To determine which element would have a full outer shell of electrons if it gained three electrons, we need to consider the electron configuration of each element listed:
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Boron (B) has 3 electrons (config: 1s² 2s² 2p¹). If it gains 3 electrons, it would have 6 electrons, which gives it the electron configuration of carbon (1s² 2s² 2p²), but not a full outer shell.
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Lithium (Li) has 3 electrons (config: 1s² 2s¹). If it gains 3 electrons, it would have 6 electrons, similar to boron, which is not a full outer shell.
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Argon (Ar) has 18 electrons (config: 1s² 2s² 2p⁶ 3s² 3p⁶) and already has a full outer shell (8 electrons in the outermost shell).
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Nitrogen (N) has 7 electrons (config: 1s² 2s² 2p³). If it gains 3 electrons, it would have 10 electrons, which results in the configuration of neon (1s² 2s² 2p⁶), thus having a full outer shell.
Therefore, the correct answer is nitrogen.
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