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Electromagnetic Radiation

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Electromagnetic Radiation

Wavelength, , lambda

The distance between two adjacent identical points of the wave.

Frequency, , nu• The number of

wave crests passing a given point per unit time.

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• Wavelength and frequency are related to the speed of light.

c =

Electromagnetic Radiation

• All light waves travel at exactly the same speed (in a vacuum) – the speed of light, c, is a constant.

C = 2.998 108 ms-1

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Electromagnetic Radiation

• All radiation may have the same speed but the energy can vary.

• The higher the frequency, the more rapidly the wave is oscillating and the higher the energy.

Energy = Planck’s constant frequency

E = h h = 6.626 x 10-34 Js

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EM Radiation - colour

Large wavelengthLow frequencyLow energy

Short wavelengthHigh frequencyHigh energy

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Atomic Spectra• When an atom is supplied with energy (eg

heat, electric current) it emits electromagnetic radiation of a relatively few, fixed frequencies that are characteristic of that element – this is the atomic emission spectrum.

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Atomic Spectra

Light emitted from a hydrogen lamp has only a few lines.

– Only light of certain energies is emitted.

– The pattern of lines is unique to hydrogen.

– Suggests the process of emitting light from the atom is quantised (comes in discrete amounts).

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Atomic Spectra

Our eye sees the combination of wavelengths:

Pass the light through a prism to see the lines:

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The Bohr Model

• Electrons in atoms can only occupy certain energy levels (orbits).

• When an electron moves from one energy level to another, energy is absorbed or emitted.

• This energy corresponds to light of a specific energy/frequency.

Niels Bohr

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The Bohr Model - Postulates

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The Bohr Model

• The number of electrons that can pack into an orbit depends on the size of the orbit.

2 electrons max

8 electrons max( 2 + 6)

18 electrons max( 2 + 6 + 10)

32 electrons max( 2 + 6 + 10 + 14)

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Electron Structures• Neutral atoms always have the same number of

electrons as the number of protons in the nucleus.

HydrogenZ = 1

1 electron

HeliumZ = 2

2 electrons

LithiumZ = 3

3 electrons

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‘Ground State’ Electron Structures

20+

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Electron Structures

Period: no of electron shells/orbits containing electrons

Group: no of electronsin outer shell/orbit

• By the end of this lecture, you should:− appreciate the relationship between frequency,

wavelength and energy− know that the speed of light (in a vacuum) is

constant− know that the Bohr model of an atom limits

electrons to orbits of certain energies− recognise the energy of light emitted from an

atom represents the energy difference between two orbits

− know the maximum number of electrons the first, second and third shell can accommodate (2, 8, 18)

− recognise the connection between an atom’s electron configuration and its position in the periodic table

− be able to complete the worksheet (if you haven’t already done so…)

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Learning Outcomes:

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Questions to complete for next lecture:

1. Which light: red or blue, has (a) the higher frequency, (b) the longer wavelength, (c) the faster speed and (d) the greater energy?

2. What is the colour of the light emitted by a ‘sodium street light’?

3. If the electron of a hydrogen atom moves from the first (n = 1) to the third (n = 3) orbit, will energy be absorbed or released from the surroundings?

4. What is the maximum number of electrons that can fit in the second (n = 2) orbit?

5. A vertical column in the Periodic Table is called a……….. and a horizontal row is called a…………..

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Questions to complete for next lecture:

6. How many electrons are in the outermost orbit of a ‘ground state’ magnesium atom?

7. What element is in Group 16 and the third period? How many electrons are in its third orbit?

8. Which element has two completely filled electron shells and just three electrons in the third shell?

9. Draw a diagram showing the electron structure of a K atom.