Final answer:
The periodic law describes the periodic repetition of chemical and physical properties of elements when they are arranged by increasing atomic number on the periodic table. Elements are grouped in rows (periods) and columns (groups) to reflect their similar properties, which are influenced by the distribution of electrons in their valence shells.
Step-by-step explanation:
When the repeating chemical and physical properties of elements change periodically with the elements' atomic numbers, it is called the periodic law. The modern periodic table is arranged in rows called periods and columns known as groups, where elements with similar chemical properties are grouped together. This organization helps predict the properties of elements based on their position in the table. For example, as we move down a group, the metallic character of the elements increases, which can be seen in group 16 where oxygen is a gas, selenium a semiconducting solid, and polonium a conductive solid. Each element in the periodic table is also represented by a box containing its atomic number, symbol, average atomic mass, and sometimes its name. Elements in the same group have the same number and distribution of electrons in their valence shells, which is why they exhibit similar chemical behavior.
Dmitri Mendeleev, the Russian chemist who first organized the periodic table, arranged elements by increasing atomic mass. However, the modern periodic table is arranged by increasing atomic number because it was found that atomic number, rather than atomic mass, reveals periodic properties more clearly. This arrangement ensures that the periodic table fits on a single page, with two of the rows, a total of 14 columns, usually written below the main body of the table.