Dot and Cross Diagrams

Choose Standard or Challenge, then start a round of ten questions. Standard question 1 shows four diagrams for a covalent molecule such as H₂O or CH₄. Pick the correct one. Question 2 asks for the number of shared pairs, lone pairs or bonding electrons in a molecule. Question 3 asks which ion forms from sodium, magnesium, chlorine or oxygen. Question 4 shows four diagrams for NaCl or MgO. Questions 5, 8 and 10 ask for electron counts. Question 6 asks you to choose a molecule diagram. Question 7 asks about an ion. Question 9 asks you to choose a molecule diagram.

Challenge keeps the same ten-question length. It starts with a molecule diagram, an electron count and an ion question. Question 4 uses O₂, N₂ or CO₂, so you need to read a double or triple bond. Question 5 counts electrons in one of these molecules. Question 6 shows CaCl₂, Na₂O or MgCl₂, each with two of one ion. Question 7 counts electrons in a molecule with multiple bonds. Question 8 asks about an ion. Question 9 asks you to name an error in a diagram. Question 10 asks you to choose a diagram for a multiple-bond molecule or repeated-ion compound.

Each question has four choices. After you answer, read the reason before you press Next. It explains the electron count and why each shell is full. A right answer gives 10 points. A correct answer within 15 seconds adds a 5-point speed bonus. Standard keeps the question open when the clock reaches zero. Challenge locks an unanswered question at zero and marks it wrong. Practice mode turns the clock off.

Press 1, 2, 3 or 4 to choose an answer on a keyboard. Press Enter to continue after the reason appears. Your best scores stay on this device, with separate scores for Standard and Challenge. Every round changes its species and answer order.

A dot and cross diagram tracks electrons in the outer shell. The marks show which atom supplied each electron. In a covalent bond, two non-metal atoms share a pair. This game uses a dot for one atom and a cross for the other. A shared pair has one of each mark. A single bond has one shared pair, a double bond has two and a triple bond has three.

Hydrogen needs two electrons in its first shell. Most other atoms in these small molecules need eight in their outer shell. In H₂, each hydrogen shares one electron. In H₂O, oxygen shares one electron with each hydrogen and keeps two lone pairs. NH₃ has one lone pair on nitrogen. HCl has three on chlorine and Cl₂ has three on each chlorine. O₂ has two lone pairs on each oxygen. N₂ has one on each nitrogen. CO₂ has two on each oxygen. CH₄ has four shared pairs around carbon.

The diagrams show only outer-shell electrons. They leave out inner shells so the shared pairs and lone pairs are easier to count. A missing lone pair can leave an atom short of a full shell. A hydrogen with a lone pair has too many electrons. In O₂, N₂ and CO₂, the number of shared pairs must match the double or triple bond.

Ionic bonding transfers electrons from a metal atom to a non-metal atom. The metal loses electrons and forms a positive ion. The non-metal gains them and forms a negative ion. In this game, brackets surround every ion and the charge sits at the top right. The metal's original outer-shell electrons are no longer shown around its ion; its full inner shells are left out of the drawing. The non-metal ion shows eight outer-shell electrons, including marks from the metal. The charges and transferred electrons balance across the compound.

The ionic examples are NaCl, MgO, CaCl₂, Na₂O and MgCl₂. CaCl₂ has two chloride ions for each calcium ion. Na₂O has two sodium ions for each oxide ion. MgCl₂ has two chloride ions for each magnesium ion. The diagrams show these ratios and the electrons transferred to each non-metal ion. Use Atomic Structure to practise electron arrangements before you try the ion questions.

The game covers AQA GCSE Chemistry 8462 sections 4.2.1.2 on ionic bonding and 4.2.1.4 on covalent bonding. AQA Combined Science Trilogy 8464 uses sections 5.2.1.2 and 5.2.1.4. Pearson Edexcel Chemistry 1CH0 and Combined Science 1SC0 use points 1.21 and 1.24 for ionic bonding and point 1.31 for covalent diagrams. OCR Gateway Chemistry A J248 covers bonds and electron transfer in C2.2d and C2.2e. Its C2.2f asks students to construct dot and cross diagrams for simple covalent and binary ionic substances. Read the AQA Chemistry specification, AQA Combined Science specification, Pearson Chemistry specification, Pearson Combined Science specification and OCR J248 specification.

• Count each atom's outer electrons before you check the bond.
• Count a shared pair as two electrons: one dot and one cross.
• Add lone pairs to the atom that owns those electrons.
• Check hydrogen has two electrons and other atoms have eight.
• For ions, count the transferred electrons and check the total charge is zero.
• For CaCl₂ and MgCl₂, check both chloride ions.

Read a diagram in two passes. First count the pairs in each bond. Then count lone pairs around each atom. For an ionic diagram, check the brackets and charge before you count the marks. Compare your result with the shown reason. Periodic Table Challenge gives more practice with the group numbers that help predict ion charges.