Position in the system of chemical sciences
The study of hydrogen opens the course of inorganic chemistry, a branch that explores all chemical elements and their compounds. Organic chemistry is an exception, as it is singled out into a separate field and examines substances with C–C and C–H bonds. This isolation is due to the colossal diversity of organic molecules, their specific physicochemical properties, and their vital biological role.
Historically (at the beginning of the 19th century), the division into organic and inorganic chemistry seemed strictly justified:
- Inorganic substances were considered predominantly solid, refractory, and non-flammable. Ionic and framework structures are typical for them.
- Organic compounds were described as gaseous or liquid, thermally unstable, easily flammable substances with a molecular structure.
Today, this division is recognized as conventional. The boundaries between the sections have blurred: many inorganic substances with a molecular structure and organic ones with an ionic structure have been discovered. In addition, any chemical element can form bonds with carbon, giving rise to an extensive class of organoelement and organometallic compounds.
Abundance in nature
Hydrogen is the primary chemical element both in its origin after the Big Bang and in its position in Mendeleev's periodic table.
- In the Universe, it is the absolute record holder. In terms of the number of atoms, it accounts for about 90% (another 10% is helium, and only 1% consists of all other elements). By mass, hydrogen makes up approximately 70% of cosmic matter.
- On Earth, the situation is different. During the formation of the planet, upon the compression and condensation of the initial gas-dust cloud, light gases were largely lost. Therefore, the mass fraction of hydrogen on Earth is less than 1% (specifically, in the Earth's crust and hydrosphere it is about 0.7%, whereas oxygen constitutes 49.2%). However, due to the extremely small mass of the atom, hydrogen accounts for a solid 17% in terms of particle count.
Atomic structure
The hydrogen atom has the simplest structure. It is a mobile system consisting of just two subatomic particles:
- One positively charged proton ($p^+$) in the nucleus.
- One negatively charged electron ($e^-$).
Such composition is characteristic of the main natural isotope of hydrogen — protium, which accounts for 99.98% of the mass of all natural hydrogen.
Electronic structure: The atom has one electron and one valence orbital.
- Electronic formula: $1s^1$.
- Energy characteristic: the energy of an electron located in the $1s$ orbital is $-1312$ kJ/mol.
Paradoxes of placement in the Periodic Table
Unlike helium, which is unambiguously classified among the noble gases, the placement of hydrogen in Mendeleev's table causes controversy. Its properties have a dual nature:
- Similarity with alkali metals (Group IA): Like lithium, sodium, or potassium, hydrogen has exactly one electron in its outer energy level.
- Similarity with halogens (Group VIIA): Like fluorine or chlorine, hydrogen lacks just one electron to complete its outer level. In addition, it forms diatomic molecules ($H_2$), which is typical for halogens ($F_2, Cl_2$), but does not occur in alkali metals.
- Similarity with carbon (Group IVA): The outer energy level is filled by exactly half (1 electron out of 2 possible for hydrogen, just like 4 out of 8 possible for carbon).
Due to this combination of traits, hydrogen cannot be definitively assigned to a single group. It is logical not to divide the hydrogen cell into parts, as this emphasizes its specificity. Notably, under extreme pressure (greater than $10^8$ kPa), hydrogen is capable of transitioning into a metallic state, which confirms its affinity with Group IA elements.
Briefly: answers to topical questions
Which isotopes of hydrogen exist in nature, and what is the composition of their nuclei?
There are three main isotopes of hydrogen in nature, which differ in the number of neutrons in the nucleus.
- Protium (${}^1_1\text{H}$) — the nucleus consists of one proton, with no neutrons present.
- Deuterium (${}^2_1\text{H}$) — the nucleus contains one proton and one neutron.
- Tritium (${}^3_1\text{H}$) — the nucleus includes one proton and two neutrons.
Protium is the most abundant natural isotope, accounting for 99.98% by mass.
What types of chemical bonds does hydrogen form in bioorganic compounds?
In bioorganic compounds, hydrogen forms covalent and hydrogen bonds.
- Covalent bond — hydrogen is always monovalent and forms a single bond with other atoms.
- Hydrogen bond — occurs between a positively charged hydrogen atom and electronegative atoms, such as nitrogen or oxygen.
Hydrogen bonds are involved in the interaction of drugs with receptors.
What are the physical properties of molecular hydrogen under standard conditions?
Under normal conditions, molecular hydrogen is the lightest colorless and odorless gas.
- Density — is $0.0893$ g/L.
- Solubility — has low solubility in water ($K_v = 0.0221$ at $0\ ^\circ C$).
- Boiling point — has one of the lowest boiling points, equal to $21.2\ K$.
What is a hydrogen bond and what is its role in the structure of biological macromolecules?
A hydrogen bond is an interaction between a hydrogen atom of one polar group and an electronegative atom, in which hydrogen acts as a "bridge" between two molecules or parts of a molecule.
In proteins, hydrogen bonds are the main factor in stabilizing the secondary structure — the helical or pleated conformation of the polypeptide chain. The bond forms between the oxygen atom of one peptide group and the hydrogen atom of another peptide group.
To memorize the position of hydrogen, imagine it as a "chameleon": it has 1 electron (like Group IA alkali metals), needs 1 electron to reach the ideal (like Group VIIA halogens), and its shell is exactly 50% filled (like Group IVA carbon).
Class and exam questions
What is the atomic composition of the main hydrogen isotope?
The main isotope (protium) consists of one proton in the nucleus and one electron in the 1s orbital. It accounts for 99.98% by mass.
Why is hydrogen 90% of the universe by atom count, but only 17% on Earth?
During the formation of Earth from the gas-dust cloud, a significant loss of light gases occurred, which is why little hydrogen remains on the planet.
Can hydrogen exhibit metallic properties?
Yes, at colossal pressure (greater than $10^8$ kPa), hydrogen transitions into a state known as metallic hydrogen.
What else to study on this topic
- Criteria for the classification of chemical sciences and the history of their development
- Specifics and properties of organoelement compounds
- Physicochemical differences between hydrogen isotopes
- Mechanisms of gas transition into a metallic state under ultrahigh pressure
- Comparative analysis of the properties of hydrogen and halogens
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Other topics in "Chapter 8. Hydrogen"
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