Chemistry I Honors History The Electron Cloud Shells and Orbitals

Some History… Remember that John Dalton described the atom as a tiny, indivisible particles in his “Atomic Theorie”. His paper did not include any mention of “sub-atomic particles” – they had not been discovered at that time.

Sub-Atomic Particles J.J. Thomson discovers the electron while experimenting with the cathode ray tube. Shortly after, the proton is identified.

Atoms have a Nucleus Ernest Rutherford discovers that the atom has a small, dense center structure that contains all of the protons. He calls this structure the “nucleus” - (undoubtedly borrowed from Biology). This changes the “vision” of the atom into an idea one that has a positively charged nucleus (because of the protons) and an “electron cloud” where the electrons exist.

Gold Foil Experiment

The Next Question Science found itself trying to answer the following question: “If the nucleus has a positive charge (protons) and the electrons are negatively charged, why don’t the electrons “fall into” the nucleus?”

The Birth of Quantum Mechanics Bohr proposed that the electrons actually move around the nucleus in fixed, circular orbits. Borrowing from astronomy, this became the “solar-system” model of the atom.

Not Just Circular Motion… Bohr’s colleague, Max Planck determines that the electrons can actually move into paths that are farther away from the nucleus – and they can also move into “orbits” that are closer to the nucleus.

Combining Thoughts Together, Bohr and Planck arrive at a conclusion that the electron cloud can be thought of as having different “layers” in which the electrons move. The electrons in these layers have specific energies.

Review of the Atom We have learned that the atom has a structure that contains a nucleus in the middle that is surrounded by an electron cloud. We are able to use the Atomic Number to determine the number of protons in the nucleus and the number of electrons in the electron cloud.

Now for a Focus on the Electron Cloud Chemistry views the electron cloud as having layers – we call these layers SHELLS. We name these SHELLS with a single letter – starting with the SHELL that is closest to the nucleus. Shells: K (closest to nucleus) L M N O P Q (farthest from nucleus)

More on Shells Electrons in these shells have certain energies associated with their distance from the nucleus. (they have energy because they are moving) So, electrons in the K Shell have less energy that the electrons in the other shells. And… electrons in the Q Shell have more energy that any other electrons.

Shells and Energy Electrons closer to the nucleus have less energy than electrons farther away from the nucleus.

Jumping Shells It is possible for electrons to jump from a closer shell to one that is farther away. The electron must absorb energy from some source – like heat, or light, or electricity. Nucleus Electron gains energy from some outside source and moves farther away from the nucleus.

And falling… It is also possible for an electron to “fall” from a far-away shell to one that is closer to the nucleus. When this happens, the electron gives off energy as light, or X- rays, or radio waves, or a number of other types of energy. nucleus Electron will give off energy as it falls to a closer shell.

State Expectation The state NCFE is going to expect you to be able to determine which form of electromagnetic energy will be given off as a specific transition occurs.

So let’s Review: Seven SHELLS that are like layers make up the electron cloud. They are K, L, M, N, O, P, and Q K shell is closest to the nucleus and its electrons have the lowest energy. Q shell is farthest from the nucleus and its electrons have the most energy. Electrons can gain or lose energy and move from shell to shell.

Now for Orbitals… Starting with the L shell (the 2 nd shell from the nucleus), the shells are sub-divided into regions called orbitals. Depending on the shell, there may be four different “types” of orbitals. The 1 st shell (K shell) only has one region because it is the closest to the nucleus. The entire shell is that single region.

Orbital Types The first type of orbital is called an “s” orbital. Each of the shells has one of these “s” orbitals. This region can “hold” two electrons. The next type of orbital is called a “p” orbital. Nature actually establishes 3 of these orbitals in each shell - starting with the 2 nd shell (the L-Shell) Each of the individual p- orbitals can hold 2 electrons – so the set of them can hold a total of 6 electrons.

Continuing… The third type of orbital is called the “d-orbital”. This type occurs in sets of 5 regions starting with the 3 rd shell (the M-Shell). Each of the individual d- orbitals can hold 2 electrons for a total of 10 electrons. The last of the orbital types is called the “f- orbital”. This type occurs in sets of 7 regions starting with the 4 th shell (the N- shell). Each of the individual orbitals can hold 2 electrons for a total of 14 electrons

First Task: Use the information presented so far to answer the questions and fill in the chart on Electron Cloud Structure.

Next Focus – Shell Capacities Because the shells contain different numbers of orbitals, they can hold different numbers of electrons. This is important – any single orbital can hold 2 electrons. – This means that an “s” orbital can hold 2 electrons. – But, a set of “p” orbitals can hold a total of 6 electrons. – A set of “d” orbitals can hold a total of 10 electrons. – And a set of “f” orbitals can hold a total of 14 electrons.

Capacities – continued… s p d f Each region can hold 2 electrons

Task #2 Now use the new information along with the previous table to fill in Table #2 on the worksheet.