Unit 2: Atoms and BondingUnit 2: Atoms and Bonding

2.62 2.62 Photons and Energy

Textbook ch 6.2 and ch Textbook ch 6.2 and ch 6.36.3

Big Idea 1: The chemical elements are fundamental building materials of matter, and all matter can be understood in terms of arrangements of atoms. These atoms retain their identity in chemical reactions. Students will be able to demonstrate understanding by laboratory investigation, analysis of data and creation of models.

SWBAT:SWBAT:•Explain what photons are and be able Explain what photons are and be able to calculate their energies given either to calculate their energies given either their frequency or wavelengththeir frequency or wavelength

•Explain how line spectra related to Explain how line spectra related to the idea of quantized energy states of the idea of quantized energy states of electrons in atomselectrons in atoms

Learning Objectives:

Max Planck• He assumed that energy can be

released (or absorbed) by atoms only in “chunks”.

• He called these chunks “quantum” .

• Quantum is the smallest quantity of energy that can be emitted or absorbed as electromagnetic radiation

• Think of it like comparing a ramp to climbing stairs. If you use the ramp your PE (potential energy) is uniform.

• If you climb stairs, you can only step on individual steps not between them. Your PE is restricted in certain values and is “quantized”

E = hE = h

EE = Energy, in units of Joules = Energy, in units of Joules

hh = Planck’s constant (6.626 x 10= Planck’s constant (6.626 x 10-34-34 J·s) J·s)

= frequency, in units of hertz (hz or sec= frequency, in units of hertz (hz or sec-1-1))

The energy (The energy (E E ) of electromagnetic ) of electromagnetic radiation is directly proportional to the radiation is directly proportional to the frequency (frequency () of the radiation.) of the radiation.

• A few years later, Einstein used Planck’s theory to explain photoelectric effect.

• Light shining on a metal surface cause electrons to be emitted.

• Einstein went on to think the light was a stream of packets of energy.

• He called these packets of energy photons.

• Energy of photon = E = E = hh

• Photon is a “particle” of light with energy E= h

Planck and Einstein at Nobel Conference

Long Wavelength

=Low Frequency

=Low ENERGY

Short Wavelength

=High

Frequency=

High ENERGY

Wavelength TableWavelength Table

• So… short wavelengths, (like X-Rays)

• …have high frequency and high Energy

• X - ray photons can cause tissue damage and even cancer

• e- can only have specific (quantized) energy values

• The e-’s energy correspond to orbits around the nucleus. Outer orbits have higher energy

• light is emitted as e- moves from higher energy level to lower energy level

PhotonE=h

Niels Bohr’s Model of the Atom (1913)

Ground State: n =Excited State: nIonized: n =

1>1

∞

Niels Bohr and Max Planck at MIT

Figure 7.10

Quantum staircase

E = h

E = h

Spectroscopic Spectroscopic analysis of the analysis of the hydrogen hydrogen spectrum… …spectrum… …produces a produces a “bright line” “bright line” spectrumspectrum

This produces This produces bandsbandsof light with of light with definite definite wavelengths.wavelengths.

Electron Electron transitionstransitionsinvolve jumps of involve jumps of

definite definite amounts ofamounts ofenergy.energy.

Flame TestsFlame Tests

sodium lithium potassium copper

Many elements give off characteristic Many elements give off characteristic light which can be used to help light which can be used to help identify them.identify them.

Let’s try…1. Calculate energy of one photon of yellow

light at 589nm

2. Calculate energy of one mole of yellow light at 589nm

ReferencesReferences

www.sciencegeek.net/Chemistry and Mr. Perekupa’s PPT Cinnaminson.com from Cinnaminson High school.

I modified the original PPTs to fit our needs in AP Chemistry.

Our textbook: Brown, Lemay et all. AP edition chemistry, 13th edition, 2015