Quantum Mechanical Model of the Atom

Just as crawling is the first developmental step to mobility, earlier models of the atom lead the way to the modern quantum mechanical model of the atom.



B2.32 Baby crawling
Bohr knew his model needed to be modified. It needed to be able to predict/explain elements other than just hydrogen. Over the next several years, many scientists, including Louis de Broglie, Erwin Schrödinger, and Werner Heisenberg, collaborated and contributed to the development of the quantum mechanical model of the atom.

Recall that in the Bohr model, the exact path of the electron was restricted to circular energy levels around the nucleus. In actuality, electrons do not travel around the nucleus in simple circular orbits. The quantum mechanical model gives the probability of finding an electron at a given point around the nucleus. This model can be portrayed as a positive nucleus surrounded by an electron cloud.

Where the cloud is densest, the probability of finding the electron is greatest and the electron is less likely to be in a less dense area of the cloud. This model is based on probability rather than certainty.

The quantum mechanical theory is very complex and includes complex mathematical equations, the understanding of quantum theory (which says that matter has properties associated with waves), and the application of the uncertainty principle.
© Wikimedia Commons
B2.33 Atom model at the American Museum of Science and Energy in Oak Ridge


  Try This

Modeling an Electron Cloud


The electron cloud can be modeled in the following way.

  1. Place a piece of paper on the floor with a red dot in the center, representing the nucleus.
  2. Standing over the paper, take a black marker with your arm extended straight out and try to drop it onto the red dot.
  3. You should observe small marks at each point the marker hits.
  4. Repeat many, many times.
  5. Click on the analysis tab to complete the analysis questions.
  1. What does each dot represent?

    Each dot represents a location where the electron could be at any given moment. Because of the uncertainty principle, there is no way to know exactly where the electron is.
  2. What does the overall pattern look like?

    The overall pattern of dots will be roughly circular.
  3. Are the dots evenly distributed?

    There will be more dots near the nucleus and progressively fewer dots as you move away from it. An electron cloud has variable densities: a high density where the electron is most likely to be and a low density where the electron is least likely to be.

  Interactive Activity

Structure of an Atom © The Concord Consortium


This simulation shows the possible location of electrons at various moments in time. You can explore the pattern of where electrons will most likely be located

Click the procedure tab to continue.

  1. Click on the play icon to open the interactive activity. The interactive can also be found at https://quick.adlc.ca/alpha
  2. Slide “Delay between finding Electrons” all the way to “long.”
  3. What do you notice about the pathway of the electrons?

    The motion is random and variable in distance from the nucleus.
  4. Check “Trace Electrons.”
  5. Click “Start.”
  6. Let simulation run for 25 s.
  7. Click “Stop”
  8. What do you notice about the distribution of the electrons?

    They are more concentrated around the nucleus.
  9. Uncheck “Trace Electrons.”
  10. Slide “Delay between finding Electrons” all the way to “short.”
  11. Click “Start.”
  12. How many electrons does it look like there are? Explain.

    It appears as many more than two electrons. This is because the electrons are moving so fast.
  13. Click “Stop.”
  14. How many electrons are there?

  15.  Check “Trace Electrons.”
  16. Click “Start.”
  17. Let simulation run for 25 s.
  18. Click “Stop.”
  19. What do you notice about the distribution of the electrons?

    They are more concentrated around the nucleus.

  Quantum Mechanical Model of the Atom Continued


The quantum mechanical theory still incorporates the concept of energy levels; the pathways are just not as restricting as Bohr’s model. It goes further to also incorporate energy sub-levels, orbitals, and electron spin.

Although Rutherford suspected there was a third subatomic particle, this was not proven until 1932. In 1932, James Chadwick bombarded beryllium atoms with high energy particles. This caused another particle—with a neutral electrical charge and the approximate mass of a proton—to come loose. This particle became known as the neutron.


B2.34 Model of an atom


Since 1932, through continued experimentation, many additional particles have been discovered in the atom, such as quarks that make up protons and neutrons. Also, new elements have been created by bombarding existing nuclei with various subatomic particles. The study of the composition of the atom continues to be an ongoing and exciting journey.

  Read This

Please read pages 25 and 32 to 33 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the quantum mechanical model of the atom. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. What particles are found in the nucleus of the atom? Describe these particles.

    The proton and the neutron. The proton is a positively charged particle, while the neutron is a neutral particle. Both the proton and neutron are of approximately the same size.
  2. What is the third particle found in the atom? Where is it located? How does it compare to the particles found in the nucleus?