Chemical Symbols, Formulas and Reactions

Chemical Symbols

A chemical symbol (sometimes known as an atomic symbol) is a code given to each element, mostly derived from the element’s name. Chemical symbols are typically one or two letters long.

If the chemical symbol of an element consists of only one letter, then that letter is capitalised.

  • For example, the chemical symbol for the element carbon is C
  • And for Hydrogen it’s H

However, if the chemical symbol has two letters, then the first letter is uppercase (capitalised) and the second letter is lowercase.

  • For example, the chemical symbol for the element Aluminium is Al
  • And for the element bromine, it’s Br

Keep in mind that some chemical symbols are derived from their Greek or Latin names. For example, the chemical symbol for the element ‘iron’ is Fe. This originates from the Latin word. ‘Ferrum‘.

Chemical Formulas

Chemical formulas show which atoms and how many atoms join up together to form a compound. When writing the formula of a molecule, we use the chemical symbols for the elements it contains.

For example, the molecule Aluminium oxide is written as Al2​O3​, containing two aluminium atoms and three oxygen atoms.

Using Numbers in Formulae

When a compound contains more than one atom of an element, we use numbers. This tells us how many atoms of an element there are in the compound. These numbers are written in subscripts, so they are smaller than the other letters and numbers in the formula.

  • For example, consider the molecule carbon dioxide. The ‘2‘ in CO2 means that there are two oxygen atoms in the molecule. As there is no number after the carbon atom, this means that there is only one carbon atom.

When using numbers in formulae, it’s important to place them as subscripts. For example, the formula for bromine oxide is written as BrO2, NOT BrO2 or BrO2.

Some formulas can be more complicated, meaning they represent molecules that contain multiple elements with more than one atom. For example, with sulphuric acid (H2SO4). In this molecule, there are two hydrogen atoms, so there is a ‘2‘, one sulphur atom, which is why there are no small numbers after sulphur. Also, there are four oxygen atoms, which explains the ‘4‘.

The formula for a given compound is always the same; that’s what makes a compound unique. For example, if carbon dioxide (CO2) had one oxygen atom instead of two, then it would be carbon monoxide (CO) instead.

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