REVIEW OF ELECTRON CONFIGURATIONS

Historically, the properties of elements and the way those elements combined with other elements were the basis for the development of the periodic table (particularly the short form in the latter half of the 19th century). When physicists developed an understanding of how atoms were constructed (particularly the configuration of the electrons), it was possible to relate that structure to the periodic table (particularly the long form). Our next lesson (Bonding) will emphasize how the electron configurations dictate the chemical properties and combining patterns of the elements.

ELECTRON CONFIGURATIONS AND THE PERIODIC TABLE

Now take a look at the periodic table shown below. (A similar table is shown in example 2 in your workbook.) It is another periodic table, but instead of having atomic weights it has the last part of the electron configuration for each of the elements.

PATTERNS

The point here is to emphasize some of the patterns that exist in the relationship between the electron configurations of the elements and the location of the elements on the periodic table and even the shape of the periodic table.

Looking at this example note how the periodic table can be broken up into s, p, d and f blocks. The first two columns of the periodic table (groups Ia and IIa) are in the s block because the elements in these two groups have their outermost electrons in s orbitals. Note that the s block has two groups because atoms can put two electrons in an s sublevel.

The p block consists of groups IIIa through the inert gases. These are the elements which have their last electrons in p orbitals. Note that there are six groups in this block because atoms can put up to six electrons into a p sublevel.

The transition elements comprise the d block. The d block has 10 columns because up to 10 electrons will fit into a d sublevel.

The f block at the bottom of the periodic table has 14 columns because up to 14 electrons can fit into an f sublevel.

Remember the pattern we worked with that resulted in the electron configurations. 1s, then 2s, 2p, 3s, then 3p, 4s, then 3d, 4p, 5s. That resulted in electron configurations that looked like this. 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰ and so on. The periodic table shows that same arrangement in a different way. The first period has 1s¹ then ². The second period has 2s¹ then ², then 2p¹, ², ³, ⁴, ⁵, and ⁶. Similarly, the third period has 3s¹ then ², then 3p¹, ², ³, ⁴, ⁵ and ⁶. The fourth period with the transition elements in the middle has 4s¹ then ², followed by 3d¹, ², ³, ⁴ is altered, then ⁵, ⁶, ⁷, ⁸, ⁹ is also altered, then ¹⁰, followed by 4p¹, ², ³, ⁴, ⁵ and ⁶. The periodic table continues on showing the pattern dictated by the electron configurations.

SHORT CUTS FOR ELECTRON CONFIGURATIONS

Next, let's use the periodic table to provide a short cut method of getting the last part of the electron configuration for nearly any element.

PRACTICE

For practice, try figuring out the last orbital electron configuration for the elements shown below (they are also listed in example 4 in your workbook). Use the periodic table in workbook example 1 for reference. Then use the periodic table in workbook example 2 to check your answers. That table includes the s electrons for the p block and d block elements. You may include them or not as you wish.

SHORT CUTS AND COMPLETE CONFIGURATIONS

This short-cut method can also be used to determine the complete electron configurations. Use the location of the element on the periodic table to figure out the last entry in the configuration, as you have just done. Use the shape of the periodic table to remind you of the order in which the sublevels fill. Do this by reading the blocks in each period in order. First 1s. Then 2s, 2p. Then 3s, 3p. Then 4s, 3d, 4p. Next 5s, 4d, 5p, and so on. Keep adding electrons to that pattern until you have reached what you know to be the end of the electron configuration for that element based on its location on the periodic table.

ORBITALS AND THE PERIODIC TABLE

The repeating nature of the electron configuration or electron structure is shown here also, this time with emphasis on the orbitals. Note that the atomic size trends are also apparent in this diagram.

If you take a look at the electron configurations that are shown there, hydrogen with its 1s1 shows the 1s orbital. Then for helium, 1s2, the 1s orbital is also shown. It is a little bit darker to indicate that there are two electrons in it.

Then the third electron goes into the 2s1 and this is shown with lithium. Notice that the 1s orbital is shown in the center and then the 2s orbital is shown around it in the lighter color and larger. With beryllium there are two electrons in the 2s orbital.

With boron if you look closely, you can see the p orbital that has the fifth electron in it. With carbon you can see that there are two of the p orbitals. With nitrogen you can see that there are three of the p orbitals shown. With oxygen there are still the same number of orbitals, but there are four p electrons. There are more and more electrons as you go from oxygen to fluorine to neon. Thus the diagrams get darker to show the higher concentration of electrons.

Then with sodium you have to start working with another s orbital and that is shown in yellow here. Magnesium shows another electron in that s orbital.

With aluminum, silicon, and phosphorus, you can see the additional p orbitals in the third energy level showing up there. Then the concentration of electrons gets more intense and thus the color gets darker and you go to sulfur, chlorine, and argon.

Again with one more electron after argon, you need to start another energy level and that is shown as the 4s energy sublevel with potassium and also the same with calcium.