Chemistry, The Numbers Game: Subscripts and Superscripts

Chemistry subscripts and superscripts.

Chemistry is all about the numbers. Copyright Image by Vincent Summers, all rights reserved.

Subscript and superscript can make all the difference when it comes to chemical formulas.

Molecules, compounds, and other chemical structures include more than one atom. Sometimes, there are multiples of one particular atom.

For instance, anhydrous aluminum chloride features one atom of aluminum joined to or combined with three atoms of chlorine. Its chemical formula reflects this: AlCl.

But – simply knowing how to use a number in this instance is not enough. It is essential to know the proper use of subscripts and superscripts.

Subscripts in Chemistry

Notice the number 3 is written as a subscript, or a number that is smaller than the other text, and below the normal text line, in the formula for anhydrous aluminum chloride above. The concept of a multiplicity of atoms is conveyed by this use of a subscript.




You can also write the formula for Sucrose or table sugar, using chemical symbols and subscript numbers. There are 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms in sucrose – the chemical formula for table sugar looks like this: C₁₂H₂₂O₁₁.

Sometimes parentheses are used to characterize a particular structure, as follows:

(CH)CCOOH

The above molecule is pivalic acid. We occasionally refer to this molecule as trimethylacetic acid, since, visually, the molecule contains 3 methyl groups CH. Written in its most elemental form, the formula for pivalic acid is CH₁₀O.

Another more complex structure – acetone, or dimethyl ketone – exemplifies the beneficial use of parentheses.

CH(CO)CH

How are the parentheses beneficial? In this instance, it is because the oxygen atom is not connected to either the leftmost or the rightmost carbon atom but only the middle carbon atom. Acetone’s chemical structure could also be written (CH)CO or (CH)C=O.

Superscripts in Chemistry

Atoms often occur as combinations of isotopes. Hydrogen, the simplest gaseous element, has one electron and one proton in all its atoms. However, a small percentage of hydrogen atoms also have a neutron in the nucleus. This form of hydrogen atom is, naturally, heavier than the hydrogen without a neutron. To distinguish them, a superscript is employed. The letter H with a left-justified superscript symbol 1, written ¹H, represents hydrogen containing one nucleon – that is one nuclear particle – a lone proton. On the other hand, the symbolism ²H represents the deuterium form of hydrogen, which contains 2 nucleons – one proton and one neutron. If there is a subscript below the superscript, it indicates the number of protons, only.

Sometimes a superscript is used in connection with atomic charge or ionization. Some atoms lose an electron or electrons and bear a positive charge, for example 2+. If instead there is an excess of electrons, the (+) symbol becomes a (–) symbol. A metallic barium ion can lose 2 electrons to become Ba²⁺.

Water of Crystallization

Sometimes compounds crystallize from water solution with some specific amount of water becoming part of the crystal structure. This water is called water of crystallization. One of the best-known examples is copper sulfate, CuSO. If prepared free of water, copper sulfate is almost white. Add water then crystallize it, however, and it becomes the pentahydrate. Since 5 molecules of water combine with each molecule of copper sulfate, the structural formula is written CuSO•5HO.

Up, Down, Left, or Right

To summarize, then, the location of the number next to an atom’s shorthand symbol is very important. The location indicates what that number refers to – number of molecules, number of atoms, number of nucleons, number of protons, and even the number of electrical charges an atom bears.

© Copyright 2015 Vincent Summers, All rights Reserved. Written For: Decoded Science

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    • Hi Meg!
      Deuterium water can be bought and will not kill you! If our bodies were composed entirely of deuterium then our bodies would become much much s l o w e r because of their larger mass. Good news: a couple ice cubes won’t hurt us and make for a good party trick. Bad news: pretty expensive.

  1. Maybe this is the place to ask a question I have held in my head for over 50 years. I seem to recall at school a teacher saying that if you were to water a plant with water consisting solely of deuterium it would die. But later, the teacher did not recall saying it, so maybe I dreamt it. Do you know whether deuterium only water is poisonous or unavailable? I know that water generally is likely to have a little deuterium and tritium in it but can you get water comprise ONLY of those forms?

    • While I can’t say I absolutely know the answer to your question, I would suggest the answer is, a plant could absorb and use pure deuterated water. It might affect the plant, but wouldn’t be fatal to it. The three forms of water can be separated, and, in fact, are for special (albeit costly) purposes.