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
- 01
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.
- 02
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.
- 03
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?
Subshell structure causes small bumps: nitrogen's half-filled 2p³ is unusually stable, while oxygen's 2p⁴ holds its first paired electron, which is easier to remove — so oxygen's first ionization energy is lower than nitrogen's. Otherwise the trends are remarkably consistent — driven by simple Coulomb attraction between nucleus and outer electrons.
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