If the nitrogen atom has electronic configuration 1s⁷, it would have energy lower than that of the normal ground state configuration 1s²2s²2p³, because the electrons would be closer to the nucleus. Yet 1s⁷ is not observed because it violates

How many moles of electron weigh one kilogram?

The point(s) in the curve $${y^3} + 3{x^2} = 12y$$ where the tangent is vertical, is (are)

If $${\overrightarrow a }$$ and $${\overrightarrow b }$$ are two unit vectors such that $${\overrightarrow a + 2\overrightarrow b }$$ and $${5\overrightarrow a - 4\overrightarrow b }$$ are perpendicular to each other then the angle between $$\overrightarrow a $$ and $$\overrightarrow b $$ is