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Chapter 5. Electrons in Atoms. The Bohr Model. An electron is found only in specific circular paths, or orbits, around the nucleus. Each orbit has a fixed energy. The orbits are called ‘energy levels.’. Energy Levels. Energy levels are like the rungs of a ladder:

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chapter 5

Chapter 5

Electrons in Atoms

the bohr model
The Bohr Model
  • An electron is found only in specific circular paths, or orbits, around the nucleus.
  • Each orbit has a fixed energy. The orbits are called ‘energy levels.’
energy levels
Energy Levels
  • Energy levels are like
  • the rungs of a ladder:
    • You can move up or down by going from rung to rung.
    • You can’t stand in-between rungs.
  • For an electron to change energy levels it must gain or lose exactly the right amount of energy.
a quantum
A Quantum
  • A quantum of energy is the amount needed to move an electron from one energy level to another.
  • The energy of an electron is said to be “quantized.”
  • Energy levels in an atom are not all equally spaced.
an airplane propeller
An Airplane Propeller
  • The blurry picture of an airplane propeller represents the area where the actual propeller blade can be found.
  • Similarly, the electron cloud of an atom represents the locations where an electron is likely to be found.
the model quantum mechanical
The Model Quantum Mechanical
  • Comes from the mathematical solution to the Schrodinger equation.
  • Determines allowed energies an electron can have & how likely it is to find the electron in various locations around the nucleus.
  • Uses probability
atomic orbitals
Atomic Orbitals
  • A region in space in which there is a high probability of finding an electron.
  • Energy levels of electrons are labeled by principal quantum numbers (n)

n = 1, 2, 3, 4 …

s orbitals
s Orbitals

are spherical

p orbitals
p Orbitals

are dumbbell- shaped

d orbitals
d Orbitals

4 out of the 5 d orbitals have clover leaf shapes

f orbitals
f Orbitals

are more complicated

atomic orbitals1
Atomic Orbitals

The number and kinds of atomic orbitals depend on the energy sub level.

  • N=1 has 1 sublevel called 1s
  • N=2 has 2 sublevels called 2s and 2p
  • N=3 has 3 sublevels called 3s, 3p, and 3d
  • N=4 has 4 sublevels 4s, 4p, 4d, and 4f

The maximum number of electrons that can occupy a principle energy level is 2n2.

(n=principle quantum #)

electron configurations
Electron Configurations
  • Electrons in an atom try to make the most stable arrangement possible (lowest energy)
  • The Aufbau Principle, the Pauli Exclusion Principle, and Hund’s Rule are guidelines that govern electron configurations in atoms
aufbau principle
Aufbau Principle
  • Electrons occupy the orbitals of lowest energy first
pauli exclusion principle
Pauli Exclusion Principle
  • An orbital can hold at most 2 electrons
  • Does it make sense that two negatively charged particles will ‘want’ to share the same space?
  • This phenomenon is made possible because electrons possess a quantum mechanical property called spin
electron spin
Electron Spin
  • Spin may be thought of as clockwise or counter-clockwise
  • An arrow indicates an electron and its direction of spin
  • An orbital containing paired electrons is written
hund s rule
Hund’s Rule
  • When filling orbitals of equal energy, one electron enters each orbital until all the orbitals contain one electron with similar spin
hund s rule1
Hund’s Rule
  • How would you put 2 electrons into a p sublevel?
  • How would you put 7 electrons into a d sublevel?
  • Now that we understand how electrons are arranged in atoms, we can begin to look at how the frequencies of emitted light are related to changes in electron energies
  • Light waves properties:
  • Amplitude – the wave’s height from zero to crest
  • Wavelength – the distance between crests
  • Frequency – the number of wave cycles to pass a given point per unit of time (Usually Hz = 1/s)
  • Wavelength has the symbol (λ) lambda.
  • Frequency has the symbol (ν) nu.
  • The speed of light is a constant (c) = 3x108 m/s
  • c = λν
  • How are wavelength and frequency related?
  • They are inversely related. As one increases, the other decreases
  • How long are the wavelengths that correspond to visible light?
  • 700-380 nanometers
electromagnetic spectrum
Electromagnetic Spectrum
  • Visible light is only a tiny portion of the electromagnetic spectrum which also includes radio waves, microwaves, infrared, visible light, ultra violet, X-rays, and gamma rays.
  • If the entire electromagnetic spectrum was a strip of professional 16 mm movie film stretching from Los Angeles to Seattle, the portion of visible light would be only ONE frame of film.
atomic spectra
Atomic Spectra
  • When atoms absorb energy, electrons move to higher energy levels
  • Electrons then lose energy by emitting light as they return to lower energy levels
  • Atoms emit only specific frequencies of light that correspond to the energy levels in the atom
  • The frequencies of light emitted by an element separate into discrete lines to give the atomic emission spectrum of the element
atomic spectra1
Atomic Spectra
  • An electron with its lowest possible energy is in its ground state
  • The light emitted by an electron is directly proportional to the energy change of the electron.
  • E = hν
  • Atomic spectra are like fingerprints: no two are alike!