Question

Why do KCL and CaCl2 produce different flame colors even though K+ and Ca+2 are isoelectronic?

Answer 1

As we move down a group, atomic size increases, so the outermost electrons become easier to remove, since they are farther from the nucleus. Therefore, ionization energy decreases as we move down a group . As we move left-to-right across a period, Z_eff increases and atomic size decreases. As a result, the outer electrons are held more tightly, and are harder to remove. Therefore, ionization energy increases as we move from left to right across a period.(Here i am not talking about ionization energy, but how much energy is required to excite an electron from a ground state)

even though K+ and Ca+2 are isoelectronic, but the number of protons in the nucleus is not same. In calcium one more proton is present. This effective charge holds the outer electron more tightly than in potassium. This creates different energy levels for each atom. Different elements have different flame colours because their electrons have different allowed energy levels.