Nature of hydrogen and its ion
The hydrogen atom has a relative electronegativity ($\chi$) of 2.2. This value is lower than that of the absolute majority of nonmetals (silicon is the only exception), but higher than that of most metals. In its compounds, hydrogen always exhibits a valency of I. Depending on its partner, it forms different types of substances:
- With nonmetals: molecular compounds (e.g., water $H_2O$, hydrogen chloride $HCl$, methane $CH_4$).
- With metals: salt-like hydrides (sodium hydride $NaH$, calcium hydride $CaH_2$).
- With noble gases: stable compounds do not exist.
The most electronegative elements (fluorine, oxygen, and nitrogen) draw electron density toward themselves, forming strongly polar bonds. As a result, the nucleus of the hydrogen atom—a proton ($p^+$)—can detach from its electron shell and transfer to the fluorine, oxygen, or nitrogen atom of another molecule. This proton transfer process lies at the very foundation of acid-base theory. As a result of this transfer, complex ions are formed, such as the hydronium cation ($H_3O^+$), which for simplicity is often referred to as the "hydrogen ion".
Difference between a proton and metal ions
It is important to understand the fundamental difference between a proton ($H^+$) and typical metal ions.
- Metal ions are particles of an atomic type. Even when losing electrons from their outer shell, they retain their inner electron shells and possess a well-defined, measurable radius.
- The hydrogen ion is exclusively a "naked" nucleus left after the loss of its single electron. It lacks an electron shell and is a point-sized subatomic particle. Calling it a "charged atom" in the classical sense is incorrect.
Due to the absence of electrons and its point-like size, the proton has the highest transfer rate between electronegative atoms compared to any other chemical process. The addition of a proton radically alters the electronic structure of a molecule or ion, sharply increasing its chemical reactivity. This is why it frequently acts as a potent catalyst, particularly in organic chemistry reactions.
Mechanism of hydrogen bonding
A hydrogen bond arises as a result of the incomplete transfer of a proton. The proton begins to interact with the electronegative atom of an adjacent molecule, forming a bond with it, while no complete rupture of its previous covalent bond (denoted by a solid line `—`) occurs. As a result, the hydrogen atom functions as a connecting "bridge" between two molecules. Unlike a covalent bond, a hydrogen bond is conventionally depicted in diagrams by a dashed line or dots: `····H`.
The main condition is that the bond forms only if hydrogen is already covalently bonded to one of the three most electronegative elements: nitrogen (N), oxygen (O), or fluorine (F).
Effect on physical properties
The presence or absence of hydrogen bonds dramatically alters the state of aggregation of a substance. This is most clearly seen when comparing water ($H_2O$) and hydrogen sulfide ($H_2S$).
In water, oxygen has a high electronegativity, so $H_2O$ molecules are tightly linked by hydrogen bonds into long chains and spatial networks. This association makes it very difficult for molecules to break away from each other and transition into a gaseous state.
In the hydrogen sulfide molecule, the sulfur atom has a lower electronegativity than N, O, or F. Under such conditions, hydrogen loses its unique mobility and behaves like an ordinary atom. $H_2S$ molecules are unable to form chains, which is why hydrogen sulfide remains a gas under normal conditions (as is quite expected for a substance with such a low molecular weight).
| Substance | $t_{boil}, ^\circ C$ | Reason for difference |
|---|---|---|
| $H_2O$ | $+100$ | Presence of hydrogen bonds (molecular association) |
| $H_2S$ | $-59,55$ | Absence of hydrogen bonds |
Biological significance
Although the energy of a single hydrogen bond is small compared to a covalent bond, their vast number ensures the incredible stability of biological macromolecules. In living organisms, it is the system of hydrogen bonds that rigidly fixes the secondary and tertiary structures of proteins (e.g., helices and pleated sheets). In addition, they specifically pair complementary nitrogenous bases in nucleic acid molecules (DNA and RNA), making reliable storage and error-free transmission of genetic information possible.
Briefly: answers to topical questions
How many hydrogen bonds form between adenine and thymine, and how many between guanine and cytosine in a DNA molecule?
In a DNA molecule, two hydrogen bonds form between the complementary nitrogenous bases adenine and thymine, and three hydrogen bonds form between guanine and cytosine.
- Adenine — Thymine — two hydrogen bonds are formed
- Guanine — Cytosine — three hydrogen bonds are formed
How does the formation of hydrogen bonds with water molecules affect the solubility of organic substances (e.g., alcohols and carbohydrates)?
The polarity of water molecules and their ability to form hydrogen bonds account for its high dissolving capacity.
To easily remember which elements hydrogen can form a hydrogen bond with, use the acronym FON (Fluorine, Oxygen, Nitrogen).
Class and exam questions
Why is water liquid at room temperature, while hydrogen sulfide is a gas?
In water, the oxygen atom forms hydrogen bonds that "stitch" molecules into chains, preventing them from evaporating (boiling point +100 °C). In hydrogen sulfide, sulfur is less electronegative, hydrogen bonds do not form, and the substance remains a gas.
How does a hydrogen ion fundamentally differ from sodium or potassium cations?
Metal ions have an electron shell and a specific atomic radius. A hydrogen ion (proton) is a "naked" nucleus without electrons, representing a point-sized subatomic particle.
Can an $H^+$ ion float freely in solution?
No, isolated protons do not exist in liquid or solid media. They always attach to molecules, forming complex particles—for example, interacting with water to yield the hydronium ion ($H_3O^+$).
What else to study on this topic
- Acid-base theory: the role of proton transfer
- Mechanisms of proton catalytic action in organic chemistry
- Chemical properties of metal hydrides and their interaction with water
- Structure and properties of the hydronium ion ($H_3O^+$)
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