Why is second electron affinity for halogens is zero?

Answer 1

All of the halogens are one electron short of having all of their atomic orbitals filled to reach an atom's state of nirvana. This explains why, in general, halide chemistry is such that halogens so willingly literally accept one electron in their ionic formulations and formally accept one electron or share a pair of electrons in the vast majority of their predominately covalent compounds. Halogens have no affinity for accepting a second electron because once a halogen atom has accepted once electron, all of its atomic orbitals each contain two electrons and are thus full. Any element with all its atomic orbitals filled has the equivalent electronic configuration of a noble gas and is in its most stable electronic state.

What follows is very important to understand. It appears that many chemistry students do not know this fact probably because most textbooks and instructors do not explicitly point it out or they do a poor job emphasizing it: Elements only possess the atomic orbitals defined by the row in which an element exists in the Periodic Table.

In many compounds, a particular element may possess one or more empty atomic orbitals in its electronic ground state. Students who have completed the first semester of general chemistry were presented with, and expected to understand, what atomic orbitals each element has. They should also know the order in which a given element's orbitals are progressively occupied by electrons when that element is in its ground electronic state and that orbitals with the lowest energy are filled first. It is also important to understand that the theoretical order of atomic orbitals in elements heavier than argon may be in a different order. This effect, when it occurs, is due to electron-electron repulsions about the element's nucleus.

Let's look at a 2nd row element as an example. How about nitrogen? Because it's a 2nd row element, nitrogen has two "shells" of atomic orbitals and a total of five orbitals; however only electrons in the outer shell of orbitals may participate in chemical bonding. The 1st shell of electrons consists only of the 1s orbital. Like all atomic orbitals, the 1s orbital can hold a maximum of two electrons, which is denoted by the superscript in the orbital's designation, as in 1s2. Starting from the 1st element in the 2nd row and counting each element up to and including nitrogen shows that the outer shell of orbitals on nitrogen contains five electrons. Assuming that no electron-electron interactions alter the respective theoretical energy levels of the five orbitals (This does not occur in any of the 2nd row elements), the atomic orbitals on nitrogen are, in increasing energy: [1s2], 2s2, 2px1, 2py1, 2pz1. The three 2p orbitals have the same energy and are filled with one electron first before any of them takes on a second electron. Note that the first p orbitals, and the ones lowest in energy, are the 2p orbitals. There is simply no such thing as a 1p orbital. The 2p orbitals could have been named 1p orbitals. Everyone who first applied quantum mechanics to the hydrogen atom in order to describe its atomic emission spectrum, and, not long thereafter, the number and energy levels of an atom's electrons, are no longer with us. Nevertheless, the reason for the seemingly strange numerical designations is almost certainly because the quantum numbers that are solutions to the wave equation corresponding to the number and shape of the atomic orbitals begin with "2" for the p orbitals, "3" for the d orbitals, etc., and perhaps the people who discovered and published all of these findings decided not to change the numerical designations.

The point I hope I made is that the five atomic orbitals shown for nitrogen are all it has. In addition to s and p atomic orbitals, there exists d and f orbitals, but not for nitrogen or any other second-row element. Therefore, once the 2s and 2p orbitals are filled, nitrogen cannot accept or share another additional electron because there is no atomic orbital in which it can be placed.