Final answer:
The gas often used between the panes of high-efficiency windows is argon, selected for its low thermal conductivity and nonreactive nature, improving insulation and energy efficiency.
Step-by-step explanation:
The gas injected between the panes of some high-efficiency windows is argon. This inert gas, which is part of the Noble Gases group, is typically used because of its low thermal conductivity and nonreactive nature. Argon's properties help to increase the insulation of the windows by minimizing heat exchange between the panes, therefore, enhancing the overall energy efficiency of the windows. Argon is more effective than nitrogen in this application due to its superior thermal insulation characteristics. Additionally, argon is the third most abundant gas in the Earth's atmosphere, which makes it relatively cost-effective to use in such applications.