Chemistry · Lesson

Chemical databases, molecular representations and identifiers

Read chemical graphs and records with explicit isotope, charge, stereochemical and provenance distinctions.

By the end, you can

Distinguish formula equality from graph identity.
Count a specified isotope inventory without inventing a spatial configuration.
Include selected protonation states and counterions in the material account.
Compare property records using their units, conditions and original sources.

A chemical record needs a defined entity

A name, formula, connectivity graph, stereochemical description and three-dimensional coordinates supply different information. Specify isotopes, charge, protonation, stereochemistry and other distinctions that matter for the question. A formula alone can correspond to several structures.

A database record can represent a defined species, a material or another curated grouping. Check its identity and provenance before treating its properties as properties of a particular sample or chemical state.

A representation should retain the distinctions needed by the question

An original comparison uses N-ethylbutan-1-amine and triethylamine. Their neutral formulas and electron counts agree, but their nitrogen attachment counts differ. Listing the heavy-atom edges makes that difference testable without relying on a drawing or a generated identifier. A string can carry structural information that the formula omits.

Isotopic identity adds another layer. A tetrahedral carbon attached to ¹H and ²H has distinguishable substituents even when an isotope-blind formula writes both as H. Counting protons and neutrons establishes the isotope inventory; it cannot recover a missing spatial arrangement. Preserve an unspecified configuration as unspecified.

A mass number counts nucleons. Exact isotopic masses additionally reflect the physical masses and binding-energy account. Do not treat the integer sum in the teaching case as an exact molecular mass or a measured mass-spectrometric peak.

A molecular notation encodes selected structural information

A connectivity representation describes atoms and bonds through a stated convention. It can include charge, isotopes and stereochemical information, but omitting such details does not prove they are chemically irrelevant. A structural string and a coordinate file have different roles.

Different valid strings can encode the same graph. A canonicalization procedure chooses a representation under its own rules; the word canonical alone does not guarantee identical strings across every implementation or version.

A chemical record and a bulk inventory have related but different roles

The selected imidazole and imidazolium forms below share the same heavy ring and electron count, while their hydrogen counts and charges differ. Adding counterions changes the full material inventory. A representation should state the selected tautomer and protonation site when those matter. The stipulated mixture has no inferred pH.

Identifier systems can normalize or layer molecular information under documented conventions. A matching key provides evidence of a match within that convention; it does not certify the observed form in an unknown sample. The original entity record and its interpretation should remain available.

For property records, first ask which quantity was reported, in which units and conditions, and where the value originated. A converted duplicate remains one origin. A calculated value and a warmer-condition measurement can be useful comparisons without forming one set of independent measurements. Agreement alone does not justify pooling or remove systematic error.

What the invented property rows actually represent

Record	Origin and kind	Temperature / K	Original D	D / m² s⁻¹
R1	run A; observation-style	298.15	2.40×10⁻¹⁰ m² s⁻¹	2.40×10⁻¹⁰
R2	run A; converted duplicate	298.15	2.40×10⁻⁶ cm² s⁻¹	2.40×10⁻¹⁰
R3	model B; calculated	298.15	2.20×10⁻¹⁰ m² s⁻¹	2.20×10⁻¹⁰
R4	run C; observation-style	310.15	2.90×10⁻¹⁰ m² s⁻¹	2.90×10⁻¹⁰

All values are invented. The two run-A rows share one origin; the model row is calculated and run C has a different temperature. This table is a provenance account, not a pooled estimate.

An identifier has a defined normalization scope

A chemical identifier can distinguish selected layers of molecular information under specified conventions. Standardization may normalize representations or group forms for a particular purpose. State what distinctions remain represented and which have been merged.

A matching identifier is evidence of a match within that convention. It does not prove equal purity, physical form, measurement conditions or every biological property. A hash or lookup key also does not supply the full scientific context of a record.

A property value needs its origin and conditions

Measured, calculated, estimated and curated values are different kinds of evidence. Temperature, pressure, solvent, chemical state, method and uncertainty can affect a property. A database’s hosting institution does not make every imported value an independent measurement.

Preserve the original record, citation, units and any transformations applied. The original representation cases below use explicitly defined graphs or symbolic entities, rather than an automatically verified identity for an unknown sample.

Worked examples

One formula can describe different graphs

Compare neutral N-ethylbutan-1-amine, represented here by CCCCNCC, and triethylamine, CCN(CC)CC. All bonds in these acyclic heavy-atom graphs are single bonds. Do their equal formulas establish equal connectivity?

Use the literal illustrative strings CCCCNCC and CCN(CC)CC. The first has a four-carbon chain and a two-carbon chain attached to N; the second has three two-carbon chains attached to N. Ordinary neutral carbon valence four and amine nitrogen valence three give C₆H₁₅N in each case.
Both have seven heavy atoms and six heavy-atom edges. Including H, each has 22 atoms and 6×6 + 15×1 + 7 = 58 electrons. The first graph has 14 C-bound H and one N-bound H; the second has 15 C-bound H and no N-bound H.
The sorted heavy-atom degree lists are [1,1,2,2,2,2,2] and [1,1,1,2,2,2,3]. A graph isomorphism preserves degrees, so these graphs cannot be isomorphic. Nitrogen has heavy degree two versus three.
These are illustrative valid structural strings, without a claim that a canonicalization program generated them. Formula and electron count are useful checks but do not identify connectivity, physical sample or measured property.

