Molecular and non-molecular structure
For a long time, the molecule was considered the universal structural unit of any substance. However, X-ray diffraction analysis has proven that many compounds, especially multi-element ones, have no discrete molecules at all in the solid state.
By type of structure, all substances can be divided into two large groups:
- Molecular structures. They consist of clearly distinguishable, isolated molecules. A typical example is water ($H_2O$).
- Non-molecular structures (network and ionic). In these, atoms form a continuous spatial network. Isolating an individual molecule in such a crystal is impossible. Examples: quartz ($SiO_2$), table salt ($NaCl$).
Examples of non-molecular structures
Let us examine the structure of typical non-molecular crystals:
Silicon dioxide (quartz, $SiO_2$) There are no separate $SiO_2$ molecules here. The structure is a giant single molecule—a continuous network of chemical bonds. Each silicon atom ($Si$) is symmetrically surrounded by four oxygen atoms ($O$), and each oxygen by two silicon atoms. All distances between neighboring atoms are completely identical.
Sodium chloride (table salt, $NaCl$) Molecules are also absent in the salt crystal. Each sodium atom ($Na$) is equidistant from six surrounding chlorine atoms ($Cl$), and each chlorine from six sodium atoms. The structure is a perfect cubic lattice.
Formula units and structural formulas
Since there are no real molecules in non-molecular substances, their chemical formulas reflect only the elementary composition—the minimum possible ratio of atoms.
Such conventional groups of atoms are called formula units (or formula units). For example, in quartz, there are four "halves" of $O$ atoms from neighboring bonds per one $Si$ atom, which totals two $O$ atoms. Hence the formula is $SiO_2$.
Important rule: it is incorrect to use classical structural formulas with dashes (e.g., `Na—Cl`) for non-molecular crystals. In solid salt, a sodium atom is bonded to six neighbors at once. The notation `Na—Cl` is valid only for the gaseous state of $NaCl$ (at temperatures above $1490\ ^\circ\text{C}$), where the salt exists as ion pairs.
How to distinguish by physical properties?
The type of structure is easily determined by the substance's properties under normal conditions:
- Substances with a molecular structure are usually gases, liquids, or volatile solids. A characteristic feature is the ability to undergo sublimation (transition from the solid state directly to the gaseous state, bypassing the liquid state). Examples: water (ice evaporates in frost), naphthalene (volatility is indicated by a strong odor).
- Substances with a non-molecular structure are always solid, non-volatile crystals with high melting points (salts, metal oxides).
Briefly: answers to topical questions
What are the crystal-chemical differences between ionic and atomic crystal lattices?
The crystal-chemical differences between ionic and atomic lattices lie in the nature of chemical bonds and the type of structural units.
| Characteristic | Ionic lattice | Atomic lattice |
|---|---|---|
| Nature of the bond | Electrostatic attraction of cations and anions | Continuous network of covalent bonds |
| Structural units | Formula units | Atoms |
| Examples | Sodium chloride (NaCl) | Silicon dioxide (SiO2), diamond |
What types of chemical bonding predominate in molecular and non-molecular compounds?
Covalent bonds predominate in molecular compounds, while ionic or network covalent bonds predominate in non-molecular ones.
- Molecular compounds are formed via covalent bonds (formation of a shared electron pair). Ion interaction with charge redistribution also leads to the formation of molecules rather than an ionic lattice.
- Non-molecular compounds are represented by ionic and atomic structures. Ionic crystals are dominated by ionic bonds arising from a large difference in electronegativities. In atomic crystals, atoms are bound by a continuous network of polar covalent bonds.
Remembering the formula unit is simple: it is not a real molecule, but merely the "minimum proportion" of the recipe (like 1:1 for NaCl or 1:2 for SiO2).
Class and exam questions
Why is the formula of quartz SiO2 if molecules do not exist?
The formula SiO2 is a formula unit reflecting only the ratio of elements in the crystal (there are two oxygen atoms per silicon atom in the shared bond network).
Is it true that the structural formula Na—Cl is always incorrect?
It is erroneous for solid table salt, where sodium is bonded to six chlorine atoms. But it is correct for the gaseous state of salt at extremely high temperatures.
What is sublimation and for which substances is it characteristic?
Sublimation is the direct transition of a substance from the solid phase to the gaseous phase. This property is characteristic exclusively of substances with a molecular structure (ice, naphthalene).
What else to study on this topic
- Crystal lattice: parameters and unit cell
- Types of structural units: atoms, molecules, formula units
- Behavior of molecular and non-molecular substances during phase transitions
- X-ray diffraction analysis in studying crystal structures
- Features of atomic crystal lattices (diamond, metals)
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Other topics in the section "Chapter 1. Substances. Properties and Structure"
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