If an element has 9 electrons, how many of them will be in the 2nd energy level?

Which other elements contain the same number of outer electrons as sodium

How does kepler second law of planetary motion overthrow one of the basic beliefs of classical astronomy

Water undergoes a large change in density at 0 ∘ c as it freezes to form ice. calculate the percent change in density that occurs when liquid water freezes to ice at 0 ∘ c given that