4) explain why the 4s sublevel fills before the 3d sublevel begins to fill as electrons are added. (4 points) 5) describe the trends in first ionization energy within groups and across periods in the periodic table. provide examples. (3 points)

4) Electrons are filled into respective orbitals based on the Aufbau rule according to which lower energy levels will fill up prior to those in the higher energies. Since 4s sublevel is lower in energy than the 3d sublevel, electrons will be added to the 4s first followed by 3d.

5) Ionization energy is the amount of energy required to release an electron from the outermost orbital.

Within a  group, the first ionization energy decreases on moving from the top to bottom. This is because the atomic size increases from top to bottom, as a result the valence electron feels less attraction towards the nucleus and can be releases easily.  For example, in alkali metals the first IE for Li = 520 kJ/mol and that of Cs = 375 kJ/mol

Across a period, the first ionization energy increases. This is because the atomic size decreases which increases the valence electron attraction towards the nucleus. For example, in the second period, first IE of Li = 520 kJ/mol and Ne = 2080 kJ/mol

it's filtration, but this is an assumption so my apologies if i got this

increased stability in the completion of fluorine's outer shell.