Final answer:
The periodic table's rows represent periods that correspond to the filling of electron subshells in atoms. Their varying lengths reflect the sequential addition of protons and electrons, leading to the occupation of different subshells and principal energy levels.
Step-by-step explanation:
The periodic table is a detailed map of chemical elements, reflecting their atomic structure and properties. The reason for the specific number of elements in each row or period of the periodic table relates to the atomic structure, specifically the electron configurations. As atoms increase in atomic number, electrons fill into subshells according to the principles of quantum mechanics, which leads to distinct periodic trends across rows and down groups in the periodic table.
For instance, the first period starts with hydrogen (Z=1) and ends with helium, marking the filling of the 1s subshell. The following periods correlate to the filling of higher principal energy levels and their respective subshells. As you move right across a period, each additional element has one more proton and electron than the previous one, resulting in the periods having varying lengths based on how electrons can be accommodated in subshells and energy levels.
In essence, each row corresponds to the filling of a principal energy level; for example, the first period correlates with the 1s subshell, while the second and third periods fill the 2s, 2p, 3s, and 3p subshells respectively. The fourth and fifth periods begin to fill the d-subshells, and by the time we reach the sixth and seventh periods, we are also filling f-subshells, which contributes to their greater length.