Question

Why do the metallic elements of a given period (horizontal row) typically have much lower ionization energies than do the nonmetallic elements of the same period?

Answer 1

Typically, the outside electrons in the non-metals "see" a higher positive charge and that makes it harder to dislodge the electron. For example, look at Na and Cl. Na has 11+ charges in the nucleus; Cl has 17+.

11* 2e shell 8e shell 1e shell
17* 2e shell 8e shell 7e shell.
So the Na (11+ charges) pulls on that outside single electron by about 1 charge (11 + shielded by 10 e = about 1)

The Cl sees about (17+ charges shielded by 10 e = about 7+ charges); therefore, the outside electrons in Cl are harder to pull away, thus a higher ionization energy.