Electrons in atoms are often categorized as either valence electrons or core electrons in order to rationalize a wide array of atomic properties.
Using only information from the periodic table and the underlying quantum-mechanical ideas about atomic structure, respond to the following prompts.
(a) Describe how valence electrons are distinguished from core electrons for (i) a main-group element such as sulfur and (ii) a first-row transition-metal element such as vanadium. Your description should connect each case to its generic electron configuration notation.
(b) Explain why the distinction between valence and core electrons is not identical for all transition-metal elements, citing specific features of their subshell energies.
(c) The second ionization energy of magnesium is substantially larger than the first ionization energy, whereas the second ionization energy of sodium is more than an order of magnitude larger than its first. Using your discussion from parts (a) and (b), explain these observations in terms of which electron (valence or core) is being removed at each ionization step.