Nature of Bonding and Basic Properties
Molecules and crystals exist due to the physical attraction and bonding of microobjects—atoms. The interactions formed in this process are called chemical bonds. Although the true nature of these bonds lies in the complex structure of the atom, at a basic level they are studied through formal characteristics.
From the analysis of molecular composition, chemists have identified two key properties of valency:
- Directionality. Bonds between atoms are not random; they possess a strict spatial orientation. It is this directionality that dictates the final geometric parameters of the molecule and the relative arrangement of its parts.
- Saturation. The ability of an atom to form bonds has a strict limit. The value of valency is numerically equal to the number of bonds that a given atom has already formed or can form.
Historically, hydrogen ($H$) became the standard for calculations. Since no more than one atom of another element ever attaches to a hydrogen atom, it is considered strictly monovalent. Values for all other elements are calculated based on the number of attached (or substituted) hydrogen atoms.
Classic examples from structural formulas:
- In hydrogen chloride ($HCl$), chlorine forms one bond with hydrogen and is monovalent.
- In the water molecule ($H_2O$), oxygen forms two bonds and is divalent.
- In ammonia ($NH_3$), nitrogen holds three hydrogen atoms and is trivalent.
- In methane ($CH_4$), carbon is bonded to four hydrogen atoms and is tetravalent.
Search the Periodic Table
The Periodic Table not only systematizes elements but also allows for the prediction of their valency based on their position in specific columns (groups). This rule works thanks to the law of periodicity: elements in the same group have a similar atomic structure.
- Groups IA to IVA: The typical valency of an element precisely matches its group number.
- Groups VA to VIIIA: The value is calculated using a simple formula: 8 − group number.
This basic rule makes it possible to quickly predict the composition of a vast number of chemical compounds. However, for the most accurate understanding of processes, a deep study of the atomic structure itself is required.
Multiple Bonds and Carbon Structure
Atoms are capable of bonding not only with single bonds but also with multiple bonds (double or triple). When these are formed, the overall valency of the element remains the same as if it were reacting with hydrogen.
- Double Bonds: Impart additional stability to molecules. In carbon dioxide ($CO_2$) with the structure $O=C=O$, carbon exhibits a valency of IV, and oxygen exhibits II. A similar double bond is present in the simple oxygen molecule ($O=O$).
- Triple Bonds: Found, for example, in gaseous nitrogen ($N_2$). The triple bond ($N \equiv N$) makes this simple substance exceptionally chemically stable (unlike ammonia, where nitrogen is also trivalent, but the bonds are single).
- Limitation: It is believed that bond multiplicity cannot exceed three. For example, carbon is tetravalent, but the diatomic molecule $C_2$ with a quadruple bond does not exist under normal conditions.
Instead of forming $C_2$ molecules, carbon forms large-scale crystal structures—diamond and graphite. Diamond has a framework crystal lattice in which carbon atoms are connected by single covalent bonds and have a tetrahedral spatial environment.
Variable Valency and the Influence of the Crystal Lattice
Most chemical elements are capable of exhibiting variable valency—that is, changing the number of bonds formed depending on the substance they are reacting with. Often this depends on the electronegativity of the partner.
Examples of Variable Valency:
- Carbon ($C$): In carbon monoxide ($CO$) its valency is II, while in carbon dioxide ($CO_2$) it is IV.
- Sulfur ($S$): When combining with less electronegative hydrogen (hydrogen sulfide, $H_2S$), sulfur is divalent (II). However, in reactions with more electronegative oxygen, it can be tetravalent in sulfur dioxide ($SO_2$) or hexavalent in sulfur trioxide ($SO_3$).
Crystal Problem: The classical definition of valency as a strict number of bonds works impeccably for substances with a molecular structure (gases, liquids). But in substances with ionic or framework structures, this rule fails.
A striking example is table salt ($NaCl$). In the gaseous state, this is a molecule with a single bond (sodium is monovalent). However, in an ionic crystal, each sodium atom is surrounded by six chlorine atoms at once (has 6 neighbors). In such cases, the value of valency reflects the internal state of the atom rather than the physical number of bonds around it.
