9701 · 3.7
Dot-and-cross diagrams flashcards
Revision flashcards for Cambridge 9701 Dot-and-cross diagrams (syllabus 3.7). Flip, recall, then mark a real past-paper question.
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What is a dot-and-cross diagram?
A diagram that represents the arrangement of outer (valence) shell electrons in atoms, ions, or molecules. Electrons from different atoms are shown as dots or crosses to indicate their origin.
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In an ionic dot-and-cross diagram, what are the three essential features?
1. Square brackets around each ion. 2. The charge shown outside the top right of the bracket. 3. The correct number of electrons in the outer shell of each ion (usually a full shell).
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What is a covalent bond?
A chemical bond formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei of the bonded atoms.
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What is a dative (co-ordinate) bond?
A covalent bond in which both electrons in the shared pair come from one of the bonding atoms. The donor atom must have a lone pair and the acceptor atom must have an empty orbital.
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Common mistake: Forgetting to show the charge on ions. Why is this critical?
The electrostatic attraction between oppositely charged ions *is* the ionic bond. Omitting the charges means you haven't correctly represented the bonding and will lose marks.
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What does 'expansion of the octet' mean?
The ability of some elements in Period 3 and below to have more than eight electrons in their valence shell when forming covalent bonds (e.g., S in SF₆ has 12).
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Give an example of a molecule with an 'incomplete octet'.
Boron trifluoride (BF₃). The central boron atom has only six electrons in its outer shell. Aluminium chloride (AlCl₃) is another common example.
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How would you represent the ammonium ion, NH₄⁺, using a dot-and-cross diagram?
A central nitrogen atom bonded to four hydrogen atoms. Three bonds are normal covalent; one is a dative bond from the lone pair on nitrogen to a H⁺ ion. The entire structure is placed in square brackets with a +1 charge.
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When drawing the dot-and-cross for the Al₂Cl₆ dimer, what types of bonding are present?
It contains both normal covalent bonds (between Al and terminal Cl) and dative covalent bonds (from bridging Cl atoms to Al atoms).
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Why do we only show outer shell electrons in these diagrams?
Because only the valence electrons are involved in chemical bonding. The inner shell electrons (core electrons) are not part of the bonding interactions and showing them would make the diagram unnecessarily complicated.
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How do you determine the number of valence electrons for a main group element?
The number of outer shell (valence) electrons is equal to its Group number in the Periodic Table. For example, oxygen in Group 16 has 6 valence electrons.