The explanation that I have heard states that when we move horizontally across the periodic table, the number of electrons increases leading to a greater force of attraction from the nucleus.
For instance, let's take the example of Lithium (atomic radius = 152pm) and Carbon (atomic radius = 77). The size of Lithium is greater than that of Carbon is because in Carbon, the number of electrons increases leading to a greater force of attraction than in Lithium. However, when the number of electrons in Carbon increases, the number of proton increases as well, right?
If in Lithium we had 3 electrons and 3 protons, then there is no net charge. Similarly in Carbon, there are 6 electrons and 6 protons, again, no net charge. Then why is it that the effective charge in Carbon is somehow greater leading to a smaller atomic radius?
Best Answer
EDIT:
I was able to narrow down an answer I think. We define effective nuclear charge as $Z_{\mathrm{eff}} = Z - S$, where $Z$ is number of protons and $S$ is the average number of electrons between the nucleus and the electron in question. Only the 1s orbital electrons have $Z_{\mathrm{eff}} = Z + 0 = Z$, ie $S = 0$ only for neutral hydrogen and helium. For all other standard elements, we have additional orbitals like 2s and 2p and 3s, etc and these all experience $S \neq 0$ so $Z_{\mathrm{eff}} < Z$.
Apparently, there is something called the Slater's rules. The shielding constant for each group is formed as the sum of the following contributions:
So for iron, here is the effective nuclear charge for different electrons.
Now let me try to address the radius issue according to Slater's rules. I make no guarantee Slater's rules are foolproof. You should investigate that yourself. I'm just going to take these rules and apply them. Let's consider fluorine. I like fluorine as an example because I always think of it as the hungriest of the elements. It wants an electron to fill it's outer shell badly. Why? So for the 2nd row, the effective nuclear charge increases as you go across the periodic table as follows:
So the effective nuclear charge does increase across the table! And thus, the electrons in the outer orbital experience a greater nuclear charge for elements on the left than on the right.
The basic idea is,
and the result is the radii decrease as you go from right to left as a result because effective nuclear charge is increasing.
EDIT 2:
I learned on Chem SE that Slater's rules are just an approximation.