Chapter 7

Part 1

Chapter 7Electronic Structure

of Atoms

Recall:

• The concept of atoms proposed by Dalton explained many important observations:

- such as why compounds always have the same composition and how chemical rxns. occur• Once chemists were convinced of the

existence of atoms…..they naturally began to ask what atoms looked like?

The studies of Thomson, Rutherford and Chadwick

• Lead to our picture of the atom: - which includes a solid dense nucleus

containing protons and neutrons about which we find the negatively charged

electrons

In this Chapter we will look at the atomic structure in more detail.

• In particular we will develop a picture of the electron arrangements in atoms

• This picture will allow us to account for the chemistry of the elements

• With this knowledge we will revisit the arrangements of the elements in the periodic table and see why there are striking differences in the chemical properties of elements in the groups

Electromagnetic Radiation and Energy

• The sun is a central source of energy

• Energy from the sun travels through space in the form of electromagnetic radiation

Forms of Electromagnetic Radiation

• Visible light is a form of electromagnetic radiation• Microwaves are another form of electromagnetic

radiation• X-rays are yet another form of electromagnetic

radiation***all of these forms of energy are different, but they are

all forms of electromagnetic radiation

Electromagnetic Radiation

• All forms of electromagnetic radiation exhibit wave-like behavior and travel at the speed of light in a vacuum (airless space)

speed of light = 3.8 x 1010cm/s or 186,000 mi/s

Waves

• Waves have 3 primary characteristics:1. Wavelength- λ (Greek letter lambda) is the distance between 2

consecutive peaks or troughs in a wave2. Frequency- υ (Greek letter nu) it indicates how many waves

pass a given point per second3. Speed- indicates how fast a given peak moves through space

• Electromagnetic radiation from the sun is divided into various classes (forms) according to λ (wavelength)

• Energy from the sun reaches the earth in the form of visible and ultra-violet light

• Hot coals emit infrared radiation• Microwave ovens use microwave radiation to heat food**** all have different wavelengths

Visible Light

• Also known as white light• When passed through a prism

it separates into a continuous range of colors with one gradually blending into another called the continuous spectrum(rainbow)

• Separation is due to different speeds in the prisim

• Each color of light has a specific wavelength

Continuous Spectrum

• Violet has the shortest wavelength• Red has the longest wavelength

Wavelength and energy

• Wavelength and energy have an inverse relationship, as shown below

• h is Planck’s constant (6.626 10-34 J·s)

• c is the speed of light

hc

EE 1

Energy and Wavelength

• Red light with the longest wavelength has the lowest energy

• Purple light with the shortest wavelength has the highest energy

The Nature of Energy

• The wave nature of light does not explain how an object can glow when its temperature increases.

• Max Planck explained it by assuming that energy comes in packets called quanta.

Photoelectric Effect

A freshly polished, negatively charged zinc plate looses its charge if it is exposed to ultraviolet light. This phenomenon is called the photoelectric effect.

The Nature of Energy

• Einstein used this assumption to explain the photoelectric effect.

• He concluded that energy is proportional to frequency:

E = hwhere h is Planck’s constant, 6.63 10−34 J-s.

The Nature of Energy

• Therefore, if one knows the wavelength of light, one can calculate the energy in one photon, or packet, of that light:

c = E = h

The Nature of Energy

Another mystery involved the emission spectra observed from energy emitted by atoms and molecules.

Heating an Element

• When an element is heated strongly to the point that the element changes phase to a gas, the gaseous atoms emit light like the sun

• One might think that the light produced would result in a continuous spectrum like light emitted from the sun…………instead…….

When an element is heated..

• Only definite or discrete colors are produced• Since colors are discrete (definite) and the colors

correspond to energies• The energy being emitted by the atoms of the

element is also discrete

An Elements Spectrum

• Is unique to that element• Are called Atomic Emission Spectra or Line

Spectra• Are the basis of a fireworks display

The Nature of Energy

• Niels Bohr adopted Planck’s assumption and explained these phenomena in this way:

1. Electrons in an atom can only occupy certain orbits (corresponding to certain energies).

The Nature of Energy

• Niels Bohr adopted Planck’s assumption and explained these phenomena in this way:

2. Electrons in permitted orbits have specific, “allowed” energies; these energies will not be radiated from the atom.

The Nature of Energy

• Niels Bohr adopted Planck’s assumption and explained these phenomena in this way:

3. Energy is only absorbed or emitted in such a way as to move an electron from one “allowed” energy state to another; the energy is defined by

E = h

The Nature of Energy

The energy absorbed or emitted from the process of electron promotion or demotion can be calculated by the equation:

E = −RH ( )1nf

2

1ni

2-

where RH is the Rydberg constant, 2.18 10−18 J, and ni and nf are the initial and final energy levels of the electron.

Electronic Structure

Good Points• Electrons in Quantized

Energy Levels• Maximum # electrons in

each n is 2n2

• Sublevels (s,p,d,f) and # electrons they hold

Bad Points• Electrons are placed in

orbits about nucleus• Only explains emission

spectra of H2

• Does not address all interactions

• Treats electron as particle

The Wave Nature of Matter

• Louis de Broglie postulated that if light can have material properties, matter should exhibit wave properties.

• He demonstrated that the relationship between mass and wavelength was

=hmv

The Uncertainty Principle

• Heisenberg showed that the more precisely the momentum of a particle is known, the less precisely is its position known:

• In many cases, our uncertainty of the whereabouts of an electron is greater than the size of the atom itself!

(x) (mv) h4

Dual Nature of Electron

Previous Concept; A Substance is Either Matter or Energy

• Matter; Definite Mass and Position Made of Particles

• Energy; Massless and Delocalized Position not Specificed Wave-like

Dual Nature of Electron

• Electron is both “particle-like” and “wave-like” at the same time.

• Previous model only considered “particle-like” nature of the electron