# Homework Help: Sizes of atoms

1. Jan 31, 2010

### chops369

1. The problem statement, all variables and given/known data
I have a few questions here that say to identify which atom from each pair is larger.

One pair is Be and B, and the other is P and Cl.

2. Relevant equations
N/A

3. The attempt at a solution
I know that Be is larger than B, and that P is larger than Cl. I'm just having trouble understanding why. Is there some rule of thumb that says that sizes of atoms increase when you go across a period? What is the reasoning behind this?

2. Feb 2, 2010

### chemisttree

Imagine you are an electron in the outermost shell of these pairs of elements. You are negatively charged and attracted to the positive nucleus. How close you are to the nucleus is determined by the magnitude of the charge exerted on you by the nucleus. All of the other electrons in the outermost shell feel the same forces as you. Now add a couple of protons and neutrons to the same nucleus. Are you more strongly or less strongly attracted to the nucleus? How would that affect your distance from the nucleus?

3. Feb 2, 2010

### chops369

So you're saying that, since the elements that are further down a period have more positive charges in the nucleus, the outer electrons are condensed inward and therefore the atom is smaller? At what point does this stop being true?

Further down in the periodic table, as the number of total electrons gets larger, won't the addition of sublevels overpower the nucleus wanting to pull the electrons closer? Judging by the comparison of some larger elements' molar volumes to those of smaller elements, I would think that must be the case.

4. Feb 3, 2010

### Bohrok

To answer the first part of your post: You can think of the electrons being drawn closer to the nucleus as you go to the right in a period because the number of protons increases and so attracts the electrons more. Once you move to a new shell, the atom is larger since the new outer shell is further away from the nucleus.