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Ch. 5: Models of the Atom
What you need to know: Chapter 5• Timeline pp. 128-129 (theory and people)• Definitions: quantum mechanical model,
atomic orbitals, speed of light, electromagnetic radiation, atomic emission spectrum (ROYGBIV), ground state, photon
• Bohr model• Electron configurations
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The Development of Atomic Models
• The timeline shoes the development of atomic models from 1803 to 1911.
5.1
This timeline needs to be in your notes!
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The Development of Atomic Models
• The timeline shows the development of atomic models from 1913 to 1932.
5.1
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5.1 The Bohr Model
Bohr proposed that an electron is found only in specific circular paths, or orbits, around the nucleus
Each orbit has a particular energy level; electrons must gain or lose energy to move from one energy level to the next
A quantum of energy is the amount of energy required to move an electron from one energy level to another energy level
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5.1 Quantum mechanical model
• Mathematically determined by Erwin Schrodinger• determines the allowed energies an electron can
have and how likely it is to find the electron in various locations around the nucleus
• An atomic orbital is often thought of as a region of space in which there is a high probability of finding an electron.
– Each energy sublevel corresponds to an orbital of a different shape, which describes where the electron is likely to be found. (s, p, and d orbitals)
(Bohr used orbits; Schrodinger used orbitals)
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Atomic Orbitals
• Different atomic orbitals are denoted by letters. The s orbitals are spherical, and p orbitals are dumbbell-shaped.
5.1
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Atomic Orbitals
• The numbers and kinds of atomic orbitals depend on the energy sublevel.
5.1
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Atomic Orbitals
• The number of electrons allowed in each of the first four energy levels are shown here.
5.1
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Electron Arrangement in Atoms
If this rock were to tumble over, it would end up at a lower height. It would have less energy than before, but its position would be more stable. You will learn that energy and stability play an important role in determining how electrons are configured in an atom.
5.2
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Electron Arrangement in Atoms
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Electron Configurations
Electron Configurations
What are the three rules for writing the electron configurations of elements?
5.2
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Electron Arrangement in Atoms
> Electron Configurations
The ways in which electrons are arranged in various orbitals around the nuclei of atoms are called electron configurations.
Three rules—the aufbau principle, the Pauli exclusion principle, and Hund’s rule—tell you how to find the electron configurations of atoms.
5.2
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Electron Arrangement in Atoms
> Electron Configurations
Aufbau Principle
According to the aufbau principle, electrons occupy the orbitals of lowest energy first. In the aufbau diagram below, each box represents an atomic orbital.
5.2
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Electron Arrangement in Atoms
> Electron Configurations
Pauli Exclusion Principle
According to the Pauli exclusion principle, an atomic orbital may describe at most two electrons. To occupy the same orbital, two electrons must have opposite spins; that is, the electron spins must be paired.
5.2
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Electron Arrangement in Atoms
> Electron Configurations
Hund’s Rule
Hund’s rule states that electrons occupy orbitals of the same energy in a way that makes the number of electrons with the same spin direction as large as possible.
5.2
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Electron Arrangement in Atoms
> Electron Configurations
Orbital Filling Diagram
5.2
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Electron Arrangement in Atoms
> Electron Configuration and Orbital Filling Practice
1. Write the electron configuration and orbital filling for
a. Li
b. Mg
c. Si
a. Li: atomic number 3
1s22s1
b. Mg: atomic number 12
1s22s22p63s2
___
c. Si: atomic number 14
1s22s22p63s23p2
____
_
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Electron Arrangement in Atoms
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Exceptional Electron Configurations
Exceptional Electron Configurations
Why do actual electron configurations for some elements differ from those assigned using the aufbau principle?
5.2
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Electron Arrangement in Atoms
> Exceptional Electron Configurations
Some actual electron configurations differ from those assigned using the aufbau principle because half-filled sublevels are not as stable as filled sublevels, but they are more stable than other configurations.
5.2
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Electron Arrangement in Atoms
> Exceptional Electron Configurations
Exceptions to the aufbau principle are due to subtle electron-electron interactions in orbitals with very similar energies.
Copper has an electron configuration that is an exception to the aufbau principle.
5.2
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Light
According to the wave model, light consists of electromagnetic waves.
Electromagnetic radiation includes radio waves, microwaves, infrared waves, visible light, ultraviolet waves, X-rays, and gamma rays.
All electromagnetic waves travel in a vacuum at a speed of 2.998 108
m/s.
5.3
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Light
Sunlight consists of light with a continuous range of wavelengths and frequencies.
When sunlight passes through a prism, the different frequencies separate into a spectrum of colors.
In the visible spectrum, red light has the longest wavelength and the lowest frequency.
ROYGBIV (red, orange, yellow, green, blue, indigo, violet)
5.3
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Light
The electromagnetic spectrum consists of radiation over a broad band of wavelengths.
5.3
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Atomic Spectra
Atomic Spectra
What causes atomic emission spectra?
5.3
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Atomic Spectra
When atoms absorb energy, electrons move into higher energy levels. These electrons then lose energy by emitting light when they return to lower energy levels.
5.3
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5.3 Atomic Spectra
a. A prism separates light into the colors it contains. When white light passes through a prism, it produces a rainbow of colors.
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5.3 Atomic Spectra
a. When light from a helium lamp passes through a prism, discrete lines are produced.
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5.3 Atomic Spectra
a. The frequencies of light emitted by an element separate into discrete lines to give the atomic emission spectrum of the element.
Mercury Nitrogen
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5.3 An Explanation of Atomic Spectra
An Explanation of Atomic Spectra
How are the frequencies of light an atom emits related to changes of electron energies?
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5.3 An Explanation of Atomic Spectra
a.In the Bohr model, the lone electron in the hydrogen atom can have only certain specific energies.
When the electron has its lowest possible energy, the atom is in its ground state.
Excitation of the electron by absorbing energy raises the atom from the ground state to an excited state.
A quantum of energy in the form of light (photon) is emitted when the electron drops back to a lower energy level.
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5.3 An Explanation of Atomic Spectra
The light emitted by an electron moving from a higher to a lower energy level has a frequency directly proportional to the energy change of the electron.
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5.3 Quantum MechanicsLight was found to have properties similar to….
a.In 1905, Albert Einstein successfully explained experimental data by proposing that light could be described as quanta of energy.
The quanta behave as if they were particles.Light quanta are called photons.
b.In 1924, De Broglie developed an equation that predicts that all moving objects have wavelike behavior.
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