Scale Drawings of Atoms and Orbitals: Hydrogen Through Krypton

This is the only atom for which Bohr's planetary model gave accurate results. The orbital radius, 0.53 Angstroms or 53 picometers, is sometimes called a "Bohr unit" and orbital dimensions are often tabulated in Bohr units rather than metric, especially in older literature. That unit system is often called "atomic units."

Helium also just has a 1s orbital. It has two electrons but also two protons. So the stronger attraction of the nucleus makes the helium s orbital only a little more than half the radius of the hydrogen s orbital.

Lithium and beryllium have a 2s as well. The shielding effect of the 1s electrons is the same as in helium but there are three protons in lithium. The 1s radius is about a third that of hydrogen.

Ionizing the elements lithium through carbon strips off the 2s and 2p electrons, leaving only the 1s shell and creating extremely small cations.

Ionizing the elements nitrogen through fluorine fills the p orbitals, causing the ion to "puff up" because the electron repulsion can partially overcome the attraction of the nucleus. With only 7, 8, or 9 protons, an extra electron makes a big difference. The result is fat anions.

Neon is the first element with a stable octet with complete p and s orbitals. The reason the octet is so stable is that the only way to add electrons is to add the next s orbital outward. There is no way to add electrons to the 2 shell of neon, period.

This will be a general pattern: f orbitals are bigger than d orbitals, which are larger than p, with s as the smallest. With increasing atomic number and nuclear charge, the discrepancy diminishes. Orbitals in shells deep within the atom are pretty similar in size.

As atomic number increases, the orbitals shrink dramatically in size as the positive charge in the nucleus increases.

Argon is the next element with a stable octet with complete p and s orbitals. The reason the octet is so stable is that the only way to add electrons is to add the next s orbital outward. It is possible to add 3d orbitals, but not until the 4s orbital is full. So it's all but impossible to make reactive ions out of the noble gases.

We can see that ionizing these elements will often strip off the 4s shell, leaving rather small ions with +2 charge. Scandium through manganese can lose some or all the d electrons also, giving charges of +3 to +7, respectively.

Ionizing gallium and germanium strips away the 4 shell electrons, creating very tiny cations. Arsenic and selenium can lose electrons to become cations, or gain electrons to become anions.