5 Steps to a 5 AP Chemistry

The energy of the electromagnetic spectrum moves through space as waves that have

three associated variables—frequency, wavelength, and amplitude. The frequency, n,is the

number of waves that pass a point per second. Wavelength,l, is the distance between two

identical points on a wave. Amplitudeis the height of the wave and is related to the inten-

sity (or brightness, for visible light) of the wave. Figure 10.2 shows the wavelength and

amplitude of a wave.

The energy associated with a certain frequency of light is related by the equation:

E=hnwhere his Planck’s constant =6.63 × 10 −^34 Js

In developing the quantum mechanical model of the atom, it was found that the

electrons can have only certain distinct quantities of energy associated with them, and that

in order for the atom to change its energy it has to absorb or emit a certain amount of energy.

The energy that is emitted or absorbed is really the difference in the two energy states and

can be calculated by:

ΔE=hn

All electromagnetic radiation travels at about the same speed in a vacuum, 3.0 × 108 m/s.

This constant is called the speed of light (c). The product of the frequency and the wave-

length is the speed of light:

c=nl

Let’s apply some of the relationships. What wavelength of radiation has photons of

energy 7.83 × 10 −^19 J?

138  STEP 4. Review the Knowledge You Need to Score High

Gamma

ray

Ultra-

X-ray violet Infrared Microwave Radio frequency

Visible

Wavelength (m)

Frequency (1/s)

1020

10 −^1110 −^910 −^710 −^510 −^310 −^1101103

1018 1016 1014 1012 1010 108 106 104

Figure 10.1 The electromagnetic spectrum.

Wavelengthl Amplitude

Figure 10.2 Wavelength and amplitude of a wave.