Online Introductory Chemistry
Dr. Walt Volland   revised July 5 , 2010 all rights reserved

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 the atomic number increases. Every step up in atomic number (a proton is added to nucleus) requires an additional electron to keep the atom neutral.

The periodic table has a series of BLOCKS. One BLOCK is made of 2 columns, a second BLOCK is made of 10 columns, a third BLOCK is made of 6 columns and fourth BLOCK is made of 14 columns.

The 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. This 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.

The f 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 elements appear between the 6s and 5d block.

Note that the 5f-block elements appear between the 7s and 6d block.

Notice that there are no elements listed with electrons in the 7p because these are only made in high energy accelerators. The highest atomic number right now is 118 which matches Group 8A (Group 18) beyond the end of the 6d block.Click here to visit physics site about super heavy elements.



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 click for more about 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 outer or 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.

Atoms with a big radius and volume like sodium, Na, are more easily ionized to produce positive ions. Atoms with a small radius and volume like fluorine, F, are more likely to attract electrons and form negative ions. click for graphic