Student Resources

Figure 2.15 The Periodic Table and Electron Configurations. Note that each of the families on the periodic table corresponds to a specific electron configuration pattern. This makes it easy to determine where the electron configuration ends, and allows us to work backwards to complete the electron configuration.

Example 1

1. What is the electron configuration of Arsenic?

Solution:

1. We can use the periodic table like a map and determine where the final electron in Arsenic is placed:

We can see that arsenic ends in the p-block at np3, and if we follow the period back to the left, we can see the n=4. Therefore, arsenic ends in 4p3. We know that according to Aufbau’s principle, that all of the lower energy subshells must be filled with electrons. We can then just walk backwards on the periodic table to read and fill in the lower energy subshells. If we walk backwards from arsenic along the previous periods of the periodic table, we run into the transition metals which represent the d-block. Remember that for the d-block subshell that the parent shell is (n-1). Since we are tracking back from arsenic in row 4, the d-block that we run into is n-1 or 4-1 = 3. So the next subshell would be 3d10, followed by 4s2, then 3p6, 3s2, 2p6, 2s2, and finally 1s2. Putting it all together, the electron configuration of arsenic is:

As = 1s22s22p63s23p64s23d104p3

Example 2

2. What stunning element ends with an electron configuration of 5d9? What is the complete electron configuration?

Solution

Again, we can use our knowledge of the periodic table to determine which element that this is represented by this configuration and help us write the full configuration.

The one thing that we have to be careful of when evaluating d-block elements, is that we remember that the d-block shell is always calculated as n-1. So for our problem, we know that our element in question end in 5d9. So the shell (n) or row that our element is in needs to be calculated using the formula n -1 = 5, which gives our final shell number of n = 6. Now we can simply follow the nd9 column down to the 6th row, and we find that Au or gold is our element. 
To complete the electron configuration, we would simply walk back along the periodic table to fill in all of the lower energy subshells with electrons. So we start from gold at 5d9 and start walking back. What we notice is that the first thing we run into is an f-block (the lanthanide series). Note that the f-block shell is represented by (n-2). Since we are still in the 6th row, this would mean that the 6-2 = 4. The f-block is the 4f subshell. Recall that the f-subshell can hold a total of 14 electrons. Thus, the electron configuration is 4f14. This is followed by 6s2, 5p6, 4d10, 5s2, and so on, all the way back to 1s2. If we write out the full electron configuration of gold, it would be:

Au = 1s22s22p63s23p64s23d104p65s24d105p66s24f145d9

 

More Review Questions

1. 
   If an element is said to have an outermost electronic configuration of ns2np3, it is in what group in the periodic table?
   

(a) Group 3A

2. 
   

(b) Group 4A

3. 
   

(c) Group 5A

4. 
   

(d) Group 7A

5. 
   
6. 
   What is the general electronic configuration for the Group 8A elements? (Note: when we wish to indicate an electron configuration without specifying the exact energy level, we use the letter “n” to represent any energy level number. That is, ns2np3 represents any of the following; 2s22p3, 3s23p3, 4s24p3, and so on.)
   

(a) ns2np6

7. 
   

(b) ns2np5

8. 
   

(c) ns2np1

9. 
   

(d) ns2

10. 
    
11. 
    The group 2 elements are given what name?
    

(a) alkali metals

12. 
    

(b) alkaline earth metals

13. 
    

(c) halogens

14. 
    

(d) noble gases

15. 
    
16. 
    Using the diagram below, identify:
    

(a) The alkali metal by giving the letter that indicates where the element would be located and write the outermost electronic configuration.

17. 
    

(b) The alkaline earth metal by giving the letter that indicates where the element would be located and write the outermost electronic configuration.

18. 
    

(c) The noble gas by giving the letter that indicates where the element would be located and write the outermost electronic configuration.

19. 
    

(d) The halogen by giving the letter that indicates where the element would be located and write the outermost electronic configuration.

20. 
    

(e) The element with an outermost electronic configuration of s2p3 by giving the letter that indicates where the element would be located.

21. 
    

(f) The element with an outermost electronic configuration of s2p1 by giving the letter that indicates where the element would be located.

22. 
    

23. 
    
24. 
    In the periodic table, name the element whose outermost electronic configuration is found below. Where possible, give the name of the group.
    

(a) 5s2

25. 
    

(b) 4s23d104p1

26. 
    

(c) 3s23p3

27. 
    

(d) 5s24d105p2

28. 
    

(e) 3s1

29. 
    

(f) 1s2

30. 
    

(g) 6s25d106p5

31. 
    

(h) 4s24p4

32. 
    

