Short answers, pulled from the story.
The Russian chemist Dmitri Mendeleev created the first periodic table to become generally accepted, in 1869. He formulated the periodic law as a dependence of chemical properties on atomic mass and used it to predict properties of some elements that were still unknown.
The periodic law states that when the elements are arranged in order of their atomic numbers, an approximate recurrence of their properties is evident. The periodic table is a graphic depiction of this law, with rows called periods and columns called groups.
118 elements are known, and they complete the first seven rows of the table. The first 94 occur naturally on Earth, while the remaining 24, americium to oganesson, occur only when synthesized in laboratories. By 2010 all 118 were known.
The periodic table is divided into four roughly rectangular areas called blocks, named s, p, d, and f after the subshells being filled. The s- and p-block elements are main-group elements, the d-block elements are the transition metals, and the f-block elements are sometimes called inner transition elements.
Helium is placed in group 18 because it is unreactive and has a full outer shell, matching the noble gases. This placement contradicts its electronic structure, since helium has two outer electrons and is an s-block element, and a 1988 proposal to move it to group 2 was rejected by IUPAC.
Atomic radii generally decrease going left to right across a period because nuclear charge increases while outer electrons stay in the same shell, and increase going down a group because the outermost electrons occupy higher shells farther from the nucleus. Ionisation energy and electronegativity rise from left to right and from bottom to top, opposite to atomic radius.