What determines the length of each period in the periodic table?

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    The periods (rows) correspond to the energy levels where electrons are found in atoms.
    The first energy level (just an s sublevel) can hold up to two electrons. This period contains only two ; H and He.
    The second energy level can hold up to 8 electrons (s and p sublevels) so this period has eight elements; Li Be B C N O F and Ne.
    The third energy level can hold up to 18 electrons (s p and d sublevels).
    Here it gets a little tricky the 3d orbital does not fill until after the 4s orbital. The third period still contains only 8 elements (Na – Ar) because the third period contains the 3s and 3p sublevels.
    The fourth period (K-Kr) has 18 elements and contains the following sublevels; 4s 3d and 4p.
    The 5th period also has 18 elements (Rb – Xe) and contains the following sublevels; 5s 4d and 5p.
    The fourth energy level can hold 32 electrons (spd and f sublevels)
    The 6th and 7th periods have 32 elements.
    Period 6 (Cs-Rn) includes elements in the Lanthanide series (La-Yb) and includes the sublevels; 6s 4f 5d 6p

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