Predict and explain trends in ionization energy, electronegativity, and metallic character across a period and down a group.
Ionization energy is the energy required to remove one electron from a neutral atom. It follows the OPPOSITE pattern of atomic radius: ionization energy increases across a period (more protons hold electrons more tightly, so removing one takes more energy) and decreases down a group (the outermost electron is farther from the nucleus and shielded by more inner electrons, so it takes less energy to pull it away).
Electronegativity measures how strongly an atom attracts electrons when it is bonded to another atom. It follows the same pattern as ionization energy: increasing across a period, decreasing down a group. Fluorine is the most electronegative element on the entire table (about 4.0 on the Pauling scale), sitting in the top right corner where the trend peaks; francium and cesium, in the bottom left, are among the least electronegative.
Metallic character -- how strongly an element behaves like a metal (losing electrons easily, forming positive ions, conducting well) -- runs opposite to both of those trends: it increases going DOWN a group and increases going LEFT across a period, since low ionization energy and low electronegativity are exactly what makes an element behave like a metal in the first place.
Mastery looks like: They correctly predict and explain relative ionization energy, electronegativity, and metallic character for pairs of elements using period and group trends.
Common stumbles: Mixing up the direction of the ionization-energy/electronegativity trend with the atomic-radius trend, since they run opposite to each other; forgetting that metallic character is the reverse of electronegativity rather than following the same direction.