Overview
This lecture covers three main periodic trends: atomic size, ionization energy, and electronegativity, focusing on how these properties change across and down the periodic table.
Atomic Size (Atomic Radius)
- Atomic size is measured as atomic radius, from the nucleus to the outermost electron.
- Atomic size increases as you move down a group (column) due to increasing principal quantum number (n) and extra electron shells.
- Atomic size decreases as you move across a period (row) from left to right because increased nuclear charge pulls electrons closer.
- The largest atoms are found at the bottom left (e.g., cesium, francium); the smallest at the top right (helium).
- Atomic radius is measured in picometers (10⁻¹² meters).
Ionization Energy
- Ionization energy is the energy needed to remove an electron from an atom.
- Lower ionization energy means an atom loses electrons easily; higher value means electrons are held tightly.
- Ionization energy decreases down a group due to increased atomic size and electron shielding.
- Ionization energy increases across a period as atoms become smaller and nuclear charge grows.
- Trend is opposite to atomic size: highest at the top right, lowest at the bottom left.
Electronegativity
- Electronegativity measures an atom’s ability to attract electrons in a bond (tug-of-war strength).
- Electronegativity increases across a period and up a group (highest at top right, excluding noble gases).
- Fluorine is the most electronegative element (score ≈ 4), while cesium is among the least.
- Noble gases don’t have electronegativity values because they rarely form bonds.
- Rule of thumb: one step down and two steps right yields similar electronegativity values.
Worked Examples & Problem-Solving
- When ranking atomic size, always reorder elements by their positions on the periodic table.
- In isoelectronic series (ions with the same electron count), more protons means smaller size.
- Large jumps in successive ionization energies reveal the number of valence electrons.
- In bonds, greatest charge separation (polarity) occurs with the largest electronegativity difference, especially H–F.
- Diatomic molecules (e.g., F₂, O₂) are nonpolar as both atoms have equal electronegativity.
Key Terms & Definitions
- Atomic radius — Distance from nucleus to outermost electron.
- Ionization energy — Energy required to remove one electron from a neutral atom.
- Electronegativity — An atom’s ability to attract bonded electrons.
- Shielding — Electron repulsion from inner shells that reduces effective nuclear charge.
- Polarity — Separation of electric charge in a bond due to electronegativity difference.
- Isoelectronic series — Group of ions/atoms with the same number of electrons.
Action Items / Next Steps
- Find or draw a blank periodic table for practicing trends.
- Review data on ptable.com for atomic radius, ionization energy, and electronegativity.
- Practice ranking elements/ions by size, ionization energy, and electronegativity.