GCSE · Chemistry · Edexcel · Spec 1CH0

Dot-and-cross diagrams of small covalent molecules

Two atoms, one shared pair, and one strict rule about who puts what in the middle. Learn to keep score and you can check any diagram.

Read it before you draw it

One, two and three shared pairs, side by side
Hydrogen, H₂ — 1 shared pairHHCarbon dioxide, CO₂ — 2 shared pairs in each overlapOCONitrogen, N₂ — 3 shared pairsNN● dot = one atom's electron ✕ cross = the other atom's electron

Showing 2 layers: Atoms and shells, Electrons

Explore

Tap a chip, or tap the diagram itself, to light up a part.

Pick a part of each diagram to see what every dot and cross means.

Your turn to draw

Water first, then carbon dioxide

Build dot and cross diagrams for water, H₂O, and then carbon dioxide, CO₂. Some steps are missing, so choose each one.

  1. Water: oxygen is in Group 6, so it has 6 outer-shell electrons, and each hydrogen has 1. Draw oxygen's as dots and each hydrogen's as a cross.The group number is your starting count.
  2. missing step
Which line is step 2?

Spot the patterns

Seven molecules, four yes/no questions

For each molecule, tick every statement that is true, then check your grid. The hydrogen exception is waiting for you.

H₂
Cl₂
H₂O
CH₄
O₂
N₂
CO₂

Check someone else's work

One of these lines breaks a checking rule

A student describes how they drew the dot and cross diagram for chlorine, Cl₂. Which line is where it goes wrong?

A student's method for Cl₂ — which line goes wrong?

Two models, same molecule

Dot and cross diagramvsDisplayed formula

Think of H–H, H–O–H, O=C=O and O=O: each one can also be drawn as a dot and cross diagram.

Focus

What it shows

Dot and cross diagram

The outer-shell electrons of each atom, with the shared pairs in the overlaps

Displayed formula

Each shared pair as a line, such as H–H, H–O–H and O=C=O

The insight

Count the lines and you have counted the shared pairs: a double line is two pairs.

The electronic structure

Dot and cross diagram

Shows it: which electrons are shared and which are not

Displayed formula

Does not show it

Ease of drawing

Dot and cross diagram

Harder to draw, because every outer-shell electron has to be placed

Displayed formula

Easier to draw

Shape and the nature of the bond

Dot and cross diagram

A 2D model: no 3D shape, and nothing to say the bond is an electrostatic force of attraction between charged particles

Displayed formula

Does not always show the shape of the molecule

What it helps with

Dot and cross diagram

Counting the electron pairs around and between atoms, which helps to work out the molecular formula of simple covalent substances

Displayed formula

A quick picture of which atoms are joined and by how many shared pairs

Predict, then check

Use the diagram to get a formula.

A diagram shows one sulphur atom in the middle with a hydrogen atom on each side, and one dot and one cross in each of the two overlaps. What is the molecular formula?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Keep score of the electrons, and every small covalent molecule becomes easy to draw and easy to check.

What you need to know

  • One atom's electrons are drawn as dots and the other's as crosses. It works either way round, if you stay consistent.
  • Draw only outer-shell electrons; the group gives the number: H 1, C 4, N 5, O and S 6, Cl and F 7.
  • Shared pairs sit in the overlap between atoms. Each pair is one dot and one cross: one electron from each atom.
  • Every atom must end with a full outer shell: two for hydrogen, eight for the other atoms in these molecules.
  • Have a goYour friend swears that hydrogen needs eight outer-shell electrons for a full shell, just like everyone else. How many does it actually need?

    Two.

    Hydrogen's full shell is two electrons. Eight is the full-shell number for the other atoms in these molecules.

  • H₂ and Cl₂ share one pair, O₂ shares two (a double bond) and N₂ shares three (a triple bond).
  • Have a goN₂ shares three pairs. In the overlap between the two nitrogen atoms, how many dots and how many crosses do you draw?

    Three dots and three crosses.

    Each shared pair is one dot and one cross, so three pairs means three of each.

