Can anyone explain me why the electron configuration of the vanadium atom is [Ar] 3d34s2 not [Ar] 4s23d3?

What is the mass defect of a mole of nuclei with 1.8 x 10^15 j/mol binding energy?

Consider the reduction reactions and their equilibrium constants. cu+(aq)+e−↽−−⇀cu(s)pb2+(aq)+2e−↽−−⇀pb(s)fe3+(aq)+3e−↽−−⇀fe(=6.2×108=4.0×10−5=9.3×10−3 cu + ( aq ) + e − ↽ − − ⇀ cu ( s ) k =6.2× 10 8 pb 2 + ( aq ) +2 e − ↽ − − ⇀ pb ( s ) k =4.0× 10 − 5 fe 3 + ( aq ) +3 e − ↽ − − ⇀ fe ( s ) k =9.3× 10 − 3 arrange these ions from strongest to weakest oxidizing agent.

As changes in energy levels of electrons increase, the frequencies of atomic line spectra they emit