Final answer:
The horizontal rows in the periodic table are called periods, which are seven in total, and each row represents a different energy level for electrons in an atom. These periods are numbered from 1 to 7 and increase in length as you move down the table.
Step-by-step explanation:
The horizontal rows in the periodic table are known as periods. The periodic table is comprised of seven such horizontal rows, each representing a different energy level that an electron can occupy in an atom.
These are numbered from 1 at the top to 7 at the bottom, with the first period containing only two elements (hydrogen and helium) and the periods increasing in length as you move down the table.
Elements within the same period have their electron shells filling sequentially as you move across the table from left to right. The rows are organized in order of increasing atomic number, meaning each succeeding element in a period has one more proton than the element before it.
Moreover, elements that show similar chemical behavior are organized into vertical columns called groups or families, which are numbered from 1-18.