Each period (horizontal row) of the periodic table corresponds to an
energy shell. For example, atoms of carbon, C, (row 2 ) have outer electrons
in the 2nd energy shell; atoms of sodium, Na, (row 3 ) have outer electrons
in the 3rd energy shell, and so on.
- When writing electron configurations, and determining which subshells
to fill, be aware of what area and row the element is in. Then remember the
following points:
• An element in the s area of row n has outer electrons in the n
subshell.
• An element in the p area of row n has outer electrons in the np
subshell.
• An element in the d area of row n has outer electrons in the (n
− 1)d subshell.
• An element in the f area of row n has outer electrons in the (n
− 2)f subshell.
Let’s put it all together and try writing the electron configuration for an atom of
fluorine.
Step 1. Where do we start? At hydrogen, of course. It’s in the s area of
row 1. Hydrogen (H) has an electron in its 1s subshell. Remember that
although helium looks like it is in the p area, it is actually part of the 1s
area.How Does This Work?
Consider an atom of phosphorus, P, (row 3). It’s in
the p area, so its outer electrons are in the 3p
subshell. What about an atom of nickel, Ni, (row
4)? It’s in the d area. That means its outer electrons
go into the (4 − 1)d or 3d subshell.Step 2. Now we have 2 electrons in the lone orbital of the 1s subshell.
Since no orbital can hold 3 electrons, we need to go to a different (higher
energy) subshell for the next addition.
Step 3. Follow the numbers to lithium (Li) and then beryllium (Be);