Dot & Cross Diagrams (AQA AS Chemistry): Revision Note
Exam code: 7404
Representing Dot & Cross Diagrams
Dot and cross diagrams are diagrams that show the arrangement of the outer-shell electrons in an ionic or covalent compound or element
The electrons are shown as dots and crosses
In a dot and cross diagram:
Only the outer electrons are shown
The charge of the ion is spread evenly which is shown by using brackets
The charge on each ion is written at the top right-hand corner
Ionic compounds
Ionic bonds are formed when metal atoms transfer electrons to a non-metal to form a positively charged and negatively charged ion
The atoms achieve a noble gas configuration
Dot-and-cross diagrams of ionic compounds in which one of the atoms transfers their valence electrons to the other
Covalent compounds
The atoms in covalent compounds will share their outer valence electrons to achieve a noble gas configuration
Dot-and-cross diagrams of covalent compounds in which the atoms share their valence electrons
Worked Example
Draw a dot and cross diagram for lithium nitride
Answer
Lithium is a Group 1 metal, so it loses its 1 electron from its outer shell
This forms a Li+ ion
Nitrogen is a Group 5 non-metal, so it needs to gain 3 electrons to complete its outer shell
This forms an N3- ion
Overall, three lithium atoms each donate an electron to one nitrogen atom, to form 3 x Li+ ions and 1 x N3-
The overall ionic compound formed is Li3N
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