Answer
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Work Step by Step
1. Evidence for putting hydrogen in Group 1A (1)
Hydrogen is sometimes placed in Group 1 because it has some similarities to alkali metals:
- Hydrogen has one electron in its outermost shell (1s¹), just like the alkali metals (ns¹).
- It can form H⁺ ions, similar to alkali metals forming M⁺ ions.
- Some hydrogen compounds resemble alkali metal compounds (e.g., NaH vs. H₂O reactions).
However: Hydrogen is not a metal and has unique properties, so it’s often placed separately in modern periodic tables.
Ways hydrogen is unlike a typical Group 1A element
Hydrogen differs from alkali metals in many ways:
Physical state:
- Hydrogen: Gas at room temp
- Alkali metals: Solid
Metallic character
- Hydrogen: Nonmetal
- Alkali metals: Metal
Density
- Hydrogen: Very low
- Alkali metals: Low to moderate, but much higher than H
Melting/boiling points
- Hydrogen: very low
- Alkali metals: Relatively low, but higher than H
Bonding
- Hydrogen: Covalent in H₂, can form ionic H⁻ in hydrides
- Alkali metals: Forms ionic compounds easily
Reactivity
- Hydrogen: Forms covalent compounds with nonmetals
- Alkali metals: Forms ionic compounds with nonmetals
3. Valence electron configuration for alkali metals
All alkali metals have one electron in their outermost s-orbital:
General configuration: ns¹
Examples:
Lithium: 1s² 2s¹
Sodium: 1s² 2s² 2p⁶ 3s¹
Potassium: [Ar] 4s¹
4. Common properties of alkali metals
- Very reactive, especially with water. Reactivity increases down the group.
- Soft metals, can be cut with a knife.
- Low density (Li, Na, K can float on water).
- Low melting and boiling points compared to most metals.
- Form ionic compounds (M⁺ ions).
- Shiny, metallic luster when freshly cut but tarnish quickly in air.
5. How pure alkali metals are prepared
Alkali metals are too reactive to be obtained by simple reduction with carbon. Common methods include:
- Electrolysis of molten salts (most common)
Example: NaCl → Na + ½Cl₂ (electrolysis of molten NaCl)
- Reduction of compounds with a stronger reducing agent (historically, for Li, using Li₂O with Mg).
6. Predicting formulas of alkali metal compounds
Alkali metals react to form 1:1 ionic compounds (M⁺) because they have one valence electron.
