose energy only in small, specific amounts called quanta • A quantum is the minimum amount of energy that can be gained or lost by an atom • Referred to as Quantized Energy • Established the relationship between Equantum= Planck’s Constant x Frequency

Notice that the amount of energy goes up as the frequency increases • According to Planck, for a given frequency, matter can emit or absorb energy in whole number multiples of the product of the constant times the frequency • Albert Einstein also joined the discussion adding that the photon is a particle of electromagnetic radiation with no mass that carries a quantum of energy

Units of Energy is joule (J) defined as a newton meter • J = kg · m2 • s2 • Planck’s Constant = 6.626 x 10-34 J·s • Some books give the units as J/Hz • Remembering Hz = waves/s the first equation is equivalent as is the following • Planck’s constant, h = 6.626 x10-34 J·s • wave

Even though the Equantumis related to Planck’s constant times the EM frequency, remember that frequency is related to the wavelength via the speed of light • Therefore we can summarize the relationship between electromagnetic energy and an electron as follows: • An EM wave of a certain frequency has only one possible wavelength

Back to Bohr • Since he observed the spectral lines given off from hydrogen gas he concluded that there were specified energy levels in an atom • He proposed the Orbit Model which is an extension of Rutherford’s Planetary model • Are we finally done?

No • That’s where we are going next-

History of the Atom…Part 2