Final answer:
Magnesium (Mg) has the least favorable (least negative) electron affinity compared to sulfur (S), chlorine (Cl), and arsenic (As), as it is positioned further left on the periodic table and has typical metal properties that do not favor the attraction of additional electrons as much as the other non-metal elements listed.
Step-by-step explanation:
To determine which element has the least favorable (least negative) electron affinity out of Mg (magnesium), S (sulfur), Cl (chlorine), and As (arsenic), we should consider the general trends in electron affinity across the periodic table. Electron affinities tend to become less negative as we move down a group due to an increase in atomic size and electron shielding, and more negative from left to right across a period as the effective nuclear charge increases. Chlorine (Cl), for example, has a very high negative electron affinity because it is in the halogen group, which is known for attracting electrons.
Given these trends, magnesium (Mg) would have the least negative electron affinity since it is further to the left on the periodic table compared to sulfur (S) and chlorine (Cl) and it is higher in its group compared to arsenic (As). It resides in the s-block with typical metal characteristics and does not attract additional electrons as strongly as non-metals like chlorine, sulfur, and arsenic.