How is the periodic table organized?

How is the periodic table organized? The periodic table is organized (arranged in order) by increasing atomic number, the number of protons in each atom, reading left to right along each row and then down to the next.

Rows are periods and columns are groups, so elements with the same number of valence electrons and similar chemical properties line up in a column; this repeating pattern is the periodic law. The table is also split into blocks (s, p, d and f) according to which subshell is filling, and colored into families such as alkali metals, transition metals, halogens and noble gases.

Metals fill the left and center, nonmetals the upper right, and metalloids such as silicon and arsenic lie along a diagonal staircase between them. The lanthanides and actinides (the f-block) are drawn in two rows below the main body so the chart stays narrow. Readers also ask “how to read a periodic table of the elements”, which has its own short answer.

Each cell shows an element's symbol, name, atomic number and atomic mass, and organizing the elements this way makes trends in atomic radius, electronegativity and reactivity easy to spot. To see the organization at work, pick an element such as chlorine: its atomic number 17 gives its order in the table, its position in period 3 shows three electron shells, and its place in group 17 shows seven valence electrons and halogen behavior. For a different question, see “how to memorize the periodic table”.

The blocks follow the subshell being filled: the s-block is groups 1 and 2 plus helium, the p-block is groups 13 to 18 (except helium), the d-block is groups 3 to 12 and the f-block is the lanthanides and actinides below. Across the table, metallic character increases toward the left and down, while electronegativity and ionization energy increase toward the upper right. A separate page on this site answers “why is it called the periodic table of elements”.

Mendeleev first organized the elements by atomic weight, and Henry Moseley's 1913 work on X-ray spectra led to the modern ordering by atomic number. That is why the table has no gaps today and why element 119 would start period 8. In the modern periodic table the elements are also classified as metals, nonmetals and metalloids, so the order itself reveals trends.