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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!