Atomic & Ionic Radius Trends in S-Block Elements: Alkali & Alkaline Earth Metals

Atomic and Ionic Radius - The atomic and ionic radii of elements play a crucial role in determining their chemical and physical properties. In S-block elements, which include alkali metals (Group 1) and alkaline earth metals (Group 2), these radii show significant trends across a period and down a group.

This guide will cover:

• The trends in atomic and ionic radii in alkali and alkaline earth metals.
• The factors affecting atomic size.
• The comparison between alkali and alkaline earth metals.
• The influence of effective nuclear charge ($Z_{\text{eff}}$) on atomic size.

Understanding Atomic and Ionic Radius

What is Atomic Radius?

• The atomic radius is the distance from the nucleus to the outermost electron shell.
• It determines how large an atom is.
• Measured in picometers (pm).

What is Ionic Radius?

• The ionic radius is the size of an atom’s ion after losing or gaining electrons.
• Cations (positive ions) are smaller than their neutral atoms due to loss of electrons.
• Anions (negative ions) are larger due to electron gain and increased electron-electron repulsion.

Atomic and Ionic Radius in Alkali Metals (Group 1)

Trend Across a Period

• Alkali metals have the largest atomic radii in their respective periods.
• This is because they have only one valence electron and weak nuclear attraction.

Trend Down the Group

• Atomic radius increases as we move down Group 1.
• Reason:
  • Increase in the number of electron shells.
  • Weaker nuclear attraction due to electron shielding.

Element	Atomic Radius (pm)	Ionic Radius (M⁺) (pm)
Lithium (Li)	152	76
Sodium (Na)	186	102
Potassium (K)	227	138
Rubidium (Rb)	248	152
Cesium (Cs)	265	167

Key Observations in Alkali Metals

1. Largest Atomic Size in Period: Alkali metals have the largest atomic size in their respective periods.
2. Increase in Atomic Radius Down the Group: Due to additional electron shells, the atomic size increases.
3. Ionic Radius is Smaller than Atomic Radius: Alkali metals form M⁺ ions, which are smaller than their neutral atoms due to loss of one electron.

Atomic and Ionic Radius in Alkaline Earth Metals (Group 2)

Trend Across a Period

• Alkaline earth metals are smaller than alkali metals in the same period.
• This is due to the higher effective nuclear charge ($Z_{\text{eff}}$), which pulls electrons closer.

Trend Down the Group

• Atomic radius increases down Group 2.
• This is because of:
  • Increase in electron shells.
  • Electron shielding effect reducing nuclear attraction.

Element	Atomic Radius (pm)	Ionic Radius (M²⁺) (pm)
Beryllium (Be)	112	31
Magnesium (Mg)	160	72
Calcium (Ca)	197	100
Strontium (Sr)	215	118
Barium (Ba)	222	135

Key Observations in Alkaline Earth Metals

1. Smaller Atomic Size than Alkali Metals: Due to higher nuclear charge.
2. Increase in Atomic Radius Down the Group: Due to more electron shells.
3. Ionic Radius is Smaller than Atomic Radius: Alkaline earth metals form M²⁺ ions, which are much smaller than their neutral atoms due to loss of two electrons.

Factors Affecting Atomic and Ionic Radius

1. Effective Nuclear Charge ($Z_{\text{eff}}$)

• Definition: The net positive charge experienced by valence electrons.
• Across a Period: $Z_{\text{eff}}$ increases, reducing atomic size.
• Down a Group: $Z_{\text{eff}}$ remains nearly constant, but electron shielding increases, leading to larger atomic size.

2. Electron Shielding Effect

• Inner electrons shield outer electrons from the nucleus.
• More inner shells → More shielding → Larger atomic radius.

3. Number of Electron Shells

• More shells = Larger atomic size.
• This explains why atomic radius increases down a group.

4. Ionization and Electron Loss

• Cation formation decreases size due to increased nuclear attraction.
• Alkali metals (M⁺) and alkaline earth metals (M²⁺) have smaller ionic radii than their neutral atoms.

Comparison of Alkali and Alkaline Earth Metals

Property	Alkali Metals (Group 1)	Alkaline Earth Metals (Group 2)
Atomic Size	Larger	Smaller
Ionic Size	Larger (M⁺ ions)	Smaller (M²⁺ ions)
Reason for Size	Weaker nuclear attraction	Stronger nuclear attraction
Increase Down the Group?	Yes	Yes

Why are Alkaline Earth Metals Smaller than Alkali Metals?

1. Higher Effective Nuclear Charge ($Z_{\text{eff}}$): Pulls electrons closer.
2. Stronger Metallic Bonding: Increases density and reduces atomic size.

Importance of Atomic and Ionic Radii in Chemistry

1. Chemical Reactivity

• Larger atomic size → More reactive (easier to lose electrons).
• Alkali metals are more reactive than alkaline earth metals due to their larger size and lower ionization energy.

2. Ionic Bonding and Compound Formation

• Smaller cations form stronger ionic bonds.
• Mg²⁺ and Ca²⁺ form stable ionic lattices, used in construction (e.g., CaCO₃).

3. Solubility of Compounds

• Smaller ions (Be²⁺, Mg²⁺) form less soluble compounds.
• Larger ions (Ba²⁺, Cs⁺) form more soluble compounds.

Frequently Asked Questions (FAQs)

Q1: Why do alkali metals have the largest atomic size in a period?

Alkali metals have low effective nuclear charge and only one valence electron, resulting in weak attraction and large atomic size.

Q2: Why does atomic size increase down a group?

• Due to increase in electron shells.
• Electron shielding weakens nuclear attraction, leading to larger atomic size.

Q3: Why are cations smaller than their parent atoms?

• Loss of electrons reduces electron-electron repulsion.
• The nucleus pulls remaining electrons closer, decreasing size.

Q4: Why do alkaline earth metals have smaller atomic radii than alkali metals?

• Higher nuclear charge ($Z_{\text{eff}}$) in Group 2 leads to stronger attraction.
• This pulls electrons closer, reducing atomic size.

Q5: Why does ionic size decrease from Li⁺ to Cs⁺?

• Increasing nuclear charge pulls electrons closer to the nucleus, decreasing ionic radius.