Chemistry I Honors

HistoryThe Electron CloudShells 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 2nd 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 1st 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 2nd 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 3rd 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 4th 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.