Nitrogen is not used for welding instead of argon primarily due to its reactivity at high temperatures. While nitrogen is cheaper, it can react with air to form harmful nitrous oxides when exposed to the high temperatures typical in welding processes, making it unsuitable for this application. Additionally, nitrogen's effectiveness as an inert gas is limited to medium and low temperatures, as it only behaves inertly due to its strong triple bond, which breaks down at elevated temperatures. In contrast, argon is a noble gas with a complete valence shell, ensuring it remains inert and non-reactive across all temperatures, making it the preferred choice for welding. Argon's density also helps it stay localized around the weld area, further enhancing its effectiveness.