The first ionization energy for Mg (738 kJ/mol) is higher than that for Na (469 kJ/mol) but the second ionization energy for Na (4562 kJ/mol) is much larger than that for Mg (1451 kJ/mol). Explain this observation.© BrainMass Inc. brainmass.com October 6, 2022, 12:10 pm ad1c9bdddf
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In order to address this problem, we'll need to know the electron configuration of the two samples being compared. The electron configurations are:
For Mg: 1s22s22p63s2
For Na: 1s22s22p63s1
The configuration shows how the electrons are arranged in the element. Mg has twelve electrons whereas Na has eleven as shown on the periodic table, represented as superscripts on the electron configurations. Looking at the electron configuration closely, both elements have their outer electrons in the 3S subshell, with Mg having two electrons and Na having one.
The first ionization energy is the energy required to remove the first electron from the highest subshell (3S). Since Mg, is on the right of Na on the periodic table, Mg has a higher nuclei charge and requires more energy to remove the 3S electron, thus the first ionization energy of Mg is higher compared to Na.
Upon removal of the first electron the corresponding atoms become ions with electron configuration shown:
Notice that the outer subshell for Na becomes the 2p with six electrons since the 3s has lost its only electron, whereas that for Mg remains the 3s with one electron.
Now, the second ionization energy will be the energy required to remove one of the 2p electron for Na+ or the 3s electron for Mg+. Since the Na+ electron is now in the 2p level, which is closer to the nucleus compared to Mg+ that is at the 3s level. The 2p electron for sodium is more tightly bound to the atom due to its closer proximity to the nucleus, and thus requires more energy compared to the 3s electron of Mg+, which is at a higher subshell; hence, the further away from the nucleus the more energy needed to remove it.
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