Answer:francium
The first ionization energy varies in a predictable way across the periodic table. The ionization energy decreases from top to bottom in groups, and increases from left to right across a period. Thus, helium has the largest first ionization energy, while francium has one of the lowest.
Explanation:
To measure a vector is to characterize both its direction (in one form or another) and a value (usually, a number). For example, velocity of a car moving on the road is a vector and, to measure it, we have to specify its direction (e.g. North or at angle 20 degrees from the North) and value (e.g. 40 m/sec).
