Periodic Trends Study Guide
Four trends explain most of the periodic table's behavior: atomic radius, ionization energy, electronegativity, and electron affinity. They all come from the same physics — nuclear charge pulling inward, shells and shielding pushing outward — so once you understand the cause, the trends become predictions instead of memorization. This guide states each rule with its direction, adds ionic radius and the famous exceptions, and targets the reversals and confusions that exams exploit.
Key formulas
Atomic radius: decreases left→right across a period; increases top→bottom down a group
Ionization energy: increases left→right across a period; decreases top→bottom down a group
Electronegativity: increases left→right across a period; decreases top→bottom; fluorine is highest at 3.98
Electron affinity: generally highest for halogens; noble gases ≈ 0; follows EN loosely with exceptions
Cations < neutral atoms < anions in size for the same element
Isoelectronic series: size decreases as nuclear charge (proton count) increases
Trend exceptions explained by electron shielding and half-filled/filled subshell stability (e.g., Mg > Al, N > O in first IE)
Topic checklist
Atomic radius
Atomic radius is half the distance between nuclei of two bonded atoms of the same element.
Across a period (L→R), added protons pull the same shell tighter — radius shrinks.
Down a group, each new electron shell is farther out — radius grows, and inner shells shield the outer ones.
Ionization energy
Ionization energy is the energy needed to remove one electron from a gaseous atom.
Across a period (L→R), stronger nuclear pull makes removal harder — IE rises.
Down a group, outer electrons are farther and more shielded — IE falls.
Electronegativity
Electronegativity is an atom's pull on shared electrons in a bond.
Across a period (L→R), EN rises; fluorine is the highest at 3.98.
Down a group, EN falls because the bonding electrons are farther from the nucleus.
Electron affinity
Electron affinity is the energy released when a gaseous atom gains an electron.
Halogens have the highest electron affinities; noble gases are near zero.
Trend roughly follows electronegativity but with notable exceptions (e.g., N, noble gases).
Ionic radius
Cations are smaller than their parent atoms (fewer electrons, same protons).
Anions are larger than their parent atoms (more electron-electron repulsion).
For an isoelectronic series (e.g., N³−, O²−, F−, Na+, Mg²+), more protons means smaller size.
Exceptions and fine points
Anomalies come from electron shielding and subshell stability: half-filled and filled subshells resist change.
Example: Mg (3s²) has higher IE than Al (3p¹); N (2p³) has higher IE than O (2p⁴).
Transition metals show irregular trends — state the general trend, then name the exception.
Common mistakes
Mistake: Treating atomic radius and ionic radius as the same thing
Fix:
Atomic radius refers to the neutral atom; ionic radius refers to the ion. Na (186 pm) vs Na+ (102 pm) are different quantities — always check which the question asks for.
Mistake: Reciting a trend backwards under exam pressure
Fix:
State the trend with its direction: 'increases left to right' and 'decreases top to bottom.' Drawing a small arrow diagram on scratch paper prevents reversals under pressure.
Mistake: Applying main-group trend rules rigidly to transition metals
Fix:
For transition metals, effective nuclear charge and shielding change irregularly; don't force the main-group trend onto them. Answer with the general trend and note the exception.
Mistake: Confusing ionization energy with electron affinity
Fix:
Ionization energy is about removing an electron (costs energy, positive values); electron affinity is about gaining one (usually releases energy). Keep the definitions separate.
Exam tips
Use periodic-trends-practice to drill ordering questions — they are the single most common trend question type.
Draw one table arrow-diagram for all four trends on your scratch sheet at the start of the exam and consult it.
Memorize the two classic IE exceptions: Mg > Al and N > O (half-filled subshell stability).
For isoelectronic series questions, count protons: more protons wins the size contest.
Pair with atomic-structure-practice — trends make far more sense once electron configurations are solid.