Explaining Trends

Part of the Atomic Structure section of Coddy's Chemistry journey. Lesson 41 of 45.

Every trend comes from the pull of the nucleus on the outer electrons. Three things decide it: nuclear charge (more protons pull harder), the number of shells (farther means weaker), and shielding (inner electrons block part of the pull).

Across a period, protons are added while the inner shells stay the same, so the pull grows: smaller atoms, higher ionization energy and electronegativity.

Sodium and chlorine as shells, both with three shells, the outer shell ringed

Na and Cl: same 10 inner electrons shielding, but 11 protons against 17

Down a group, a new shell and more shielding win over the extra protons: larger atoms, lower ionization energy and electronegativity. One extra shell usually beats a few extra protons.

Challenge

Lithium, beryllium, sodium, magnesium and potassium: across the period the nucleus pulls harder, and down the group each new shell adds distance. Put them in order of atomic radius, the smallest first.

Try it yourself

Periodic tablePut the elements in order