In 1913 and 1914, Englishman Henry Moseley made important discoveries working with an X-ray technique. He discovered a feature of atoms that became known as an atomic number.
At this point, suffice it to say that each chemical element has a characteristic number, the atomic number. When chemical elements are organized in ascending order of atomic number, their properties occur periodically, that is, elements with similar properties are repeated regularly.
This regularity of nature is known as periodic law of the elements.
Other scientists have refined Mendeleev and Moseley's findings. These enhancements led to the modern periodic table of elements, which appear in the table below.
In it, horizontal lines are called periods and columns (vertical) are called groups, or families.