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Electron Configuration Solitaire

If you are having some trouble using the periodic table to determine electron configurations and don’t want to memorize the energy level diagram. There is one more way to help you determine the correct electron configuration of an element. This is also a good method to use to double check you work, when you are using a periodic table to help you determine electron configurations. The method is ‘electron configuration solitaire’. I call this method solitaire because that is what it reminds me of every time I draw this chart. Essentially, you are going to lay out all of the electron subshells into neat rows (left to right that are arranged by subshell type (s, p, d, and f). The first row will contain all of the s-subshells arranged from the lowest to the highest shell (n) number. So the first row would look like this:

Next align all of the other subshells (p, d and f) in neat rows directly under their appropriate shell lane. Once your diagram is complete, it will look like you have just dealt yourself a game of solitaire. In reality, you have just created a simple tool to enable you to write out any electron configuration. 

To use your chart, you need to draw in diagonal arrows in the direction of electron filling. The final diagram tool should look like this:

It is read from left to right, starting at the bottom of each subsequent arrow. 

Example

1. 
   What is the electronic configuration of Bromine?
   

Solution

For using the chart above, it is only necessary to determine how many electrons should be present in the element bromine. If we look back at the periodic table, we will see that there are 35 electrons. I find it also really quick to construct the electron configurations if I use a combination of the solitaire method and the periodic table method. I like to use the periodic table to find the end place of my electron configuration. If we look back again at bromine, we see that it is in the p-block, ending at 4p5. So we can follow the chart above, until we reach 4p5. If you don’t use the periodic table for this, you should keep track of your electrons and stop the configuration once you have placed all 35 electrons. Either method should give you the same result. Now just read off the appropriate shell/subshells and add in the appropriate amount of electrons for the superscripts:

Br = 1s22s22p63s23p64s23d104p5

Remember also, that adding up your electrons is a good quality check to make sure that the configuration matches the required number of electrons. In this case:

2 +2 + 6 + 2 + 6 + 2 + 10 + 5 = 35

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Electron Configuration Shorthand

Writing out electron configurations can fast become tiresome, especially for larger elements like gold. Thus, chemists have made electron configuration shorthands for elements so that they wouldn’t have to keep rewriting all of the same subshell beginnings that make up most of the elements. This shorthand uses the Noble Gases as the shorthand reference point. Thus, when you are using the periodic table to determine an electron configuration, you only need to walk back on the periodic table until you run into your first Noble Gas. At that point the shorthand is written by placing the Nobel Gas in a hard bracket followed by the remaining electron configuration. Recall that the electron configuration that we constructed for gold (Au) was quite cumbersome and looked like:

Au = 1s22s22p63s23p64s23d104p65s24d105p66s24f145d9

With this electron configuration, if we use the periodic table to walk back, we run into our first noble gas at 5p6. (Hint: All the noble gases end at np6, so when you are walking back in you electron configurations, you should look for the first np6 that you see.) The noble gas at this position is Xenon (Xe). Thus, we can shorthand the Au configuration to look like:

Au = [Xe]6s24f145d9

Since xenon has all of the electrons up to 5p6, we can replace everything up to that point with the symbol for xenon. Essentially what we are then saying is that the electron configuration for gold looks like xenon + the 6s24f145d9
Learning the shorthand can save you a lot of time! 

Extra Practice: 

What are the shorthand electron configurations of:

Br = 1s22s22p63s23p64s23d104p5

and

As = 1s22s22p63s23p64s23d104p3

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Electron-Dot Symbols

Although it is entirely possible to define the number of valence electrons in an atom through numbers, sometimes it is helpful to have a graphical representation. The graphical notation used for valence electrons is called an Electron-Dot Symbol. To draw an electron dot symbol, start with the abbreviation for the element of interest as the center, signifying the nucleus of the atom. From there, identify the number of valence electrons the atom has, and then add a single dot for each electron around the nucleus. Students often want to place these electron dots around the nucleus randomly, but it is useful for us to pair electron dots together as we might in an orbital. Following the same rules discussed above about orbital filling, the electron dot symbols for multiple atoms can be seen in the figure below. The easiest way to fill the orbitals is to start at one edge, add a dot, and fill in the rest of the dots one at a time by rotating clockwise on the edges of a square boxing in the nucleus until you have used all of the valence electrons. Note that each side should never have more than two dots, and that you should follow Hund’s rule when constructing your Electron-Dot Symbol. Each side should get one electron before the electrons are paired up.

Example

How many valence electrons does bromine have? draw the electron dot representation of bromine.

Solution

For the first part, we need to analyze the electron configuration of bromine and determine the valence shell and the electrons housed there:

We can see that the outer most shell is level 4, and that there are a total of 7 electrons housed there. Thus, bromine has 7 valence electrons. The electron dot symbol should look like:

Notice that this electron dot symbol for bromine looks just like the electron dot symbol for fluorine that is drawn above. This is because they are both in the halogen family. Recall that families share related properties because they have the same valence shell electron configuration. All of the halogens will have 7 electrons in their valence shell. 
Since the periodic table is set up so that elements with similar electron configurations are aligned into family groups, it is also easy to use the periodic table to predict the valence electrons and draw the electron dot symbols for each of the family columns. This is shown in Figure 2.16.