Outer electrons don't feel the whole nucleus. Inner electrons block part of it.
An outer electron is attracted by all Z protons but repelled by the electrons between it and the nucleus. What it actually feels is the effective nuclear charge: Zeff = Z − S, where S is the shielding. Core electrons shield almost perfectly. Electrons in the same shell barely shield each other at all.
Across a period →
Z goes up by 1, but the new electron joins the same shell and barely shields. Zeff rises, so the atom shrinks and holds its electrons tighter.Down a group ↓
A new shell is added. Zeff is roughly similar, but the electrons are much farther out, so the atom grows and holds them more loosely.Everything follows
Radius, ionization energy, electron affinity, and metallic character all come from Zeff and distance.Zeff for any element, 1–36
Blue: Slater Zeff felt by the outermost electron. Gray: atomic radius (covalent, pm). The two run opposite each other: every time Zeff climbs across a period, the radius drops. Then a new shell resets both.
The jump tells you how many valence electrons there are.
Pull electrons off one at a time. Each one costs more, since there are fewer electrons left to share the nuclear pull. But once you break into the core, the cost explodes, because those electrons sit a full shell closer with almost no shielding.
Cations shrink, anions swell.
Lose electrons and the remaining ones feel a stronger pull, often losing a whole shell too, so cations are smaller than their atoms. Gain electrons and the extra electron–electron repulsion puffs the cloud out, so anions are bigger. In an isoelectronic series (same number of electrons), more protons means smaller.
Energy change when an atom gains an electron (kJ/mol)
More negative means more energy released, so the atom "wants" the electron more. Halogens are the most eager, since one electron completes their p subshell. Noble gases and Group 2 resist (+), since the electron would have to start a new subshell.
Note Cl (−349) beats F (−328). Fluorine is so small that the incoming electron gets crowded by the electrons already there. A classic exam question.
Metallic character runs opposite to ionization energy.
Metals (lower left)
Lose electrons easily → cations. Shiny, conductive. Metal oxides are basic: CaO + H₂O → Ca(OH)₂.Nonmetals (upper right)
Gain or share electrons. Nonmetal oxides are acidic: CO₂ + H₂O → H₂CO₃, SO₃ + H₂O → H₂SO₄.Periodic trend reasoning
Acidic oxides in your water
CO₂ is a nonmetal oxide, so it's acidic. That's why purified water sitting open to air drifts down to about pH 5.6, and why carbonated product is tart before any flavor is added. Going the other way, lime (CaO) and other metal oxides are bases. Calcium oxide reacting with water releases a lot of heat (ΔH ≈ −65 kJ/mol), which is why quicklime is handled as a hazard and not just a powder.
Common mistakes
- Saying atoms get bigger across a period because "they have more electrons." More protons in the same shell pull everything in.
- Forgetting that the second ionization energy of Na is huge. Na⁺ already has a noble-gas core.
- Comparing ion sizes without checking electron counts. In an isoelectronic series, just compare protons.
- Calling a more negative electron affinity "smaller." It means more energy is released, so a stronger attraction.
- Missing the exceptions to the IE trend: Be > B and N > O, from starting a new subshell and from electron pairing.