It's Elementary :

 Orbitals
 
Tool: Interactive Periodic Table

Orbitals and Electrons
An orbital is a region of probability in which the electron can be found. These regions have very specific shapes, based on the energy of the electrons that will be occupying them. Try to think of an orbital as a loveseat. An orbital, like a loveseat, can hold up to two occupants, in this case electrons. The loveseats can be different "styles" based on the energy of the electrons that will occupy them and the energy level they are found in. The lowest-energy "style" orbital is the s orbital. It is sphere-shaped and is always the first orbital filled in any energy level. The first energy level has only one orbital, so it must be an s orbital. All of the elements in the first two columns on the left side of the Periodic Table are filling the s orbital of their highest energy level with their final (highest energy) electron. This makes the first two columns of the table the s block of elements.

The S Orbital
The sphere shaped s orbital is the first place an electron can be located in any atom. Most of any atom is really empty space! Now let's try filling in the electrons in a couple of atoms. Starting with hydrogen, with one proton it will have one electron. That electron has to go into the first energy level, in the 1s orbital. Helium, with two protons, will have two electrons. These are placed one at a time into the 1s orbital. This makes helium a very contented atom. It has a complete energy level and does not want to gain or lose any electrons. The helium atom is not reactive. In fact, helium is the first of a group of elements often referred to as the inert or noble gases.


The shape of S-orbitals in a sphere. Electrons can move anywhere within the sphere.
The P Orbital
Once the s orbital has been filled for any energy level, electrons start filling in the next higher orbital "style" -- the p orbital. The p orbitals are shaped like propeller blades, with one set lined up along the x axis of the atom (horizontally). A second p orbital is lined up along the y axis of the atom (vertically). The third and final p orbital is lined up along the z axis (from front to back through the atom).

This makes a total of three p orbitals to be filled in any given energy level from the second up. Each of these three p orbitals needs to be filled with electrons in the second and higher energy levels. But there is another rule to follow in placing the electrons in the p orbitals. This is Hund's Rule.

Hund's rule states that each p orbital must receive one electron before any p orbital can receive a second filling electron. You could think of this as the good mother rule. Everybody gets one before anybody gets seconds.


Illustraion showing S and P orbitals
Filling the Orbitals
Let's try a sample filling of electrons in p orbitals. To do this we will have to go all the way over to element number five, boron. This means there are five protons and five electrons in the atom. Remember, the first orbital of any energy level is an s orbital. This means that the elements lithium and beryllium, while they need to put electrons in the second energy level, must fill their 2s orbital with their highest energy electrons. In boron, the first two electrons are placed into the 1s orbital. The first energy level cannot hold any more electrons, and there are still three to place in the atom. These electrons will have to be placed in the next higher energy level. The first orbital to be filled in the second energy level is the 2s orbital. This still leaves one electron to be placed. The last electron has to go into a p orbital. Boron is the first element of a section of the periodic table that could be called the "p block." It is made up of the elements that are filling p orbitals with their final electrons. In the table, there is a rectangular segment located to the right of the drop-down central portion. There are six columns of elements in this rectangle. These are the elements with filled p orbitals -- the p block. (Helium is an exception, as the first energy level does not have any p orbitals.)

The D Orbital
There are two other "styles" of orbitals that are too complex to go into detail about here. They are the d orbitals, of which there are five, and the f orbitals, of which there are seven. The Periodic Table provides a section for each of these groups of orbitals. The 10 electrons of the five d orbitals are filled by the elements found in the dropped central section of the table. This section is referred to as the d block elements, or the transition metals. The two rows found separated at the bottom of the table are the elements of the f block. These elements fill the highest energy levels of the f orbitals when they place their last and highest-energy electrons at the ground state.

 The periodic table with the P-Block highlighted

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