When we move across the period i.e. from left to right, the atomic size decreases due to increase in atomic number(Nuclear pull increases).But when we move from top to down in a group, in this case also there is an increase in the atomic number i.e. increase in nuclear pull then why the size increases?If the logic is the increase in number of shells then in the both cases it can be applied.

Asked by  | 18th Nov, 2012, 09:46: AM

Expert Answer:

As we go from left to right in a period, the atomic size or atomic radius decreases with increase in atomic number. Increase in atomic number leads to an increase in the number of protons and electrons in the atoms.Now due to the large nuclear charge (positive charge on the nucleus), the electrons are pulled in more close to the nucleus and the size of atom decreases. But when we go from top to bottom in a group, the atomic size increases. This is because new shells are being added as we go down the group. This increases the distance between the outermost electrons and the nucleus so that the atomic size increases in spite of the increase in nuclear charge.

Answered by  | 19th Nov, 2012, 10:18: AM

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