How does atomic size evolve with respect the position of an element in the Periodic Table?

Question

Peyton Adams · Accepted Answer

It is a fact that atomic size decreases across a Period....

It is a fact that atomic size DECREASES across a Period, a row of the Periodic Table, from left to right as we face the Table, and INCREASES down a Group. And this is something I would expect a novice student of chemistry to grasp in that it reflects the structure of the Periodic Table.

Incomplete electronic shells shield the nuclear charge very imperfectly, and as we add protons to the nuclear core (thereby changing the identity of the element) the added electrons shield the nuclear charge VERY imperfectly....

And this results in a contraction of atomic radius across the Period..

But when a valence shell is completed (i.e. at the Noble gases, which are the smallest atoms of their Periods), the shielding of nuclear charge becomes effective. The next electron adds to a new valence shell. And the macroscopic observable is the LARGE size of the alkali metal atoms in relation to the succeeding atoms in their Periods.

And so for sulfur, sodium, and chlorine ATOMS, we would predict that atomic size decreases in that order.....We would also predict that sodium atom is LARGER than lithium atom, but we do not really have a handle when we compare the atomic size of lithium versus that of sulfur, and chlorine... From the graph, lithium is larger than sulfur, and chlorine, even tho' lithium is a second Period atom.

Henry Antrim · Answer

undefined Atomic size generally decreases across a period (from left to right) and increases down a group (from top to bottom) in the periodic table. This trend is primarily influenced by changes in the effective nuclear charge and the number of electron shells.

1. Across a period (from left to right):
   - As we move across a period, the number of protons in the nucleus increases, leading to a greater positive charge in the nucleus. This increase in positive charge pulls the electrons closer to the nucleus, resulting in a decrease in atomic size.
   - Additionally, within the same energy level, electrons are added to the same electron shell, which increases electron-electron repulsions, further contracting the electron cloud and reducing atomic size.

2. Down a group (from top to bottom):
   - As we move down a group, the number of electron shells increases. Each successive shell is farther from the nucleus, leading to an increase in atomic size.
   - Moreover, as more electron shells are added, the shielding effect increases. Electrons in inner shells shield the outer electrons from the full effect of the nuclear charge, reducing the attraction between the nucleus and the outer electrons, thus contributing to the increase in atomic size.

In summary, atomic size decreases across a period due to increasing nuclear charge and electron-electron repulsions within the same electron shell. Conversely, atomic size increases down a group due to the addition of electron shells and increased shielding effect from inner electrons.