Molecules and chemical formulas

Chemical formulas and their notation

All substances and molecules are depicted in writing using chemical formulas. They consist of chemical element symbols and numerical subscripts that indicate the exact number of atoms of each element (traditionally, the subscript "1" is omitted).

There is no strict universal rule for the sequence in which elements are written, but certain conventions have been established:

• Oxygen is most commonly written last.
• Hydrogen is placed first in inorganic acids (e.g., $H_2SO_4$), whereas in organic compounds it is conventionally written immediately after carbon.

Molecules themselves are classified into types depending on the number of atoms. Diatomic molecules can form simple homonuclear substances (oxygen $O_2$ and nitrogen $N_2$) or complex heteronuclear ones (carbon monoxide $CO$, hydrogen chloride $HCl$). Water, for example, is a triatomic molecule.

Proof of substance structure using water as an example

Conventional microscopy does not allow for the visualization of non-gigantic molecules such as $H_2O$. The primary method that proves the existence of molecules and precisely determines the arrangement of atoms in a solid is X-ray structural analysis (X-ray diffraction).

Analysis of the structure of solid water (ice) revealed two types of distances:

1. Intramolecular (100 pm): Two hydrogen atoms are chemically bonded to each oxygen atom, forming an intact molecule.
2. Intermolecular (176 pm): The distance to the atoms of a neighboring molecule is significantly greater, with no strong chemical bond between them.

It is this substantial difference in distances (100 pm versus 176 pm) that serves as irrefutable proof that water consists of discrete molecules rather than a continuous atomic lattice.

Furthermore, a chemical compound is fundamentally different from a mixture. A mixture contains different substances whose separation is a physical process. The decomposition of water (e.g., via electrolysis into 1 g of hydrogen and 8 g of oxygen out of every 9 g of water), on the other hand, is the destruction of the molecules themselves, which is a chemical process.

Spatial geometry of molecules

The spatial structure of various molecules varies greatly, which determines their properties:

• Water ($H_2O$): Has an angular structure.
• Carbon dioxide ($CO_2$): Unlike water, this is a linear molecule, with all three atoms lying on a single straight line.
• Benzene ($C_6H_6$): A planar molecule. The carbon atoms lie in a single plane and form a regular hexagon.
• Ammonia ($NH_3$): Resembles a trigonal pyramid, with a nitrogen atom at its apex and hydrogen atoms at the base corners.
• Ethane ($C_2H_6$): Consists of two $CH_3$ fragments, the key feature of which is the ability to rotate independently of each other around the $C-C$ bond axis.
• Glucose ($C_6H_{12}O_6$): Has a cyclic structure (five carbon atoms and one oxygen atom form a hexagon). This conformation is not planar but distorted (resembling a bent sheet of paper), with two opposite corners bent upward and downward.

Methods of graphical modeling

To visualize molecules, models are used in which atoms are represented by spherical particles and their sizes correspond to actual research data. It is worth keeping in mind that when depicting 3D lattices on a plane, certain distances may appear distorted due to perspective effects.

Main methods of depiction:

1. Space-filling model — clearly demonstrates the relative sizes of atoms.
2. Deformed-atom model — reflects the mutual overlapping of electron clouds upon the formation of a chemical bond (atomic deformation).
3. Ball-and-stick model — the radii of the spheres are artificially reduced, and lines indicate the presence of bonds, allowing for a better view of the spatial arrangement of elements.
4. Structural formula — the most conventional representation, containing only element symbols and dashes.

In some educational models, a specific color code is used: carbon is indicated in white (⚪), hydrogen in black (⚫), oxygen in grey (🔘), and nitrogen in dark grey (🌑).

Class and exam questions

What else to study on this topic

• Features of homonuclear and heteronuclear diatomic molecules.
• The mechanism of water molecule decomposition by electrolysis.
• Principles of conformational changes in the ethane molecule during rotation around the C-C bond.
• Visual distortions when constructing crystal lattice projections.

Other topics in the section "Chapter 1. Substances. Properties and Structure"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.