Chapter 18

Pre-IB/Pre-AP CHEMISTRYChapter 4 Arrangement of Electrons in AtomsSection 1 ObjectivesBe able to define: electromagnetic radiation, electromagnetic spectrum, wavelength, amplitude, frequency, photoelectric effect, quantum(pl. quanta), photon, ground state, excited state, line emission spectrum, continuous spectrum, energy level.Section 1 ObjectivesBe able to explain the mathematical relationship between speed, wavelength, and frequency of a wave.Be able to describe what is meant by the wave-particle duality of light.Be able to discuss how the photoelectric effect and the line emission spectrum of hydrogen lead to the development of the atomic model.Section 1 ObjectivesBe able to describe the Bohr model of the atom.

WaveA wave is a method of transferring energy. This transfer does not require matter as a medium.

WaveSome waves travel through matter (sound, water waves, etc.).

WaveSome waves do not require matter and can travel through empty space (light).

Wave PropertiesWaves can be described by their wavelength, amplitude, and frequency.WavelengthA crest is the highest point on a wave.A trough is the lowest point on a wave.CrestTroughWavelengthWavelength is simply the length of a wave. It is the distance between two crests or two troughs.Wavelength is measured in m, mm, or nm.WavelengthCrestTroughAmplitudeAmplitude is simply the height of a wave. It is the distance between the crest and trough of a wave.Amplitude is measured in units of distance.AmplitudeFrequencyFrequency is the number of waves passing a given point in a given time.Frequency describes the energyof a wave.FrequencyFrequency describes the energy of a wave: the higher the frequency, the greater the energyof that wave.FrequencyFrequency is measured in hertz or cycles per secondor vibrations per second or 1/sec or sec-1 - they all mean the same thing.FrequencyAs the wavelength increases, frequency decreases. This is called an inverse relationship.Wave PropertiesWavelength and amplitude give waves their distinctive properties. For example, the loudness of a sound wave is its amplitude, the color of visible light is its wavelength.Types of wavesElectromagnetic waves do not require a medium or matter in order to travel. Light is an example.

LightLight is an electromagnetic wave.Visible light is a small part of the electromagnetic spectrum that humans are able to see.

LightThe electromagnetic spectrum consists of different kinds of light of different wavelengths.

EM Spectrum

EM Spectrum

EM Spectrum

EM Spectrum

EM Spectrum

EM Spectrum

EM Spectrum

Light InteractionsWhite light is light consisting of all colorsof visible light. These colors are visible in a rainbow or through a prism.

VelocityThe velocity of a wave is a product of its frequency and wavelength.v= flv = velocityf = frequencyl = wavelengthVelocityThe velocity of light through a vacuum(c) is about 3.0 x 108 m/sec. It is slightly slower through matter.Photoelectric EffectPhotoelectric effect refers to the emission of electrons from a metal when light shines on the metal.

Photoelectric EffectIt was found that light of a certain frequency would cause electrons to be emitted by a particular metal. Light below that frequency had no effect.

Emission SpectraIf an object becomes hot enough it will begin to emit light.

Emission SpectraMax Planck suggested that hot objects emit light in specific amounts called quanta (sing. quantum).

Emission SpectraPlanck showed the relationship between a quantum of energy and the frequency of the radiation.Equantum= hfEquantum= energy of a quantum in joulesh = Plancks constantf = frequencyWave-Particle DualityEinstein later said that light had a dual nature it behaved as both a particle and a wave.

Wave-Particle DualityEach particle of light, Einstein said, carries a particular quantum of energy.

Wave-Particle DualityEinstein called the particles of light photons which had zero mass and carried a quantum of energy. The energy is described as:Ephoton= hfPhotoelectric EffectEinstein explained photoelectric effect by saying in order for an electron to be ejected from a metal, the photon striking it must have enough energy to eject it.

AttractionDifferent metals have stronger attraction for their electrons than other. Therefore, some must absorb more energy than others to emit electrons.

Ground StateThe lowest energy state of an atom is called its ground state.

Excited StateWhen a current is passed through a gas at low pressure, the atoms become excited.

Excited StateAtoms in an excited state have a higher potential energy than their ground state.

Excited StateAn excited atom will return to its ground state by releasing energy in the form of electromagnetic radiation.

Emission SpectraElements will emit radiation of certain frequencies. This reflects the energy states of its electrons and is called a bright-line or emission spectrum.

Emission SpectraThe emission spectrum of an element is like its fingerprint.

SodiumHeliumMercuryEnergy LevelsStudying the emission spectrum of hydrogen lead Niels Bohr to the idea of energy levels.

Energy LevelsThe spectrum Bohr and others observed was the result of excited electrons releasing photons as they returned to their ground states.

48Energy LevelsThe difference in the energy of photons was reflected in the different frequencies of light they observed.

Section 2 ObjectivesBe able to define: diffraction, interference, Heisenberg Uncertainty Principle, Quantum Theory, quantum numbers, principal quantum number, angular momentum quantum number, magnetic quantum number, spin quantum number.Be able to distinguish between the Bohr model and the quantum model of the atom.Section 2 ObjectivesBe able to explain how the Heisenberg Uncertainty Principle and the Schroedinger Wave Equation led to the idea of atomic orbitals.Be able to list the four quantum numbers that describe each electron in an atom.Section 2 ObjectivesBe able to relate the number of sublevels corresponding to each of an atoms main energy levels, the number of orbitals per sublevel, and the number of orbitals per main energy level.

Electrons as WavesFrench scientist Louis De Broglie demonstrated that electrons had a dual nature also.

Electrons as WavesDe Broglie showed that electrons behaved as waves confined to the atom. The energy of those electrons could be found like that of waves: E = hfElectrons as WavesElectron beams were shown to exhibit the wave properties of diffraction and interference.

Heisenberg UncertaintyWerner Heisenberg tried to find the location and velocity of electrons in the atom.

Heisenberg UncertaintyHeisenberg found that it is impossible to simultaneously determine the position and velocity of an electron or any other particle (The Heisenberg Uncertainty Principle).

Schrdinger Wave EquationErwin Schrdinger said that electrons had a dual nature(like light) and treated them as waves.

Quantum TheorySchrdingers wave equation and Heisenbergs Uncertainty Principle laid the foundation of modern quantum theory.

Quantum TheoryQuantum theory describes mathematically the wave properties of electrons and other very small particles.

Quantum TheoryAccording to the Bohr model we should be able to predict the location and velocity of an electron at any time.

Quantum TheoryQuantum theory disagrees with the Bohr model and says that electrons can be found in regions of high probability but cannot be pinpointed.

OrbitalsQuantum theory describes electrons as inhabiting a three-dimensional region around the nucleus that indicates their probable locations. These regions are called orbitals.

OrbitalsScientists use quantum numbers to describe orbitals. These numbers describe the properties of the orbitals and the electrons which occupy them.

s and p orbitals

d orbitals

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