Periodic Table of Elements

"When do we introduce the Periodic Table of Elements? Why is not showing up anywhere in the AMI (primary and elementary) albums?"

There are a few reasons, and a few ways to approach this topic. 
If your child is interested in this topic, by all means, go for it! Interest over-rides any "universal child" observation. 
But it's not there for everyone, because it's not a key. The periodic table of elements is actually the end-result of a process. A process that the child needs to go through as well in order to fully appreciate it and learn the most from it. 
It becomes a key later - in adolescence! After they have experienced it! 
How to approach the periodic table of elements - all of these are found in the primary and elementary albums: 
  • Primary and Lower elementary: sorting objects by observable characteristics (color, shape, size, sinking/floating); explore the states of matter; explore magnetism; describe the characteristics of various objects; measure and compare using a variety of tools; explore substances vs. solutions
  • Upper elementary: continue looking at characteristics that are less obvious; consider volumes of solids; magnetic fields; affects on non-magnetic objects; which objects are good/poor conductors of electricity (electricity is not explicit in the elementary albums, but is something to be explored at the time a child expresses interest; and it *should* come up as the child explore each key further); in the final couple of tellings of the Story of God With No Hands, highlight those particles - those basic substances that make up all of creation. You can have resources in the room that share information about the elements; but try to avoid the "official chart" for now if at all possible. 
  • Adolescence: Now we start to look at chemical properties, if the child hasn't gone there already; start sorting various substances by their chemical properties; build atoms and molecules; use the term 'elements'; pull out a set of cards that has information on the elements for sorting purposes. Don't show the periodic table until the child has created a few different organizations of it himself (based on magnetism; based on atomic weight; based on other characteristics of the child's choosing) - what are the patterns? What are the similarities and differences? Then pull out the chart itself; but also pull out (or pull up online) samples of other methods of organizing the elements - there are MANY! And they are BEAUTIFUL! 
When an elementary child has an interest in this area of study, just pull out those cards and go from there.