IntoChemistry · General Chemistry

Periodic Trends: Atomic Radius, Ionization Energy, and Electronegativity

Understand why properties change across rows and down columns of the periodic table.

◎ Clear explanations◉ Worked examples▤ Practice problems⬇ Free resources

Don’t memorize arrows without explanations

Periodic trends arise from interactions between nuclear charge, distance of the outer electrons from the nucleus, electron shielding, and electron arrangement. Simple direction arrows are a helpful first approximation, not absolute rules.

Illustrated Periodic Trends: Atomic Radius, Ionization Energy, and Electronegativity study overview — IntoChemistry visual guide. Follow the worked examples below for the complete reasoning.

Visual trend map

Atomic radius increases left and down, while first ionization energy and electronegativity generally increase right and up. — Use these directions for introductory predictions, then check actual data for exceptions.

Three major properties

Atomic radius: a measure of atomic size, defined in different ways. It usually decreases across a period and increases down a group.
First ionization energy: energy to remove the first electron from a gas-phase atom. It generally increases across a period and decreases down a group.
Electronegativity: relative tendency to attract shared electrons in a chemical bond. It generally rises toward fluorine; individual scales and bonding environments can affect the reported value.

Worked comparisons

Example 1

Na versus Cl: larger atom?

Both are in period 3.
Across the period the nuclear attraction felt by valence electrons generally becomes stronger.
Na lies farther left and has the larger atomic radius.

Na is larger.

Example 2

Li versus K: higher first ionization energy?

Both are group 1 elements.
Potassium’s valence electron is farther from the nucleus and more shielded.

Li has the higher first ionization energy.

Example 3

Be versus B: identify an exception

The general trend predicts increasing first ionization energy across period 2.
However, the first electron removed from B occupies a 2p orbital whereas Be has filled 2s subshell character.

Be has a higher first ionization energy than B, an important exception.

Common exceptions

For first ionization energy, B is lower than Be and O is lower than N.
Electronegativity values are scale-dependent, and noble gases are often omitted from introductory charts.
Cation and anion radii require more care than neutral-atom radius comparisons.

Quick check

Why is potassium larger than sodium?

K occupies another principal electron shell. Its outer electrons are farther from the nucleus on average.

Interactive Periodic Trends: Atomic Radius, Ionization Energy, and Electronegativity Practice

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