AP Chemistry · Unit 1 · Topic 1.7
Periodic Trends
A periodic-table arrow is a useful summary, but it cannot explain an exception. The stronger approach is to identify the electron being discussed and compare its attraction to the nucleus. Nuclear charge, average distance, shielding, and electron repulsion work together to produce the observed pattern.
Start with competing effects
Protons attract electrons, while other electrons partly shield that attraction and repel one another. Across a main-group period, increasing proton number is not completely canceled by the added electrons in the same principal shell. Valence electrons generally feel greater effective nuclear attraction, so atomic radius usually decreases and first ionization energy usually increases.
Why moving down a group is different
Down a group, valence electrons occupy higher principal shells, farther from the nucleus and shielded by additional inner electrons. The larger nuclear charge does not by itself guarantee stronger attraction to those distant electrons. Atomic size generally increases, while first ionization energy generally falls. Always specify the property: electron affinity does not follow a perfectly smooth version of the same arrow.
Compare carefully matched species
| Comparison | Prediction | Key reason |
|---|---|---|
| Na atom versus Cl atom | Na is larger | Same period; Cl has stronger effective attraction |
| Li atom versus K atom | K is larger | Additional occupied shells |
| O²− versus F− | O²− is larger | Same electron count; oxygen has fewer protons |
For an isoelectronic series, holding electron count fixed makes nuclear charge especially informative. The species with more protons generally pulls that common electron population into a smaller region.
Worked reasoning: the isoelectronic ranking
Order N3−, O2−, F−, and Na+ from largest to smallest radius. Each has ten electrons. Their proton counts are 7, 8, 9, and 11. Stronger nuclear attraction therefore gives N3− > O2− > F− > Na+. This argument is more controlled than mixing unrelated atomic and ionic trend arrows.
Exceptions reveal subshell structure
Boron's first ionization energy is lower than beryllium's because the electron removed from boron occupies a higher-energy 2p subshell rather than 2s. Oxygen's is lower than nitrogen's because removing a paired 2p electron reduces electron-electron repulsion. These are not failures of electrostatics; they show why the specific electron arrangement matters.
Successive ionization data as evidence
Imagine successive ionization energies of 600, 1,200, and 7,000 kJ mol−1 in a simplified dataset. The large increase after the second removal suggests that two relatively accessible valence electrons have been removed and the next electron is much more tightly bound. For a typical main-group atom, this supports two valence electrons, but it does not identify a unique element by itself.
Write an explanation with a causal chain
A complete comparison names the relevant shell or electron count, identifies the difference in attraction or repulsion, and connects that difference to the property. Avoid “it wants an octet” as a force explanation. Ionization is an energy-requiring removal process, and the ionization energy is positive even when an atom commonly forms a cation in compounds.
Practice, then check your reasoning
Which has the larger radius, Mg²⁺ or Na⁺?
Answer and reasoning: Na⁺: both have ten electrons, but Mg²⁺ has the larger nuclear charge.
A large jump occurs between the third and fourth ionization energies. What main-group interpretation is supported?
Answer and reasoning: Three valence electrons are relatively accessible; the fourth removal reaches a more tightly bound core electron.
Scientific references: OpenStax: periodic variations; OpenStax: subshell occupancy. OpenStax Chemistry 2e authors: Paul Flowers, Klaus Theopold, Richard Langley, and William R. Robinson. Consult source pages for their license terms. Accessed September 13, 2026. Explanations, comparison tables, and practice tasks were independently written for this site; numerical teaching datasets are illustrative unless explicitly identified otherwise.
Official curriculum reference: College Board AP Chemistry course page. The explanation and worked example are independently written for this study site.