What do you notice about electron configuration of the atoms as you move from the period 1 to period 2, and then period 3?

In modern periodic table, elements are arranged in increasing order of atomic number. There are 7 periods and 18 groups in modern periodic table.

As one moves along the period from left to right, electrons in the valence shell increases. On other hand, as you move from the period 1 to period 2, and then period 3, with in group, valence shell electronic configuration remains same, but principle quantum number associated with valence electrons increases.

For instance, H, Li, Na belongs to group 1, but the are placed in different period. Electronic configuration of these elements are as follows:
H = 1s1 (1st period)
Li = 1s2 2s1 (2nd period)
Na = 1s2 2s2 2p6 3s1 (3rd period)

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