Chapter 5

Electrons In Atoms

Topics to Be Covered

5.1 Light and Quantized Energy 136-145

5.2 Quantum Theory and the Atom 146-155

5.3 Electron Configuration 156-162

Section 5.1

Light and Quantized Energy

The Atom & Unanswered Questions

Early 1900s Discovered 3 subatomic particles Continued quest to understand atomic

structure Rutherford’s model

Positive charge in nucleus Fast moving electrons around that No accounting for differences and

similarities in chemical behavior

The Atom and Unanswered Questions

Example: Lithium, sodium, and potassium have

similar chemical behaviors (explained more in next chapter)

Early 1900s Scientists began to unravel mystery Certain elements emitted visible light

when heated in a flame Analysis revealed chemical behavior

depends on arrangement of electrons

The Wave Nature of Light

Electromagnetic radiation A form of energy that exhibits wavelike

behavior as it travels through space Visible light is a type of ER

Characteristics of Waves

All waves can be described by several characteristics

1. Wavelength2. Frequency3. Amplitude

Wavelength

Represented by lambda λ Shortest distance between

equivalent points on a continuous waves

Measure crest to crest or trough to trough

Usually expressed in m, cm, or nm

Frequency

Represented by nu ν The number of waves that pass a

given point per second Given in the unit of hertz (Hz) 1 Hz = 1 wave per second

Amplitude

The wave’s height from the origin to a crest or from the origin to a trough

Wavelength and frequency do not affect amplitude

Speed

All electromagnetic waves in a vacuum travel at a speed of 3.00 x 108 m/s This includes visible light

The speed of light has its own symbol C

C= λν

Electromagnetic Spectrum

Also called the EM spectrum Includes all forms of

electromagnetic radiation With the only differences in the

types of radiation being their frequencies and wavelengths

Electromagnetic Spectrum

Figure 5.5

Problems

Page 140 Calculating Wavelength of an EM

Wave

Particle Nature of Light

Needed to explain other properties of light Heated objects emit only certain

frequencies of light at a given temperature

Some metals emit electrons when light of a specific frequency shines on them

Quantum Concept

When objects are heated they emit glowing light

1900 Max Planck began searching for an

explanation Studied the light emitted by heated

objects Startling conclusion

Quantum Concept

Planck discovered: Matter can gain or lose energy only in

small specific amounts These amounts are called quanta Quantum—is the minimum amount of

energy that can be gained or lost by an atom

Example

Heating a cup of water Most people thought that you can add

any amount of thermal energy to the water by regulating the power and duration of the microwaves

In actuality, the temperature increases in infinitesimal steps as its molecules absorb quanta of energy, which appear to be a continuous manner

Quantum Concept

Planck proposed that energy emitted by hot objects was quantized

Planck further demonstrated mathematically that a relationship exists between energy of a quantum and a frequency

Energy of a Quantum

Equantum=hv

Equantum represents energy h is Planck’s constant v represents frequency

Planck’s Constant

Symbol = h 6.626 x 10-34 J*s J is the symbol for joule

The SI unit of energy The equation shows that the energy

of radiation increases as the radiation’s frequency, v, increases.

Planck’s Theory

For given frequencies Matter can emit/absorb energy only in

whole number multiples of hv 1hv, 2hv, 3hv, 4hv etc.

Matter can have only certain amounts of energy

Quantities of energy between these values do not exist

The Photoelectric Effect

Photoelectric effect electrons, called photoelectrons are emitted from a metal’s surface when light of a certain frequency, or

higher than a certain frequency shines on the surface

Light’s Dual Nature

Einstein proposed in 1905 that light has a dual nature

photon—a massless particle that carries a quantum of energy

Energy of a Photon

Ephoton=hv

Ephoton represents energy h is Planck’s constant v represents frequency

Light’s Dual Nature

Einstein proposed Energy of a photon must have a certain

threshold value to cause the ejection of a photoelectron from the surface of a metal

Even small #s of photons with energy above the threshold value will cause the photoelectric effect

Einstein won Nobel Prize in Physics in 1921

Sample Problems

Page 143 Sample Problem 5.2 Calculating Energy of a Photon

Atomic Emission Spectra

See page 145

Section 5.2

Quantum Theory and The Atom

Bohr’s Model of the Atom

Dual-nature explains more Atomic Emission Spectra

Not continuous Only certain frequencies of light

Explained the Atomic Emission Spectra

Energy States of Hydrogen

Bohr proposed certain allowable energy states

Bohr proposed electrons could travel in certain orbitals

Energy states of Hydrogen

Ground State Lowest allowable energy state of an

atom Orbit size

Smaller the orbit, the lower the energy state/level

Larger the orbit, the higher the energy state/level

Energy states of Hydrogen

Hydrogen can have many excited states It only has one electron

Quantum Number Number assigned to each orbital n Look at Table 5.1

The Hydrogen Line Spectrum

Hydrogen Ground State Electron is in n=1 orbit Does not radiate energy

Hydrogen Excited State Energy is added to the atom from

outside source Electron moves to a higher energy orbit

The Hydrogen Line Spectrum

Only Certain Atomic Energy Levels Possible

Example Our Classroom

Balmer Series Electron transitions from higher-energy

orbits to the second orbit Account for visible lines

The Hydrogen Line Spectrum

Lyman Series Ultraviolet Electrons drop into n=1 orbit

Paschen Series Infrared Electrons drop into n = 3 orbit