Periodic blocks identify the subshell being filled: s,p,d or f. The main-group left columns are s-block, the right-hand region is p-block except helium, and the central region is d-block. Helium has an s configuration despite its position. The f-block is commonly displayed as two rows below the main body. These contain the lanthanide and actinide series.
Valency describes combining capacity, not necessarily an ionic charge. In common introductory period−3 examples, Na, Mg, Al and Si have valencies 1,2,3 and 4; typical simple compounds of P,S and Cl show 3,2 and 1. Some elements have additional valencies, so use the specified compound or pattern rather than assuming one universal value.
Example 1
A periodic table highlights the two rows usually placed below the main body. Name the block.
They are the f-block region, associated with filling f subshells in the lanthanide and actinide series.
Example 2
Use valencies Si=4 and Cl=1 to predict the simplest covalent formula.
One silicon forms four bonds to four chlorine atoms, giving SiCl4.
Watch out: Valency is combining capacity; covalent valency and signed ionic charge are different descriptions.