For the carbonate ion, there are 3 resonance structures, and each of the 3 oxygen atoms carries a formal charge of –1 in two out of the three structures. This means that there is a –2/3 charge on each O atom. There are no formal charges in H₂O or O₂. and in O₃, the central O has a positive charge and each outer O has a negative charge, so there is an average charge of –1/2 on the two outer ones.

N has 5 valence electrons. In the ion, N has 4 bonds, and thus 8 bonding electrons and 0 nonbonding electrons. Thus, formal charge = 5- (1/2)*8 - 0 = 1.

Neon has an electron configuration of 1s²2s²2p⁶, just like O^2-. Sodium has one extra electron, while magnesium has two extra electrons, beyond the neon configuration. Finally, the electron configuration for Cl^- is 1s²2s²2p⁶3s²3p⁶. It is one period below flourine; F^- would be isoelectric with neon.

Electronegativity increases moving from the left side of the periodic table to the right, and decreases moving down through a group (Excluding the Group 18 noble gases).

Thus the correct answer is  (the furthest left), followed by , , , and  (the most electronegative element).

Charge stability of halide ions increases with increasing atomic size ( is more stable than , which is more stable than , which is more stable than ).

This is not to be confused with electronegativity trends, which would predict the ordering of acids opposite the correct answer.

Atomic size (and thus the propensity to release protons () into solution forming anionic halides) increases moving down Group 7, so the correct ordering is: .

This trend is tue, however, keep in mind that  and  are all considered strong acids. This means that they dissociate completely in solution and have negative pKa values.

Anions have at least one more electron than their neutral atoms. Adding an electron creates a shielding effect on other electrons in the same shell, decreasing the nuclear charge felt by each valence electron. As a result, the electrons will lie slightly further from the nucleus, corresponding to an increase in atomic radius. If there are no other electrons than the newly added electron in the valence shell, then the added electron was added to the now highest shell, which would significantly increase the size of the atom.

A cation is a positively charged ion; calcium is the only species of the answer choices that has a positive charge. Negatively charged ions, such as the fluoride ion are known as anions.

isotopes have more neutrons, but their protons are the same

The number following the compound is the isotope number, giving the number of protons and neutrons the element contains per atom. To get the number of neutrons you take the isotope number and subtract the atomic number of that element. 

O^2- is an ion, and not an isotope; an isotope has the same number of protons and electrons, but a different number of neutrons