Chemistry
Periodic Trends
Periodic trends are systematic patterns in atomic properties across the periodic table. Atomic radius shrinks left-to-right and grows top-to-bottom. Ionization energy and electronegativity do the opposite. All four trends derive from one cause: how strongly the nucleus pulls on outer-shell electrons.
How to Approach Periodic Trends
Atomic radius
Decreases left-to-right (more protons pull the same shell tighter). Increases top-to-bottom (each new period adds a shell). So Fr is the largest non-radioactive atom; He is the smallest.
Ionization energy
Energy to remove the outermost electron. Increases left-to-right (tighter hold). Decreases top-to-bottom (outer shell is farther away). Noble gases have the highest IE; alkali metals the lowest.
Electronegativity
Tendency to attract electrons in a bond. Increases left-to-right and decreases top-to-bottom. F is the most electronegative element (3.98 on the Pauling scale); Cs is among the least.
Frequently Asked Questions
Why does atomic radius shrink left-to-right?+
Each new element adds a proton (increasing nuclear charge) without adding a new shell. The outer electrons feel a stronger pull, contracting the atom.
What's electron affinity?+
Energy released when an atom gains an electron. Generally more negative (more energy released) toward the upper right. Cl has high electron affinity; noble gases have nearly zero.
Why are there exceptions to the trends?+
Half-filled and fully-filled subshells (like N's 2p³ or O's 2p⁴) cause small bumps. Otherwise the trends are remarkably consistent — driven by simple Coulomb attraction between nucleus and outer electrons.
Related Topics
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