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Chapter 5 Lecture

Basic ChemistryFourth Edition

Chapter 5 Electronic Structure and Periodic Trends5.2 Atomic Spectra and Energy Levels

Learning Goal Explain how atomicspectra correlate with the energy levelsin atoms.

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Atomic Spectra

White light from the Sun or a light bulb passes through a prism or raindrops, producing a continuous spectrum, like a rainbow.Heating atoms of an element also produces light, such as the•yellow color of sodiumstreetlights•red color of a neon sign

Colors are produced whenelectricity excites electronsin noble gases.

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Photons

The light emitted when atoms are heated is a stream of particles called photons. Photons •are a packet of energy known as a quantum•travel at the speed of light as an energy wave•have both particle and wave characteristics

The energy of a photon is directly proportional to its frequency.

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Photons in Our World

Photons play an important role in our modern world. They are used in lasers to •read pits on compact discs (CDs) and digital versatile discs (DVDs)•scan bar codes on merchandise labels

In hospitals, high-energy photons are used in treatments to reach tumors without damaging surrounding tissues.

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Learning Check

Compare the frequency and wavelength of photons of ultraviolet light with that ofinfrared light.

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Solution

Compare the frequency and wavelength of photons of ultraviolet light with that ofinfrared light. Ultraviolet light:wavelength, 10−8 m

frequency, 1016 HzInfrared light: wavelength, 10−4 m

frequency, 1013 HzUltraviolet light has shorter wavelengths and higher frequencies than infrared light.

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Atomic Spectra

When the light emitted from heated elements is passed through a prism, it does not produce a continuous spectrum.

Heated elements produce an atomic spectrum that consists of different colors separated bydark areas.

Only certain wavelengths of light are produced by these heated elements.

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Atomic Spectra

Lines produced in atomic spectra are associated with changes in the energies of electrons.

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Electron Energy Levels

In an atom, • each electron has a specific energy level• energy levels are assigned values called

principal quantum numbers (n), (n = 1, n = 2. . . )

Principal Quantum Number (n)1 < 2 < 3 < 4 < 5 < 6 < 7

Electrons in lower energy levels are closer to the nucleus.

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Electron Energy Levels

• Electrons can only havecertain energy values; wetherefore say the energy ofan electron is quantized.

• Energy levels increase inin energy as the value of“n” increases.

An electron can have only the energy of one of the energy levels in an atom.

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Changes in Energy Levels

• When electrons changefrom a lower to higherenergy level, they absorbthe energy equal to thechange in energy levels.

• When electrons changefrom a higher to lowerenergy level, they emitenergy equal to thechange in energy levels.

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Chemistry Link to the Environment, CFL

The compact fluorescent light(CFL) is replacing standard lightbulbs because it has a longer lifeand uses less electricity.

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Chemistry Link to the Environment, CFL

When a switch is turned on, electrons move between electrodes, colliding with mercury atoms in a mixture of gas.

When electrons in the mercury atoms absorb energy from the collisions, they move to a higher energy level.

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Chemistry Link to the Environment, CFL

As the electrons fall to lower energy levels emitting ultraviolet radiation, they strike the phosphor coating inside the bulb emitting fluorescent light.

Fluorescent bulbs are more efficient, using up to 70% less energy than their incandescent counterparts.

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Learning Check

In each of the following energy level changes,indicate if energy is

(1) absorbed (2) emitted(3) not changed

A. An electron moves from the first energy level (n =1) to the third energy level (n = 3).

B. An electron falls from the third energy levelto the second energy level.

C. An electron moves within the thirdenergy level.

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Solution

In each of the following energy level changes,indicate if energy is

(1) absorbed (2) emitted(3) not changed

A. An electron moves from the first energy level (n =1) to the third energy level (n = 3).

(1) The electron is moving from a lower to a higher energy level; energy is absorbed.

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Solution

In each of the following energy level changes,indicate if energy is

(1) absorbed (2) emitted(3) not changed

B. An electron falls from the third energy level to the second energy level.

(2) The electron moves from a higher to a lower energy level; energy is emitted.

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Solution

In each of the following energy level changes,indicate if energy is

(1) absorbed (2) emitted (3) not changed

C. An electron moves within the thirdenergy level.

(3) Energy is not changed when electrons stay in the same energy level.