Atomic Radius and Ionic Radius — Revision Notes

⚡ 30-Second Revision

Atomic Radius: — Half internuclear distance.

- Covalent ($r_{cov}$): Bonded atoms, electron overlap. - Metallic ($r_{met}$): Metal lattice. - Van der Waals ($r_{vdW}$): Non-bonded atoms, no overlap. - Order: $r_{vdW} > r_{met} > r_{cov}$.

Ionic Radius: — Radius of an ion.

- Cation ($M^+$): Smaller than parent atom ($M$). - Anion ($X^-$): Larger than parent atom ($X$).

Trends:

- Across Period (L to R): Atomic/Ionic radius $\downarrow$ (due to $\uparrow Z_{eff}$). - Down Group (T to B): Atomic/Ionic radius $\uparrow$ (due to $\uparrow$ shells).

Isoelectronic Species: — Same number of electrons. Size $\propto \frac{1}{\text{Nuclear Charge (Z)}}$.
Factors: — Nuclear charge (Z), No. of shells (n), Shielding effect, Electron-electron repulsion.

2-Minute Revision

Atomic and ionic radii are crucial periodic properties. Atomic radius is not a fixed value; it's defined as covalent, metallic, or van der Waals radius. Covalent radius is for bonded atoms, metallic for metal lattices, and van der Waals for non-bonded atoms in contact.

Remember the order: van der Waals > metallic > covalent for the same element. The key factors influencing atomic radius are nuclear charge (pulls electrons closer), number of electron shells (increases size), and the shielding effect (reduces effective nuclear charge).

Across a period, atomic radius decreases due to increasing effective nuclear charge. Down a group, it increases due to the addition of new electron shells. Ionic radius refers to the size of ions. Cations (positive ions) are always smaller than their parent atoms because they lose electrons and experience a greater effective nuclear charge.

Anions (negative ions) are always larger than their parent atoms due to gaining electrons, which increases electron-electron repulsion. For isoelectronic species (same number of electrons), the ionic radius decreases as the nuclear charge increases.

This inverse relationship is a frequent NEET question. Always consider these factors and trends when comparing sizes.

5-Minute Revision

To master atomic and ionic radii for NEET, start with the definitions and types. Atomic radius is an effective measure, not a precise boundary. Covalent radius is half the bond length in a molecule (e.g., $r_{Cl} = 99\,\text{pm}$ in $Cl_2$). Metallic radius is half the internuclear distance in a metal crystal (e.g., $r_{Cu} = 128\,\text{pm}$). Van der Waals radius is for non-bonded atoms in contact (e.g., $r_{Cl} = 180\,\text{pm}$). Crucially, $r_{vdW} > r_{met} > r_{cov}$.

Periodic Trends:

1
Across a Period (Left to Right): — Atomic radius generally decreases. Example: $Li (152\,\text{pm}) > Be (112\,\text{pm}) > B (85\,\text{pm})$. This is because nuclear charge (Z) increases, pulling valence electrons closer, while electrons are added to the same shell, leading to an increased effective nuclear charge ($Z_{eff}$). The shielding effect within the same shell is minimal.
2
Down a Group (Top to Bottom): — Atomic radius generally increases. Example: $Li (152\,\text{pm}) < Na (186\,\text{pm}) < K (227\,\text{pm})$. This is due to the addition of new principal electron shells, which are further from the nucleus, and increased shielding by inner electrons.

Ionic Radii:

Cations ($M^+$): — Always smaller than their parent atoms ($M$). Loss of electrons reduces electron-electron repulsion and increases $Z_{eff}$ on remaining electrons. Example: $Na (186\,\text{pm})$ vs. $Na^+ (102\,\text{pm})$.
Anions ($X^-$): — Always larger than their parent atoms ($X$). Gain of electrons increases electron-electron repulsion, expanding the electron cloud. Example: $Cl (99\,\text{pm})$ vs. $Cl^- (181\,\text{pm})$.

Isoelectronic Species: These have the same number of electrons. Their size is determined solely by nuclear charge. Higher nuclear charge means stronger pull on the electron cloud, hence smaller size. Example: For 10 electrons, $N^{3-} (Z=7) > O^{2-} (Z=8) > F^- (Z=9) > Na^+ (Z=11) > Mg^{2+} (Z=12) > Al^{3+} (Z=13)$.

NEET Focus: Practice comparing sizes of various species, especially isoelectronic ones. Understand the reasons behind each trend and exception (e.g., transition metal contraction due to poor d-electron shielding).

Prelims Revision Notes

Atomic and ionic radii are key periodic properties. Atomic radius is not a fixed value but is defined contextually:

Covalent Radius ($r_{cov}$): — Half the internuclear distance of two identical atoms covalently bonded. Example: $r_{Cl} = 99\,\text{pm}$.
Metallic Radius ($r_{met}$): — Half the internuclear distance of two adjacent metal atoms in a crystal lattice. Example: $r_{Cu} = 128\,\text{pm}$.
Van der Waals Radius ($r_{vdW}$): — Half the internuclear distance of two identical non-bonded atoms in closest contact. Example: $r_{Cl} = 180\,\text{pm}$.

Order of Radii: For a given element, $r_{vdW} > r_{met} > r_{cov}$.

Factors Affecting Atomic Radius:

1
Nuclear Charge (Z): — Higher Z pulls electrons closer, decreasing radius.
2
Number of Electron Shells (n): — More shells mean larger radius.
3
Shielding Effect: — Inner electrons shield outer electrons, reducing effective nuclear charge ($Z_{eff}$), thus increasing radius. Poor shielding (e.g., d-electrons) leads to higher $Z_{eff}$.

Periodic Trends of Atomic Radius:

Across a Period (L to R): — Decreases. $Z_{eff}$ increases as electrons are added to the same shell, pulling them closer.
Down a Group (T to B): — Increases. New electron shells are added, increasing distance from nucleus.

Ionic Radius: Radius of an ion.

Cations ($M^+$): — Always smaller than parent atom ($M$). Loss of electrons, increased $Z_{eff}$. Example: $Na > Na^+$.
Anions ($X^-$): — Always larger than parent atom ($X$). Gain of electrons, increased electron-electron repulsion. Example: $Cl < Cl^-$.

Isoelectronic Species: Same number of electrons. Ionic radius is inversely proportional to nuclear charge (Z). Higher Z means smaller size. Example: For 10 electrons: $N^{3-} > O^{2-} > F^- > Na^+ > Mg^{2+} > Al^{3+}$.

Important Exceptions/Notes:

Transition Metals: — Irregular trend across period due to poor shielding by d-electrons, leading to relatively constant radii in the middle of the series.
Lanthanide Contraction: — Poor shielding by 4f electrons causes a smaller than expected increase in atomic/ionic radii after lanthanides, affecting 5d series elements.

Vyyuha Quick Recall

Cats Shrink, Ants Grow. Period Decreases, Group Increases. Iso-Nuclear Shrinks.