Question

Looking across period 4 of the periodic table, potassium (atomic number 19) is followed by calcium (atomic number 20), which is followed by scandium (atomic number 21). Which element should have the largest atomic radius?

Answer 1

Atomic radius decreases in size across the period.
It increases down the groups.

Since we're looking at 3 elements in a period, that means the leftmost element will have the highest atomic radius (assuming neutral atom).