Periodicity of Element Properties
Quick Notes
- Periodicity is the recurring trend in element properties across periods and down groups in the periodic table.
- Key periodic trends:
- Atomic radius: decreases across a period, increases down a group
- Ionic radius: decreases across a period (within a type), increases down a group
- Ionization energy: increases across a period, decreases down a group
- Electron affinity: generally becomes more negative across a period, less negative down a group
- Electronegativity: increases across a period, decreases down a group
Full Notes:
Periodicity refers to the recurring patterns in physical and chemical properties of elements that emerge due to their electron configurations.
These trends allow predictions about unknown elements and the reactivity or bonding of known ones.
Atomic Radius
Definition: Distance from the nucleus to the outermost electron shell.
| Trend | Reason |
|---|---|
| Across a period: Decreases | More protons = stronger nuclear attraction and electrons pulled closer |
| Down a group: Increases | More electron shells means outer electrons are farther from the nucleus |
Ionic Radius
Cations (positive ions) are smaller than their atoms.
Anions (negative ions) are larger than their atoms.
| Trend | Reason |
|---|---|
| Across a period: Decreases (within same ion type) | Increasing nuclear charge pulls electrons closer |
| Down a group: Increases | More electron shells = larger radius |
First Ionization Energy
Definition: The energy required to remove one mole of electrons from one mole of gaseous atoms.
| Trend | Reason |
|---|---|
| Across a period: Increases | Greater nuclear charge and decreasing radius means electrons more tightly held |
| Down a group: Decreases | Outer electrons are further from the nucleus with more shielding |
Note: Small drops (e.g., between Groups 2 and 13) occur due to subshell structure and electron repulsion.
Electron Affinity
Definition: The energy change when an atom gains one mole of electrons in the gaseous state.
| Trend | Reason |
|---|---|
| Across a period: Becomes more negative | Atoms more easily gain electrons to fill p subshells |
| Down a group: Becomes less negative | Added electron enters a shell farther from nucleus |
Note: Trends are not perfectly smooth; there are irregularities.
Electronegativity
Definition: The ability of an atom to attract electrons in a covalent bond.
| Trend | Reason |
|---|---|
| Across a period: Increases | Atoms have higher nuclear charge and smaller radii |
| Down a group: Decreases | Electrons are farther from the nucleus with more shielding |
Most electronegative element: Fluorine (F)
Summary
- Atomic radius decreases across a period and increases down a group
- Ionic radius decreases across a period (same ion type) and increases down a group
- Ionization energy increases across a period and decreases down a group
- Electron affinity becomes more negative across a period and less negative down a group
- Electronegativity increases across a period and decreases down a group