Fluorine, chlorine, and bromine all have seven valence electrons. which element has the largest atomic radius?

The largest atomic radius correspond to Bromine. The order is

Fluorine > Chlorine > Bromine.

Explanation:

Atomic radius is a property that has the following trend: When you look a group in the periodic table, atomic radius increase from top to down; when you look along a period atomic radius decrease from left to right.

Fluorine, Chlorine and Bromine belong to the same group, so you can expect that the atomic radius will increase from Fluorine to Bromine. This is related with the fact that in the same sense there is an increase in the number of electrons that occupies outermost orbitals giving place to an increase in atomic radius.

in scientific notation, the value of the  speed of light in a vacuum

  will be written as 2.995 * 10^8. when this value is represented in the form of

  a * 10^b, then, a = 2.995 and b = 8.

the value of the speed of light in a vacuum given above can also be rounded up to approximately 300000000. in this case, the scientific notation will be written as 3 *10^8.