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When comparing elements based on the different periodic trends, how do you decide which of the two elements diagonal to one another is going to have a larger value than the other? Like, for example, when comparing the ionization energies of lithium and magnesium, fluorine and arsenic, selenium and iodine, etc. without knowing the actual ionization energy values.
I believe that going down a group has more significance than across a period for the trends. For example, with atomic radius, Arsenic would have a larger radius because it has the 4s 3d and 4p orbitals in addition to the orbitals in fluorine. The same goes for Selenium and Iodine. Although Iodine's nucleus has a stronger pull on it's electrons, because it has a greater positive charge, the addition of the 5s, 4d, and 5p orbitals have a greater affect on size. The same would go for ionization energy and electron affinity, but in reverse. So since As has a larger atomic radius, it has a lower ionization energy, since the positive charge of the nucleus has a lesser affect on the electrons (since they're further away). HOWEVER, despite all of this, I don't think we would be tested on diagonally located elements, just because the justification could be tricky. I think if you generally understand the concept of each trend, you're pretty set.
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