Writing chemical formula

What is the arms and link method?

The arms and link method is a simple way for students to learn how to write chemical formula. Each chemical species is drawn so that the number of arms matches its combining power or valency.

Metals are drawn with arms to the right and non-metals with arms to the left.

For example magnesium metal, chemical symbol Mg has a combining power of 2 and has two arms drawn to the right.

Chlorine is a non-metal. It has the chemical symbol of Cl and has a combining power of 1 and is drawn with one arm in the opposite direction.

A squiggly 'link' is drawn to link the arms of the different chemical groups. 

Simply keep adding chemical groups until all the arms match up!

Watch the video below.

The number of individual chemical species is then counted and the chemical formula quickly determined.

If there is more than one type of chemical in a compound the number is written as a subscript. There are two chlorine atoms so a small ₂ is used in the formula.

The chemical formula of magnesium chloride is MgCl₂

Using a simple Periodic Table, students are able to quickly relate the group number to the combining power or valency of an element. The combining power of polyatomic groups can also quickly be shown. See Chemical formula worksheets for International spelling (IUPAC).

This method has a big advantage of simplicity and can be taught to junior high or middle school students. They do not become confused with ionic charges or superscripts. These concepts can be introduced later on.

Using arms and links to write chemical formula

The main advantage of using the arms-link method is that students are not confused with ionic charges which can be introduced later. All students, from low ability to high achievers, can quickly learn to write chemical formula. Refer to Table 1 for specific examples.

Table 1. Determining Chemical Formula by the arms-link method

Element or polyatomic group	Element or polyatomic group	Compound	Chemical formula	Chemical name
			NaCl	Sodium chloride
			MgO	Magnesium oxide
			Na2O	Sodium oxide
			MgCl2	Magnesium chloride
			Mg(OH)2	Magnesium hydroxide
			Al2(SO4)3	Aluminum sulfate
			NH4NO3	Ammonium nitrate
			AlPO4	Aluminum phosphate
			MgCO3	Magnesium carbonate

Further questions

1. What is a polyatomic group?

A polyatomic group is a group of elements that are chemically bonded together. They act together as one.

Examples

Hydroxide group,

Carbonate, group,

Nitrate group,

2. When are brackets needed?

Brackets are only used in a chemical formula if the number of polyatomic groups is greater than one.

Examples

a) Sodium hydroxide, NaOH

The polyatomic group is the hydroxide, OH group.  Since it only appears once no brackets are needed.

b) Magnesium hydroxide, Mg(OH)₂

The polyatomic group is the hydroxide, OH group. It appears twice, so brackets are needed.

c) Magnesium nitride, Mg₃N₂

There are no polyatomic groups in magnesium nitride, so no brackets are needed.

Chemical Formula Worksheets

For more help download the Chemical Formula Worksheets