Briefly: answers to topical questions
What is the main difference between an element's valency and its oxidation state?
The main difference is that valency characterizes the ability of atoms to bond with each other and, in substances with a molecular structure, usually coincides with the number of chemical bonds formed by the atom. The oxidation state is a formal charge of an atom calculated under the assumption that all bonds in the molecule are ionic.
| Characteristic | Valence | Oxidation State |
|---|---|---|
| Definition | The ability of atoms to bond with one another; the valency value equals the number of chemical bonds formed by the atom | The formal charge of an atom assuming all bonds are ionic |
| Physical Meaning | In ionic and framework structures, it may denote a specific state of the atom rather than the actual number of bonds | The number of electrons partially or fully transferred to more electronegative atoms or accepted from less electronegative ones |
| Bonds Between Identical Atoms | Can be taken into account when determining the number of bonds of an atom | Do not affect the oxidation state |
How does the transition of an atom to an excited state affect its valency capabilities?
The transition of an atom to an excited state can expand its valency capabilities, provided the excitation energy is not excessively high. The valency of an atom is determined by the number of its unpaired electrons in the ground or excited state.
Upon excitation, electron pairing can be disrupted and an electron can transition to a free orbital: for example, for Group IV elements, the transition is described by the scheme $ns^2np^2 \rightarrow ns^1np^3$, resulting in 4 unpaired electrons and a valency of IV. In halogens, except for fluorine, the presence of free $d$-orbitals allows them to transition to excited states and exhibit higher valencies; for chlorine, valencies of III, V, and VII are indicated.
However, increasing valency via an excited state is not possible for all elements: nitrogen cannot increase its valency this way due to the absence of a $d$-subshell.
Which chemical elements, besides hydrogen, possess a strictly constant valency?
Besides hydrogen, sources indicate fluorine as an element with a strictly constant valency: it always exhibits a valency of I.
For all halogens, the monovalent state is most characteristic, but chlorine, bromine, and iodine in compounds with oxygen and fluorine can exhibit higher valencies due to the presence of free $d$-orbitals and transition to an excited state. Variable valency is characteristic of most elements.
How does the valence bond theory (VBT) explain the directionality of a covalent bond?
The directionality of a chemical bond is explained by the tendency of atoms toward maximum interaction of their electronic structures. An atom forms bonds strictly in the directions that provide the greatest overlap of electron clouds. This property determines the spatial arrangement of atoms and the geometric characteristics of molecules. To explain the structure of molecules whose bond angles differ significantly from straight angles, the concept of hybridization is used—the transformation of initial atomic orbitals into identical orbitals of an intermediate type that dictate a specific geometry (e.g., tetrahedral or linear).
For elements on the right side of the Periodic Table (groups VA–VIIIA), the "Octet" rule applies: valency = 8 − group number.
Class and exam questions
Why was hydrogen chosen as the standard for valence?
No more than one atom of another element is ever attached to a hydrogen atom. That is why it was chosen as the absolute unit — it is always monovalent.
Do quadruple chemical bonds exist?
No, chemical bonds with a multiplicity greater than three do not occur. Even tetravalent carbon does not form a $C_2$ molecule with a quadruple bond, but instead forms crystal lattices (diamond, graphite).
Is valence always equal to the number of chemical bonds?
This is true only for substances with a molecular structure. In ionic and framework crystals, valence refers to the state of the atom, while the actual bonds (neighbors) can be much greater.
What determines the variable valence of an element?
It often depends on the electronegativity of the partner element. For example, sulfur forms compounds with different valences with hydrogen ($H_2S$, valence II) and oxygen ($SO_3$, valence VI).
What else to study on this topic
- The influence of electronegativity on compound formation
- Spatial structure of molecules and bond directionality
- Features of framework and ionic crystal structures
- The relationship between valence and the electronic structure of an atom
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