Basic chemistry help for students and teachers

 

Interactive Chemistry Worksheets for Students

 

Periodic table

Naming compounds

Basic formula

Simple compounds

Ionic compounds 1

Ionic compounds 2

Chemical suffixes

Old chemical names

Hydrocarbons - Alkanes

Alkanes, alkenes & alkynes

Alkanols to alkanoic acids

 

Common compounds

Common formula quiz

Acids, bases and salts

Covalent compounds

Printable worksheets
 

 

 

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.

magnesium ion

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.

chloride ion

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.

magnesium chloride

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
sodium ion chloride ion sodium chloride

NaCl

Sodium chloride

magnesium ion oxide ion magnesium chloride

MgO

Magnesium oxide

sodium ion

sodium ion

 

oxide ion

sodium oxide

 

Na2O

 

Sodium oxide

 

manesium ion

chloride ion

chloride ion

magnesium chloride

 

MgCl2

 

Magnesium chloride

 

magensium ion

hydroxide ion

hydroxide ion

magnesium hydroxide

 

Mg(OH)2

 

Magnesium hydroxide

 

aluminum ion

aluminum ion

sulfate ion

sulfate ion

sulfate ion

 

aluminum sulfate

 

 

Al2(SO4)3

 

 

Aluminum sulfate

ammonium ion nitrate ion ammonium nitrate

NH4NO3

Ammonium nitrate

aluminum ion

phosphate ion aluminum phosphate

AlPO4

Aluminum phosphate

magnesium ion carbonate ion magnesium carbonate

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, hydroxide ion

Carbonate, group,carbonate ion

Nitrate group,nitrate ion

 

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