Radiant Energy

Light Waves are a form of Electromagneticradiation: (X-rays, gamma rays, radio waves)

Consists of oscillating electric and magnetic fields at right angles with respect to each other and direction of wave

4 Characteristics of a wave

  • Amplitude
  • Wavelength
  • Frequency
  • Speed

Amplitude

Height of wave (distance from crest to trough)

crest = wave peak

trough = lowest point of a wave

Determines:

  • Light brightness and Sound loudness

Wavelength

Distance from crest –to - crest or trough – to – trough

Expressed in centimeters (cm) or nanometers (nm)

1 nanometer = 10 –9 meter (1 billionth)

Symbol = λ ( Lambda)

Frequency

How fast a wave oscillates up & down

# waves passing a point in a given time

Symbol = ν

Unit : Cycles per second ( s –1 or 1/s )

1 Reciprocal Second = Hertz ( Hz)

FM Radio stations broadcast at a Megahertz frequency (frequency modulation)

Mega = 10 6 ( million)

1 MHz = million cycles per second

Speed

How fast a wave moves through space

Units = meters / second

Speed of light = 3 .00 x 10 8 meters / second

Symbol = c

Speed of light is a constant

Relationship between wavelength and Frequency

Wavelength = speed of light / frequency

Equation: λ = c / v

Wavelength increases as frequency decreases

Wavelength as frequency are inversely related

Calculate the wave length of a radio wave knowing the frequency of 99.0 MHz

Electromagnetic Spectrum

A range of frequencies or wavelengths of electromagnetic radiation

Ranges from: long-wave length radio waves to

short-wavelength cosmic rays

Array of visible colors = visible spectrum

Visible spectrum is a small portion of the electromagnetic spectrum

( ROY-G-BIV )

( 700 nm – 400 nm)

Different colors have different wavelengths

Quantum Theory

Spectrum of radiation emitted by an object changes with its temperature

A solid metal glows red at 750 °C,

Then white at 1200 °C why?

Planck’s Theory:

Atoms in a solid vibrate at a definite frequency,

with only certain energies

Energies (heat, light) absorbed or emitted by atoms are restricted to certain quantities

Discrete pieces of energy are called quantum

Quantum means “ fixed amount ”

Energies absorbed or emitted by atoms are quantized into discrete packets or pieces called quanta

Energy absorption or emission by an atom is not continuous across the electromagnetic spectrum

Planck’s Equation

E = h v

E = Energy ( Joules) J

v = Frequency of radiation ( Hz or 1 /s )

h = Planck’s constant

h = 6.63 x 10 –34 J ∙ s (Joule-second)

Energy is directly related to Frequency

Einstein proposed light consists of quanta of energy

Light energy behaves as tiny particles called photons

A photon’s energy is: E = h v ( v = c / λ )

Bohr’s Model of the hydrogen atom

To get spectral lines, electrons’ energies in an atom are quantized (restricted to certain quantities)

Electrons exist in energy levels or orbits labeled by a quantum number, n

n= 1 is the lowest energy level called the ground state; orbit closet to the nucleus

n = 2, 3, 4, …. are higher energy levels called

excited states farther from nucleus

As an electron absorbs a quantum of energy,

it “jumps” to a higher energy level

Ground state + energy ------Excited state

(n=1) (n=2, 3, 4, …)

Radiation (light) is emitted when an electron falls back to its ground state

Bohr’s model of a Hydrogen atom shows the connection between the wavelength of light an element emits and its atomic structure

Probability and Atomic Orbital

“Quantum Mechanical” model of an atom

Electrons have no precise orbits

Electron motion is described by the probability of finding electrons in regions of space around the nucleus

This probability is viewed as a cloud of negative charge

High electron cloud density = high probability region

Low electron cloud density = low probability region

Electron cloud density decreases as distance from nucleus increases

Therefore, further away from the nucleus there is less of a chance of finding an electron

Atomic orbital

A space around an atom’s nucleus where an electron with a given energy is likely to be found.

Orbital Characteristics:

  • Shape
Size

Energy

Orbital shape

s spherical

p dumbbell

d four-leaf clover

f complex

An electron’s energy determines its orbital

Electron Spin

Clockwise or counterclockwise (cw or ccw)

Spin generates a magnetic field

Pauli Exclusion Principle

Each orbital can hold only 2 electrons with opposite spins

One electron has a c.w. spin the other electron has a c.c.w spin

These electrons are called “ paired electrons”

A single electron in an orbital is unpaired

Dual nature of Energy

Wave or Particle?

De Broglie – matter waves

Wave-like behavior of particles

Light travels through space behaves as a wave

Light interacts with matter behaves as a stream of particles

Light – wave characteristics ( λ , v, amplitude)

Light - consists of photons

Photons are quanta of energy with particle characteristics (speed, mass, momentum)

de – Brogile Equation

λ = h / mc

λ = wavelength of a moving particle

C = speed

M = mass

Mass of an object must be very small for its wavelength to be observed

Examples: electrons, photons

Energy Levels

Electrons exist in energy levels or shells outside the nucleus

The Main or “principal; energy level” indicated by a quantum number, n = 1, 2, 3, 4….

Principal energy level divided into sublevels

# Sublevels = n for that energy level

n = 1, has one sublevel

n = 2, has two sublevels

n = 3, has three sublevels

electrons in each sublevel have the same energy

Sublevels are labeled with:

n ( principal quantum number)

spdf ( sublevel type)

2P = “ P” sublevel in the principal energy level 2

3S = “S” sublevel in the principal energy level 3

Each sublevel consists or one or more orbitals

Summary:

Each principal energy level consist of sublevels

Each sublevel consists of orbitals

To specify the location of an electron:

Principal Quantum # n (state)

Sublevel (spdf) (city)

Orbital (street)

Summary of Energy Levels, sublevels and Orbitals:

Principal sublevels orbitals

Energy Level

n = 1 1S 1S (one)

n = 2 2S, 2P 2S (one) +

2P (three)

n = 3 3S, 3P, 3d 3S (one) +

3P (three) +

3d (five)

n = 4 4S, 4P 4d, 4f 4S (one) +

4P (three) +

4d (five ) +

4f (seven)

note: # sublevels = Quantum number

# of orbitals in a sublevel is an odd number

How do the 2P and 3P orbitals differ?

  • Same shape (dumbbell)
  • Same orientation ( Px, Py , Pz)

Answer: Size

3P is larger than 2P

an electron in a 3P orbital has more room to move and larger energy

Electron energy increases as quantum number “n” increases