a) Determine the core charge and valence shell for \(\mathrm{F}^{-}\), and \(\mathrm{F}^{2-}\). Drawing diagrams may be helpful. b) Based on your answer to part a, explain why an attempt to add an electron to \(\mathrm{F}^{-}\) does not result in the formation of \(\mathrm{F}^{2-}-\) that is, the "added" electron does not stay "attached" to the original \(\mathrm{F}^{-}\) ion. c) The ions formed in molecules from Group 17 atoms (the halogens, such as \(\mathrm{F}\) ) are almost exclusively \(\mathrm{X}^{-}\) ions rather than \(\mathrm{X}^{2-}\) ions. Explain this result based on your answers to parts a and b.

The term core charge is central to understanding how atoms become ions. In relation to an atom, core charge refers to the effective positive charge experienced by electrons in an atom's valence shell. It is determined by the number of protons in the nucleus minus the number of inner-shell electrons that shield the valence electrons from the full attraction of the nucleus.

For instance, in the case of Fluorine (F), there are 9 protons in the nucleus, and electrons in inner shells slightly reduce the positive charge experienced by the valence electrons. With the formation of the F^- ion, the atom has gained an additional electron in its valence shell, but the core charge seen by those electrons remains at +9. This core charge influences how tightly the nucleus holds the valence electrons and is a significant factor in the stability of ions.

• An atom with a high core charge will strongly attract electrons.
• An atom's ability to gain additional electrons is often limited by its core charge.

Thus, core charge plays a vital role in ion formation and the chemical reactivity of elements.

The valence shell is the outermost shell of an atom that contains electrons. It plays a pivotal role in chemical bonding and the formation of ions, since the electrons in this shell are the ones involved in forming chemical bonds.

In the context of the exercise, for a neutral Fluorine atom, the valence shell corresponds to the second electron shell which includes the subshells 2s and 2p. It is in this shell that Fluorine has seven electrons needing only one more to achieve a stable, filled shell resembling that of a noble gas. When Fluorine gains an electron to become an F^- ion, its valence shell expands to include one more electron filling the 2p subshell resulting in a stable and energetically favorable electron configuration.

• The valence shell determines how atoms interact.
• A full valence shell usually indicates chemical stability.
• The desire of an atom to fill its valence shell drives ion formation.

Hence, understanding the valence shell is crucial for explaining why certain ions are more stable than others and why elements such as halogens typically form single negative ions rather than double negatives.

The concept of effective nuclear charge (Zeff) refers to the net positive charge experienced by an electron in the valence shell of an atom. It is the charge that an electron 'feels' from the nucleus, taking into account both the actual nuclear charge (the number of protons) and the shielding effect of other electrons, particularly those in the inner shells.

As we move out from the nucleus, the effective nuclear charge experienced by an electron decreases because of the increased distance and the shielding effect of the inner-shell electrons. This is of immense importance when we consider why a F^- ion cannot easily become a F^2- ion. The F^- ion has electrons all within the second shell experiencing a substantial Zeff due to proximity to the nucleus. This makes it energetically favorable and chemically stable.

• The effective nuclear charge determines an electron's attraction to the nucleus.
• Higher Zeff equates to tighter hold on valence electrons.
• Atoms with high Zeff on their valence electrons are less likely to form negative ions with additional electrons beyond a stable octet configuration.

Therefore, effective nuclear charge explains the relative stability of ions by illustrating how the force of attraction between the nucleus and valence electrons governs electron gain or loss.