Periodic Trends
- Explain trends in atomic radius, ionization energy, and electronegativity
- Use effective nuclear charge (Zₑff) to justify each trend
- Rank atoms and ions by size and reactivity
One idea explains almost every trend: Zₑff
Effective nuclear charge (Zₑff) is the net positive pull an outer electron actually feels after inner electrons shield it. Across a period, protons are added but shielding barely changes, so Zₑff rises and electrons are pulled in tighter. Down a group, each new shell adds distance and shielding, loosening the grip.
Atomic radius
Atomic radius decreases across a period (rising Zₑff pulls the same shell inward) and increases down a group (electrons occupy higher shells farther from the nucleus). So the biggest atoms sit at the bottom-left, the smallest at the top-right.
Ionization energy & electronegativity
Both track the opposite of radius. Ionization energy (the energy to remove an electron) and electronegativity (the pull on a bonded pair) increase across a period and decrease down a group — a tightly held electron is hard to remove and strongly attracted. Fluorine, top-right, is the most electronegative element.
Rank Na, Mg, and K by atomic radius, largest first.
- 1.Na and Mg are in the same period; Mg has higher Zₑff, so Mg is smaller than Na.
- 2.K is one period below Na (extra shell), so K is larger than Na.
- 3.Combine: K > Na > Mg.
Which element has the highest first ionization energy?
Cations are always smaller than their parent atom (they lost a shell or lost electron–electron repulsion); anions are always larger (added repulsion spreads the cloud out). For isoelectronic ions, more protons = smaller.
Which is the correct size ordering of these isoelectronic species (all have 10 electrons)?
Whenever a free-response asks you to justify a trend, name the mechanism — “higher Zₑff” or “additional shell / more shielding.” Stating the trend alone rarely earns the point; the reasoning does.
Answer the 2 checkpoints as you read.
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