Which of the following is closest to the first ionization energy of a rubidium (Rb) atom?

• A. 738 kJ/mol
• B. 403 kJ/mol
• C. 520 kJ/mol
• D. 1312 kJ/mol

Answer: (B)

Verified step by step guidance

1. Understand that the first ionization energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of cations.
2. Recall that rubidium (Rb) is an alkali metal in Group 1 of the periodic table, which generally have relatively low first ionization energies compared to other elements.
3. Recognize that ionization energy trends generally decrease down a group because the outermost electron is farther from the nucleus and more shielded by inner electrons, making it easier to remove.
4. Compare the given values to typical first ionization energies of alkali metals: lithium (~520 kJ/mol), sodium (~496 kJ/mol), potassium (~419 kJ/mol), and rubidium should be slightly lower than potassium due to its position below it in the periodic table.
5. Conclude that the value closest to rubidium's first ionization energy should be the lowest among the options, which is 403 kJ/mol, consistent with the trend of decreasing ionization energy down Group 1.