List in order of increasing radius:

a) Rb, K, C5, Kr

For this one I got C, Kr, K, Rb

b) Ar, Cs, Si, Al

This I got: Ar, Si, Al, Cs

Does anyone know if this is correct? I don't have the solution for these in my book :S

  • $\begingroup$ What do you know about the periodic trends in atomic radii? There are two useful generalizations and it looks like you have followed them. $\endgroup$
    – Ben Norris
    Commented Dec 3, 2012 at 2:21

2 Answers 2


There is a trend that follows an arrow from the lower left of the periodic table to the upper right. As the atomic number increases along a period, the radius for each element decreases. Here is a useful image of this:

Atomic Radii

Notice that, with the exception of the noble gases, you can draw an arrow from the lower left to the upper right. The justification for what you see here is that as the principal quantum number increases, the probability that the electrons are more distance from the center of the atom also increases. The higher the principal quantum number, the higher the energy of the electron and therefore its probable distance from the center of the atom. Therefore, as you move down a group, the radius increases.

However, another factor is that as the number of protons increase in an atom, so does the positive charge which happens to pull the electrons toward the center of the atom. So, there is the impact of the protons causing a decrease in radius (probability of the electron being closer to the center) and the probability that the electrons are in an orbital that is farther from the center.

The transition metals, however, have d orbital electrons. These electrons enter into a shell that is closer to the nucleus and the change is less noticeable. That is part of the reason why they do not seem to change as much as the other atoms, according to my understanding.


A) Rb:265 pm K: 243 pm C: 67 pm Kr: 88 pm

B) Ar: 71 pm Cs:298 pm Si: 111 pm Al: 118 pm

Where Pm is picometer =1.0 × 10^-12 meter

  • 2
    $\begingroup$ Could you provide some conceptual reasoning in your answer as to why the order of radii is what it is? $\endgroup$ Commented Dec 2, 2012 at 22:23

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