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Potassium is placed below sodium in group 1. It, therefore has one more electron shell. Na atom has three electron shells (2,8,1); K atom has four (2,8,8,1). So, potassium atom is bigger than sodium atom.

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Potassium atom is in 4th period and sodium is in 3rd period, the number of outermost orbits is greater in potassium than sodium.So the nuclear attractions decreases in potassium which ultimately leads to the larger size of it.