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Electron configuratons and the periodic table 

It will be helpful now to look at electron configurations also in the context of the periodic table.  The periodic table can be broken into “blocks” that show what the last electron added to the electron configuration is. 

The alkali metal electron configurations (group 1) always end with “s¹” and the alkaline earth metals (group 2) always end with “s²”.  These 2 groups are know as “s block” elements.  “P block” elements are all those in groups 13-18 and always end with 1 or more “p” electrons.  For example, all the elements in group 13, beginning with boron, end with “p¹”.  All of the elements in group 16, beginning with oxygen, end with “p⁴”.  The transition elements are called “d block” elements and always end with 1 or more “d electrons”.   For example, the all of the elements in group 3 beginning with scandium, all end in “d¹”.   The “f  block” elements are those at the bottom of the periodic table that we call the lanthanide and actinide groups.  These elements’ electron configurations always end with one or more “f  electrons.” 

Practice Your Electron Configurations-choose an element

 

It can be very useful  to determine the last electron added (the outermost electron) to any element since this will always be a valence electron.  We can do this by looking at the periodic table and finding 2 things: the period number (row number) and the block that the element is contained in.   Let’s try this with the element calcium.  From a periodic table, we see it is in row (period) 4 and we see that it is in the “s” block.  In fact calcium is  the second element in the “s” block, so the last electron is an “s²” electron.  Since it is in period 4, the last electron added is 4s².   Now try finding the last electron added (the outermost electron) to the element iodine.  We can see that it is in period 5 and in the “p” block.   It is the 5^th element in the “p” block, so the last electron in iodine is 5p⁵.  One more example will show a break in the trend we have set up.  Find the last electron in the element nickel.  We notice that it is in period 4 and it is the 8^th element in the “d” block.  We would think that it’s last electron would be 4d⁸, but in fact it is 3d⁸.  Remember that because of the orbital overlap we talked about earlier, the “d orbital” principal energy levels are always one behind the “s” and “p” principal energy levels.  As a result, its electron configuration is [Ar]4s²3d⁸ and the last electron added to nickel is a 3d⁸ electron.  See the following periodic table for these last electrons in the electron configuration.  A few others are included so you can see the patterns. 

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