Exceptions in Periodic Trends

[Alan Wu]

I remember there were exceptions in periodic trends in ionization energy and something else. They are due to penetration of the s-orbital over the p-orbital and Hund's Rule. May someone please give a recap on these exceptions?

[Julia Mazzucato 4D]

Yes, there are a few exceptions to the general trends we see on the periodic table.
Electronegativity increases as you move to the right of the PT from the metals up to the halogens, but the noble gases do not have electronegativities (unless forced under very extreme pressure/conditions) because they do not bond in covalent bonds.
With ionization energy, it generally increases as you move to the right and decreases as you move down the PT, but there are some exceptions. These exceptions are due to electron configuration. Some elements have their electrons configured/paired in a way that taking an electron from them in a bonding situation would either make the element more stable or less stable. This is why oxygen has a lower IE than Nitrogen despite being to its right: taking an electron from oxygen would make it more stable (because it would have three electrons with the same spin in three different p orbitals), whereas taking an electron from nitrogen makes it less stable.
Electron affinity also has a similar reason for its exceptions to ionization energy: the electron configurations. If adding an electron to the element will make the atom less stable, then the electron affinity is lower. This is why nitrogen's EA is lower than carbon's, and neon's is far lower than all others in its row.

[Maika Ngoie 1B]

Yes, there are a few exceptions to the general trends we see on the periodic table.
Electronegativity increases as you move to the right of the PT from the metals up to the halogens, but the noble gases do not have electronegativities (unless forced under very extreme pressure/conditions) because they do not bond in covalent bonds.
With ionization energy, it generally increases as you move to the right and decreases as you move down the PT, but there are some exceptions. These exceptions are due to electron configuration. Some elements have their electrons configured/paired in a way that taking an electron from them in a bonding situation would either make the element more stable or less stable. This is why oxygen has a lower IE than Nitrogen despite being to its right: taking an electron from oxygen would make it more stable (because it would have three electrons with the same spin in three different p orbitals), whereas taking an electron from nitrogen makes it less stable.
Electron affinity also has a similar reason for its exceptions to ionization energy: the electron configurations. If adding an electron to the element will make the atom less stable, then the electron affinity is lower. This is why nitrogen's EA is lower than carbon's, and neon's is far lower than all others in its row.

This idea is called electron repulsion, and an easy way to remember where it occurs is to look at the end of each block (s, p, d, f) as well as the midsection of each block because that is when half of the orbital is filled.