Online Introductory Chemistry
Dr. Walt Volland

Electron Configuration, Block classifications, Atomic radius trends and the Periodic Table

 

The periodic table shape results from the build up of electrons in the shells and subshells of the atoms as atomic number increases. Every step up in atomic number requires an additional electron to keep the atom neutral.

The periodic table has a BLOCK of two columns, a BLOCK of 10 columns, a BLOCK of 6 columns and a BLOCK of 14 columns. These BLOCK sizes match the number of electrons that fit into the s,p d and f subshells. The s subshell can hold two electrons. This matches the size of the s-block. The p subshell can hold up to six electrons. The matches the size of the p-block. The d subshell can hold up to 10 electrons. this matches the size of the d-block with 10 columns. Thef subshell can hold up to 14 electrons. The 14 columns of the f-block match the filling of the f subshell.

Note that the 4f-block of elements appears between the 6s and 5d block.
Note that the 5f-block of elements appears between the 7s and 6d block.
 

Notice that there are no elements with electrons in the 7p because the highest atomic number right now is 112 which matches the end of the 6d. There are no neutral atoms with enough electrons to use the 7p subshell.

 

ATOMIC RADII

The atomic radii for the elements show a repetitive decrease from the start to the end of each period. The decrease occurs because electrons are entering the same shell on atoms, but the positive charge on the nucleus is increasing. This increase in + charge draws electrons closer.

The groups all show larger radii at the bottom of a group and smaller radii at the top. This should seem reasonable because the electron count increases down a group so the electron-electron repulsions increase. Also, the atoms at the bottom of a group have valence electrons in shells that have larger "n" values. The distance between the nucleus and the electron increases as "n" increases. There are more inner electrons between the nucleus and the valence electrons. The screening done by the greater number of inner electrons is more important at the bottom of the group because there are more of them.