Shielding vs. proton, which is stronger

[Zechuan Rao 1K]

I've been looking at an atomic radius diagram and find something rather counter intuitive. The atomic radius of Sodium atom is much larger than that of Iodine, which has way more shells than Na. So is it because it is a metal and Iodine is not? so looked again and found K is also larger than Sr, and both of them are metal. So why is that under certain circumstances is the adding a electron in the orbit (to the right of periodic table) override the effect of adding of a shell (down the period table).

[Maxwell Yao]

Hi,

When you add a shell it makes sense that the K (potassium) would be larger than Na (sodium) because they are in the same column. And this makes sense because one of the trends is that atomic size decreases down the column; however, when you compare Iodine to Sodium, even though Iodine has more shells, the electrostatic attraction between the protons and the electrons in Iodine are a lot stronger than it would be in Sodium which is why it does indeed have a smaller atomic radius than sodium. That's the reason why atomic radius decreases across the row.

But in general, I'm assuming we typically are only going to get asked to compare elements that are in (or close to) the same column or the same row because it wouldn't make sense for us to be asked like "Which one is bigger, Tellurium or Lithium?" since that's just pure memorization and doesn't really apply the period trend rules we learned in class.

Hope this helps.

[Hanyi Jia 3B]

When we move across a period, the increase in the nucleus charge plays the dominant role in the attraction forces between the nucleus and valance electrons. (Atomic number increases across the period indicates that the charge of nucleus increase.

In terms of the shielding effect though, I think we are taught that it stays constant across the period because electrons are added to the valence shell whereas only core electrons participate in the shielding. However, I am not really sure about that for the transition medals though. We usually skip them when we talk about a number of valance electrons. I would argue that the shielding effect on the electrons of K is smaller than that of Ga.

Ultimately, whether or not the shielding effect changes across the period, the increase in nuclear charge overrides the pattern.