sizes of ions and atoms
 

How can I use the periodic table to identify the relative sizes of ions and atoms? If U know, let me know................thanx

going down the periodic table, the size increases (thius should be obvious. you have more protons and neutrons, hence a bigger neuclues, and more electrons around it)

Going from left to right, the size decreases. Why? because as you go across from left to right, you have more protons in the nucleus. This in turn pulls the electrons tighter to the nucleus (since protons are positively charged, and nuclei are negatively charged)

The first explanation (going down the periodic table) can't really be obvious, because it's contradicted by the second explanation, which says more protons = smaller atoms.

What I'd say is that as you go from one element to the next down a group you are adding a new shell of electrons. It's this extra shell that makes the atoms bigger going down a group.

As you go along any period from left to right you are dealing with the same electron shell which is pulled smaller by the extra charge from more protons in the nucleus.

Here's a neat pic from Dr. Tom Zamis, University of Wisconsin.

Here are ion sizes from Dr. Jaime Toro at West Virginia University. Atoms that become positive ions shrink while atoms that become negative ions grow.

Hmm, just found this. It's amazing seeing how tiny Al³⁺ is compared with Cl^-. AlCl₃ seems weird now with a tiny Al attached to 3 Cl. Like a tennis ball holding on to 3 soccer balls.

I'm confused with these sizes. Radius of H is 30 picometers in the first picture. Google says 1 picometer = 0.01 angstroms. That means the radius of H is 0.3 angstroms. But in the second picture carbon and silicon ions are smaller than this. That doesn't make sense because H has one electron 1s1 but Si4+ is 1s2 2s2 2p6 so it must be bigger.

I'd say it's because Si has much more positive charge than H. (14 protons v 1 proton.) This extra charge pulls the electrons in Si⁴⁺ into a smaller volume than occupied by one electron orbiting the hydrogen nucleus