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In understanding the concept of a base, it's crucial to grasp how electron donating ability plays a significant role. A base is fundamentally about electron donation—providing a pair of electrons to accept a proton. This ability is strongly influenced by the electronic properties of the substance in question.

When a chemical species has a high electron donating ability, it becomes a strong base. This is because the ease with which it can share electrons to bond with a proton is greater. Among the options given in the exercise, \(\text{NH}_2^-\) stands out because it possesses an extra electron pair ready for donation.

• Electron donating ability is key in determining basic strength.
• Anions, such as \(\text{NH}_2^-\), typically donate electrons more readily due to additional electrons.
• Neutral molecules like \(\text{PH}_3\), \(\text{AsH}_3\), and \(\text{SbH}_3\) lack the extra electrons available in anions, hence reduced ability to donate.

The periodic table is organized into groups that reflect the properties of elements. Elements in the same group often exhibit similar chemical characteristics, yet these characteristics can vary in intensity depending on their position within the group.

In the context of the exercise, elements \(N\), \(P\), \(As\), and \(Sb\) are all in Group 15 of the periodic table. As we move down this group from nitrogen to antimony, a noticeable trend occurs—electron donating ability generally decreases because:

• The atomic radius increases, causing electrons to be held less tightly.
• The energy required to donate electrons rises as atoms become larger.
• Electron shielding effects increase, further reducing effective nuclear charge.

These factors combine to make \(\text{NH}_2^-\) a considerably stronger base compared to \(\text{PH}_3\), \(\text{AsH}_3\), and \(\text{SbH}_3\), given that nitrogen's smaller size enables it to effectively donate electron pairs.

Anions are negatively charged ions, meaning they have more electrons than protons. This excess of electrons is crucial when discussing bases and their strength, as these extra electrons enable anions to act as strong electron pair donors. In the problem and its solution, \(\text{NH}_2^-\) is highlighted as an anion.

Characteristics of anions that affect their behavior as bases include:

• Additional electrons that contribute to higher electron density.
• Increased ability to share electrons due to their negative charge.
• Potentially higher basicity compared to neutral molecules.

Thus, the anionic nature of \(\text{NH}_2^-\) significantly contributes to its high basic strength, surpassing the other molecules listed in the exercise. Its ability to donate electrons readily makes it the strongest base in the given set of options.