Dot-and-Cross Diagrams and Lone Pairs
A dot-and-cross diagram shows where every outer electron sits once a bond has formed, including the pairs that are not bonding anything. Those lone pairs are the part of the picture most people leave out, and they decide what the molecule does next.
What a learner can do afterwards
- Draw a dot-and-cross diagram for a simple molecule and count bonding pairs and lone pairs separately
- Explain why a diagram with the right number of electrons can still be wrong if the pairs sit on the wrong atom
- Use the group number of an atom to predict how many bonds it usually forms
1 · Read
A dot-and-cross diagram places one mark for each outer electron around each symbol. Dots show one atom's electrons and crosses show the other atom's, so you can see who brought what. Count every mark once you finish.
Pairs that sit between two atoms are bonding pairs; they are the shared bond. Pairs that sit on a single atom are outer electrons that touch nothing else and are called lone pairs. Name the two kinds separately or you will miscount the picture.
Use the group number as a shortcut before you draw. Group 7 atoms like chlorine hold seven outer electrons and usually form one bond. Group 6 atoms like oxygen usually form two bonds, and group 5 atoms like nitrogen usually form three. Check your diagram against that number.
The common error is a diagram with the right total but pairs on the wrong atom. A second common error is leaving lone pairs out of the picture. Lone pairs decide what the molecule does next, so draw them where they truly sit.
Count every electron, place every pair, and check bonds against the group number.
More to read
Learn and try: Lewis diagrams and the octet rule
Look at the main group elements in the first three rows of the periodic table, and draw each one with dots around its symbol, one dot for each outer electron. This is called a Lewis diagram. Now the patterns jump out. As you move across a row, each element has one more dot than the one before it. When the row ends, the pattern starts again on the next row. As you move down a group, every element has the same number of dots, because elements in the same group have the same number of outer electrons. This is why group number is such a handy shortcut. Group 1 atoms have one outer electron, group 7 atoms have seven, and group 0 (the noble gases) already have a full eight. An atom usually bonds however many times it needs to reach eight, so a group 7 atom like chlorine usually forms one bond, a group 6 atom like oxygen usually forms two, and a group 5 atom like nitrogen usually forms three. Use that shortcut to check your dot-and-cross diagrams before you even start drawing.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.