Position of Hydrogen in Periodic Table — Revision Notes

⚡ 30-Second Revision

Electronic Config: — $1s^1$
Similar to Group 1 (Alkali Metals): — One valence electron, forms $H^+$, valency 1.
Dissimilar to Group 1: — Non-metal, diatomic ($H_2$), high Ionization Enthalpy ($1312,\text{kJ/mol}$), forms $H^-$.
Similar to Group 17 (Halogens): — Needs one electron for duplet, forms $H^-$, non-metal, diatomic ($H_2$).
Dissimilar to Group 17: — Lower Electron Affinity, lower Electronegativity, no lone pairs.
Unique: — Forms both $H^+$ and $H^-$. Anomalous position.

2-Minute Revision

Hydrogen's position in the periodic table is unique due to its $1s^1$ electronic configuration. It exhibits a dual nature, sharing properties with both Group 1 alkali metals and Group 17 halogens. Like alkali metals, it has one valence electron and can form a unipositive ion ($H^+$).

However, it differs significantly by being a non-metal, existing as a diatomic gas ($H_2$), and having a much higher ionization enthalpy. Conversely, hydrogen resembles halogens by needing one electron to complete its duplet, forming a uninegative ion ($H^-$), and being a non-metallic diatomic molecule.

Yet, it's less electronegative and has lower electron affinity than halogens. This ability to both lose and gain an electron, coupled with its distinct physical properties, makes it an 'anomalous' element.

For NEET, remember the specific similarities and dissimilarities with both groups, especially the high ionization enthalpy and the ability to form both $H^+$ and $H^-$ ions.

5-Minute Revision

Hydrogen, with its atomic number 1 and $1s^1$ electronic configuration, occupies a truly unique spot in the periodic table. This uniqueness stems from its ability to mimic both Group 1 (alkali metals) and Group 17 (halogens) elements, while also possessing distinct characteristics.

Similarities with Alkali Metals: Both hydrogen and alkali metals have one electron in their outermost shell and tend to lose it to form a unipositive ion (e.g., $H^+$ and $Na^+$). They both exhibit a valency of 1 and combine with electronegative elements. For example, hydrogen forms $HCl$, similar to how sodium forms $NaCl$.

Dissimilarities with Alkali Metals: Crucially, hydrogen is a non-metal, existing as a diatomic gas ($H_2$), whereas alkali metals are reactive solids. Hydrogen's ionization enthalpy ($1312,\text{kJ/mol}$) is significantly higher than that of alkali metals, meaning it's much harder to remove its electron. Unlike alkali metals, hydrogen can also gain an electron to form a hydride ion ($H^-$).

Similarities with Halogens: Hydrogen needs one electron to complete its duplet ($1s^2$), much like halogens need one electron for their octet. Both readily form uninegative ions ($H^-$ and $Cl^-$) and exist as diatomic molecules ($H_2$ and $Cl_2$). Both are non-metals and combine with electropositive elements (e.g., $NaH$ and $NaCl$).

Dissimilarities with Halogens: Hydrogen is less electronegative and has a lower electron affinity than halogens. It also lacks the characteristic lone pairs of electrons found in halogens.

Key Takeaway for NEET: The ability of hydrogen to form both $H^+$ and $H^-$ ions, its high ionization enthalpy, and its non-metallic diatomic nature are the most critical points. Understand *why* these properties lead to its anomalous position. Questions often test these specific comparative points.

Prelims Revision Notes

Hydrogen (H) - Atomic Number 1, Electronic Configuration $1s^1$.

Similarities with Group 1 (Alkali Metals):

Valence Electrons: — Both have one valence electron ($1s^1$ for H, $ns^1$ for alkali metals).
Ion Formation: — Tendency to lose one electron to form a unipositive ion ($H^+$ or $M^+$).
Valency: — Exhibits a valency of 1.
Combination: — Combines with electronegative elements (e.g., $HCl$, $H_2O$, $NaCl$, $Na_2O$).
Reducing Agent: — Both act as reducing agents.

Dissimilarities with Group 1 (Alkali Metals):

Nature: — Hydrogen is a non-metal; alkali metals are metals.
Physical State: — Hydrogen is a diatomic gas ($H_2$); alkali metals are monatomic solids.
Ionization Enthalpy: — Hydrogen has very high IE ($1312,\text{kJ/mol}$); alkali metals have very low IE.
Ion Formation: — Hydrogen can form $H^-$ (hydride ion); alkali metals do not.
Oxides: — Hydrogen forms neutral oxide ($H_2O$); alkali metals form basic oxides.

Similarities with Group 17 (Halogens):

Electron Deficiency: — Both need one electron to achieve stable configuration (duplet for H, octet for halogens).
Ion Formation: — Tendency to gain one electron to form a uninegative ion ($H^-$ or $X^-$).
Molecular State: — Both exist as diatomic molecules ($H_2$ or $X_2$).
Nature: — Both are non-metals.
Combination: — Combine with electropositive elements (e.g., $NaH$, $CaH_2$, $NaCl$, $CaCl_2$).
Ionization Enthalpy: — Both have relatively high ionization enthalpies.

Dissimilarities with Group 17 (Halogens):

Electron Affinity: — Hydrogen has lower electron affinity than halogens.
Electronegativity: — Hydrogen is less electronegative than halogens.
Lone Pairs: — Hydrogen has no lone pairs; halogens have three lone pairs.
Oxidation States: — Halogens show multiple oxidation states; hydrogen primarily $+1, -1$.

Conclusion: Hydrogen's unique properties lead to its anomalous position. It's often placed separately or at the top of Group 1, acknowledging its distinct character. Focus on the reasons for these similarities and dissimilarities for NEET.

Vyyuha Quick Recall

Hydrogen's Position is Always Debated: Always Loses Electrons (like Group 1), Always Gains Electrons (like Group 17), but Distinctly High Ionization Enthalpy.