Same formula and electron inventory; distinct explicitly defined molecular graphs.

An isotope inventory still leaves a configuration unspecified

Assign a neutral tetrahedral ¹²C with four single bonds to ¹H, ²H, ¹⁹F and ³⁵Cl. Find its isotope inventory and identify what remains unspecified.

Proton numbers are 6,1,1,9,17, totaling 34. Neutrality gives 34 electrons. Neutrons are 6,0,1,10,18, totaling 35. There are five atoms and four bonds.
The integer nucleon sum is 12 + 1 + 2 + 19 + 35 = 69. This is a count, not an exact isotopic mass. The isotope-blind formula CH₂ClF hides the distinction between the two hydrogen isotopes.
The four attached groups are isotopically distinct, allowing two tetrahedral configurations. Connectivity alone assigns neither configuration nor an R/S descriptor. A spatial arrangement and the applicable stereochemical convention are needed.

34 electrons and 35 neutrons; integer nucleon sum 69; isotope and spatial identity are separate specifications.

Counterions belong in the material inventory

Stipulate 1.20 mmol neutral 1H-imidazole C₃H₄N₂, 0.800 mmol 1H-imidazol-3-ium C₃H₅N₂⁺ and 0.800 mmol Cl⁻. Use the selected imidazole tautomer [nH]1ccnc1, adding a proton at its other, pyridine-like nitrogen for the cation. What inventory follows?

The neutral ring has 3×6 + 4 + 2×7 = 36 electrons. The cation has 3×6 + 5 + 2×7 − 1 = 36. Cl⁻ has 17 + 1 = 18. Equal ring-species electron counts do not make their formulas or charges equal.
There are 2.00 mmol ring entities. Atom amounts are C 6.00 mmol, H 1.20×4 + 0.800×5 = 8.80 mmol, N 4.00 mmol and Cl 0.800 mmol.
Net charge amount is +0.800 − 0.800 = 0 mmol elementary charges. Electron amount is 2.00×36 + 0.800×18 = 86.4 mmol electrons.
These are stipulated amounts, without a pH or equilibrium calculation. An identifier may normalize tautomer or proton information under its convention; inspect the relevant layers rather than reading a charged formula from one layer alone.

2.00 mmol ring entities, net charge zero and 86.4 mmol electrons across the declared ring species and counterions.

Four records do not imply four measurements

Four invented diffusion-coefficient records concern the same selected species and matrix. R1 cites run A at 298.15 K: 2.40×10⁻¹⁰ m² s⁻¹. R2 cites the same run: 2.40×10⁻⁶ cm² s⁻¹. R3 is a calculated model B value at 298.15 K: 2.20×10⁻¹⁰ m² s⁻¹. R4 cites run C at 310.15 K: 2.90×10⁻¹⁰ m² s⁻¹. Can all four be averaged as independent same-condition measurements?

Since 1 cm² = 10⁻⁴ m², R2 converts to 2.40×10⁻¹⁰ m² s⁻¹, equal to R1. They repeat one origin, not two independent observations.
The four rows represent three origins: run A, model B and run C. R3 is calculated; R4 has a different temperature. Neither their independence nor a common measurement distribution is supplied.
Keep the original record, origin, conditions and unit conversion. An uncertainty and sampling model would be needed for meaningful pooling; arithmetic agreement cannot supply one.

Do not pool the four rows as four independent measurements at one temperature.

Try it yourself

Add one H+ formally at nitrogen in each graph. Find each formula, charge, electron count and N-bound H count.
Show the answer and reasoning
Both become C₆H₁₆N⁺ with 58 electrons; N-bound H counts are two and one.
A proton adds a hydrogen nucleus and positive charge, not an electron; the heavy-atom graph is retained.
Replace only 35Cl by 37Cl. Which inventories change?
Show the answer and reasoning
Electrons remain 34; neutrons become 37 and integer nucleon sum 71.
Two additional neutrons change the isotope inventory; nuclear charges, electron count and connectivity remain the same.
Keep total ring entities at 2.00 mmol but stipulate cation fraction 0.650 and one chloride per cation. Recalculate amounts, H atoms and electrons.
Show the answer and reasoning
Neutral/cation/chloride amounts 0.700/1.30/1.30 mmol; H atoms 9.30 mmol; electrons 95.4 mmol; net charge zero.
The assigned cation amount is 2.00 * 0.650; neutral is the remainder. Counterions change the total electron and atom inventory even though ring-species electron counts agree.
If R2 instead says 2.40e-10 cm2 s-1 while still citing run-A, convert it and compare with R1.
Show the answer and reasoning
2.40×10⁻¹⁴ m² s⁻¹, or 10⁻⁴ of R1; flag a conflict against the shared origin.
The squared-length conversion multiplies by 1e-4.

What to carry forward

A molecular formula does not uniquely determine connectivity.
Isotopes, charge and configuration are distinct parts of entity identity.
Counterions belong to the specified material inventory.
Preserve property origins and conditions before combining values.

References

Teaching resources and reference material supporting this lesson’s explanations and examples.