Molecules

The trivial or common name of the chemical.

Trivial names are not recognised according to the rules of a formal system, such as IUPAC (International Union of Pure and Applied Chemistry) nomenclature.

The common name of many chemicals predate our modern understanding of atoms, molecules and their chemical bonds.

Provides details about how a molecule’s atoms are arranged in 3-dimensional space. Atoms are represented by coloured spheres and bonds are represented by sticks that join the atoms together.

In contrast, the space-filling model of a molecule displays the atoms at their full size relative to the distance between the centres of the atoms.

The atoms are drawn smaller than their actual size to make the angles and the number bonds between them visible

The atoms are traditionally coloured according to the CPK colour scheme. Carbon atoms are coloured black or grey, oxygen atoms red, hydrogen white, and so on. The CPK scheme is named after three scientists who pioneered the use of accurate molecular models; Robert Corey, Linus Pauling, and Walter Koltun.

An abstract representation of a molecule that uses letters and numbers.

Atoms are represented by their element symbols. The number of atoms of each kind is represented by a subscript numeral, or numerals, after the element symbol. The numeral is omitted when there is only one atom of an element in the molecule.

This modern form of chemical notation was invented by Jöns Jacob Berzelius in 1813 and modified by Justus von Liebig.

Berzelius used superscript numerals and wrote CO² and H²O for carbon dioxide and water. A decade later and for reasons unknown, von Liebig began using subscript numerals instead, which became the accepted notation so that all scientists write H₂O and CO₂ today.

Displays the atoms at their full size relative to the distance between their centres. The space-filling model therefore provides more accurate information about the surface, shape and size of the molecule.

When atoms form chemical bonds, they overlap. That means the number of bonds between any two atoms, and the angles they form are not easily discerned. It is therefore more difficult to distinguish single bonds from, double bonds and triple bonds in space-filling model.

The structural formula is composed of element connected with lines to show which atoms are connected to each other.

Single bonds are represented by a single line ( – ). Double bonds are represented by two parallel lines ( = ). Triple bonds are represented by three parallel lines ( ≡ ).

The structural formula provides more information about the structure of a molecule than the molecular formula, but no information about the exact angles between bonds or the 3-dimensional shape of the molecule.

A unique number assigned to by the Chemical Abstract Service (a division of the American Chemical Society).

A CAS number designates only one chemical substance.

CAS numbers have no chemical significance.

The CAS Registry contains more than 200 million substances and almost 70 million protein and DNA and RNA acid sequences.

CAS Numbers are assigned in sequential order when a new substance is identified for inclusion in the registry.

The United Nations and most of the world’s governments, including the Australian government, use CAS numbers to identify chemicals.

For example, the Australian Inventory of Industrial Chemicals uses CAS numbers to identify the 40,000 chemicals that are manufactured or imported (introduced) into Australia for industrial use.

CAS Number parts and check digit

CAS Numbers consist of three parts separated by hyphens. The third part is a single digit checksum, which is calculated using an algorithm performed using the digits of the first and second parts of the CAS number. Check digits are used for error detection in systems where numbers sometimes need to be entered into a computer manually.

The CAS Number check digit is calculated by taking the digit that immediately precedes it multiplied by 1, then adding the preceding digit multiplied by 2, then adding the preceding digit multiplied by 3, and so on, until the first digit of the CAS number is reached. The result of this operation is then used to compute the sum modulo 10. Modulo 10 means the remainder left over after the result is divided by 10.

For example, the CAS number of water is 7732‑18‑5. Therefore, the checksum digit for water is:

( 8 × 1 ) + ( 1 × 2 ) + ( 2 × 3 ) + ( 3 × 4 ) + ( 7 × 5 ) + ( 7 × 6 ) = 105

105 mod 10 = 5

The CAS number of trinitrotoluene (C₇H₅N₃O₆), better known as the explosive, TNT, is 118‑26‑7. Therefore, the checksum digit for TNT is:

( 6 × 1 ) + ( 9 × 2 ) + ( 8 × 3 ) + ( 1 × 4 ) + ( 1 × 5 ) = 57

57 mod 10 = 7

A four digit numbers assigned by the United Nations Committee of Experts on the Transport of Dangerous Goods.

The Australian Dangerous Goods Code (ADG) requires the UN number to be displayed on an Emergency Information Panel, along with the proper shipping name and appropriate HAZCHEM code.

The HAZCHEM codes displayed on emergency information panels are also known as the Dangerous Goods Emergency Action Code (EAC). This two or three character codes provides a short and simple directive to emergency responders of how to contain a chemical spill or emergency.

The Emergency Action Codes are published by the National Chemical Emergency Centre (NCEC) in the United Kingdom.

A HAZCHEM code consists of a single number (1 to 4) followed by either one or two letters, depending on the hazard. The first digit indicates the type of fire suppressant to be used and will be a number between one and four:

1. Coarse water spray
2. Fine water spray
3. Normal protein-based foam
4. Dry agent (no water must come in contact with the substance)

The second character is one of the capital letters, P, R, S, T, W, X, Y, or Z. Each letter denotes the type of personal protective equipment to be worn by responders and how the chemical should be diluted or contained.

The third character, if present, will be the letter E, with the meaning, “consider evacuation”.

The appearance, odour, melting point, boiling point and density of a chemical are important pieces of information for hazard identification and risk assessments.

Dangerous goods are divided into nine classes:

1. Explosives
2. Gases
3. Flammable liquids
4. Flammable solids
5. Oxidising substances and organic peroxides
6. Toxic substances
7. Radioactive materials
8. Corrosive substances
9. Miscellaneous dangerous or environmentally harmful goods

Each dangerous goods class has subclasses with specific colour coded, diamond shaped labels.

The United Nations created the GHS to classify chemicals and communicate risks according to the types of hazard they present.

Australia first introduced GHS Version 3 in 2017 and adopted GHS Version 7 in January 2023. Australian consumer goods may still display the appropriate Australian Dangerous Goods class label instead of the GHS pictogram.