Question

An electron can be removed from Na or Rb by electromagnetic radiation (the photoelectric effect). Which element, Na or Rb, would require the shortest wavelength to remove the electron? Explain your answer.

Answer 1

The first ionization energy of sodium, for example, is the energy it takes to remove one electron from a neutral atom.

Na(g) + energy Na⁺(g) + e^-

The second ionization energy is the energy it takes to remove another electron to form an Na²⁺ ion in the gas phase.

Na⁺(g) + energy Na²⁺(g) + e^-

The third ionization energy can be represented by the following equation.

Na²⁺(g) + energy Na³⁺(g) + e^-

The energy required to form a Na³⁺ ion in the gas phase is the sum of the first, second, and third ionization energies of the element.

It doesn't take much energy to remove one electron from a sodium atom to form an Na⁺ ion with a filled-shell electron configuration. Once this is done, however, it takes almost 10 times as much energy to break into this filled-shell configuration to remove a second electron. Because it takes more energy to remove the second electron than is given off in any chemical reaction, sodium can react with other elements to form compounds that contain Na⁺ ions but not Na²⁺ or Na³⁺ ions.