  • H₂O and H₂S have two shared pairs, CH₄ has four, and CO₂ has two in each carbon–oxygen overlap.
  • To check a diagram: the right number of electrons for each group, one electron from each atom in every pair, and full shells.
  • Have a goA diagram of water shows the oxygen atom with only 4 electrons drawn on it before sharing. Which checking rule has it broken?

    The right number of electrons for its group.

    Oxygen is in Group 6, so it starts with 6 outer-shell electrons, not 4.

  • A displayed formula shows each shared pair as a line, such as H–H, H–O–H and O=C=O.
  • The big plus of a dot and cross diagram: you can count the electron pairs, which helps work out a molecular formula.
  • It is a 2D model, so it does not show 3D shape, and nothing in it says a bond is an electrostatic force of attraction.

The big picture

A dot and cross diagram keeps score of the electrons in a covalent molecule: dots for one atom, crosses for the other, and one dot and one cross in every shared pair. Draw it from each atom's group, then check that every atom ends with a full outer shell.

Key points

1A dot and cross diagram uses dots for one atom's outer-shell electrons and crosses for the other's.
2Each shared pair in an overlap is one dot and one cross, one electron from each atom.
3Every atom must finish with a full outer shell: two for hydrogen, eight for the other atoms in these molecules.
4One shared pair is a single bond, two is a double bond and three is a triple bond.
5A displayed formula shows each shared pair as a line; a dot and cross diagram shows the electrons themselves.
6A dot and cross diagram is a 2D model, so it does not show 3D shape, and nothing in it indicates the electrostatic nature of a bond.

Worked example

Problem

Draw the dot and cross diagram for methane, CH₄, and check it.

⚠ Watch out

Putting both electrons of a shared pair from the same atom, or treating the diagram as a picture of the molecule's 3D shape. Every pair is one dot and one cross, and the diagram is a 2D model that does not show shape.

🧠

Memory hook

Every overlap is a handshake, and a handshake needs one hand from each person. One dot, one cross, never two of the same.

✓

Check yourself

Without looking back: for H₂S, how many outer-shell electrons does each atom start with, how many shared pairs go in the overlaps, and how many outer-shell electrons does each atom finish with?

Flashcards

(13)
In a dot and cross diagram, what do the dots and the crosses stand for?
Electrons from different atoms: dots for one atom, crosses for the other. It does not matter which, as long as you are consistent.
Which electrons does a dot and cross diagram of a covalent molecule show, and where do you get the number?
The outer-shell electrons of each atom. The number comes from the atom's group.
How many outer-shell electrons: hydrogen, carbon, nitrogen, oxygen and sulphur, chlorine and fluorine?
Hydrogen 1, carbon (Group 4) 4, nitrogen (Group 5) 5, oxygen and sulphur (Group 6) 6, chlorine and fluorine (Group 7) 7.
What does every shared pair contain?
One dot and one cross: one electron from each atom.
CCl₄: how many shared pairs, and how many outer-shell electrons does carbon end with?
Four shared pairs (four chlorine atoms around a central carbon), so carbon ends with eight.
How is the O₂ overlap drawn, and what bond does it make?
Two dots and two crosses, which are two shared pairs: a double bond.
CO₂: how many shared pairs between carbon and each oxygen?
Two. Every atom in the molecule ends with eight outer-shell electrons.
Three checks for a dot and cross diagram
The right number of electrons for each group, one electron from each atom in every shared pair, and a full outer shell for every atom.
What is wrong with a diagram where a shared pair is two dots from the same atom?
It is incorrect. Shared electrons must not all come from the same atom.
What does a displayed formula use to show a shared pair?
A line. Examples: H–H for hydrogen, H–O–H for water and O=C=O for carbon dioxide.
Give the advantages of a dot and cross diagram.
It shows the electrons being shared by each atom and the shared pairs, and lets you count the electron pairs around and between atoms, which helps work out the molecular formula.
Give the limitations of a dot and cross diagram.
It is a 2D model, so it does not show 3D shape, and nothing indicates that a bond is an electrostatic force of attraction between charged particles.
Displayed formula versus dot and cross: which is easier to draw, and what does it leave out?
The displayed formula is easier to draw, but it does not show the electronic structure or always the shape of the molecule.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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