Question

the number of electrons increases in the outermost shell as we move from left to right in a period why does the atomic size of an atom decreases

Answer 1

because as the number of electrons increases the nuclear acting on them also increases. therefore atomic size decreases
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Answer 2

As we move from left to right in a periodic table, the size of the atom decreases with the increase in no. of electrons in outermost shell this is because:

As a new electron is added in the outermost shell, the force of attraction between the nuclei and the outermost shell keeps on increasing which leads to the decrease in size of the atom.