Which element has the largest atomic radius and which has the smallest: Na, Mg, and Be? Mg > Na > Be Why was this the correct answer? what method can I use to avoid getting tricked by one of these questions?
Atomic size decreases across a Period, and increases down a Group. Between Na, Mg, and Be, sodium should have greatest atomic size.
Given that atomic size decreases across a Period, the sodium atom is larger than that of magnesium. Both third row metals are larger than second row beryllium, inasmuch as the third Period metals build on the atomic radius (the electron shell) already possessed by the second Period.
You have to know that for a given Period, atomic size decreases across from left to right. And you will be given a Periodic Table in any Chemistry exam.