A table of elements (see Appendix 5) arranged in order of increasing proton number to show the similarities of chemical elements with related electronic configurations. (The original form was proposed by Dimitri Mendeleev in 1869 using relative atomic masses.) In the modern short form, the lanthanoids and actinoids are not shown. The elements fall into vertical columns, known as groups. Going down a group, the atoms of the elements all have the same outer shell structure, but an increasing number of inner shells. Traditionally, the alkali metals were shown on the left of the table and the groups were numbered IA to VIIA, IB to VIIB, and 0 (for the noble gases). All the elements in the middle of the table are classified as transition elements and the nontransition elements are regarded as main-group elements. Because of confusion in the past regarding the numbering of groups and the designations of subgroups, modern practice is to number the groups across the table from 1 to 18. Horizontal rows in the table are periods. The first three are called short periods; the next four (which include transition elements) are long periods. Within a period, the atoms of all the elements have the same number of shells, but with a steadily increasing number of electrons in the outer shell. The periodic table can also be divided into four blocks depending on the type of shell being filled: the s-block, the p-block, the d-block, and the f-block.
There are certain general features of chemical behaviour shown in the periodic table. In moving down a group, there is an increase in metallic character because of the increased size of the atom. In going across a period, there is a change from metallic (electropositive) behaviour to nonmetallic (electronegative) because of the increasing number of electrons in the outer shell. Consequently, metallic elements tend to be those on the left and towards the bottom of the table; nonmetallic elements are towards the top and the right.
There is also a significant difference between the elements of the second short period (lithium to fluorine) and the other elements in their respective groups. This is because the atoms in the second period are smaller and their valence electrons are shielded by a small 1s2 inner shell. Atoms in the other periods have inner s- and p-electrons shielding the outer electrons from the nucleus. Moreover, those in the second period only have s- and p-orbitals available for bonding. Heavier atoms can also promote electrons to vacant d-orbitals in their outer shell and use these for bonding. See also diagonal relationship; inert-pair effect.
https://www.webelements.com/ The WebElements table produced by Mark Winter at the University of Sheffield
https://www.meta-synthesis.com/webbook/35_pt/pt_database.php Over 30 different forms of the periodic table in the Chemogenesis web book by Mark R. Leach