Explain why a graph of ionization energy versus atomic number (across a row) is not linear. Where are the exceptions? Why are there exceptions?

Ionization energy is the energy required to remove an electron from an atom or ion in its gaseous state. In other words, it is the energy needed to overcome the electrostatic attraction between the electron and the nucleus.

Across a row in the periodic table (from left to right), the ionization energy generally increases. This is because the atomic number increases, resulting in a greater positive charge in the nucleus, which in turn attracts the electrons more strongly. At the same time, electrons are being added to the same energy level (principal quantum number), so the increase in shielding effect is relatively small, and the effective nuclear charge increases.

The ionization energy depends on several factors, including: 1. Atomic number: As mentioned earlier, greater atomic number means higher ionization energy due to the increased positive charge in the nucleus. 2. Shielding effect: The inner (core) electrons shield the outer electrons from the full positive charge of the nucleus, decreasing the ionization energy. 3. Electron configuration: The stability provided by filled and half-filled subshells affects the ease with which electrons can be removed.

The graph of ionization energy versus atomic number is not linear because of the combined influence of atomic number, shielding effect, and electron configuration. As one moves across a row, the subtle interplay of these factors leads to fluctuations in the ionization energy, resulting in a nonlinear trend.

Notable exceptions to the general trend of increasing ionization energy across a row are observed between Group 2 and Group 3, as well as between Group 5 and Group 6 elements: 1. The ionization energy of Group 3 elements (e.g., boron in the 2nd row) is slightly lower than that of Group 2 elements (e.g., beryllium). This is attributed to the fact that in Group 3 elements, the electron being removed is from the higher energy and less stable p-subshell, as opposed to the s-subshell in Group 2 elements. 2. The ionization energy of Group 6 elements (e.g., oxygen in the 2nd row) is slightly lower than that of Group 5 elements (e.g., nitrogen). This is because in Group 6 elements, the electron being removed is from an already partially filled p-subshell, which experiences some electron-electron repulsion, making it somewhat easier to remove compared to a half-filled p-subshell in Group 5 elements.

In summary, the graph of ionization energy versus atomic number is not linear due to the interplay of factors like atomic number, shielding effect, and electron configuration. Exceptions to the general trend occur between Group 2 and Group 3 elements, as well as between Group 5 and Group 6 elements, because of differences in electron configuration and subshell stability.