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The horizontal rows of the periodic table are called periods. The atomic number increases from left to right in a period.

Atoms in the same period have the same number of electron shells.

Nitrogen ($$\ce{N}$$), oxygen ($$\ce{O}$$) and fluorine ($$\ce{F}$$) all belong to the second period. All three elements have their valence electrons found in the second electron shell.

The period in which an element is found reflects the number of the electron shell holding the valence electrons.

Beryllium is found in the second period, and so its valence electrons are found on the second electron shell. Magnesium and calcium are found in the third and fourth period respectively.