Ch 7 · Dot-and-cross diagrams · AS & A Level Chemistry 9701

Topic 7 of 7 in Chemical bonding — AS & A Level Chemistry 9701 — 48 lessons.

Introduction

A dot-and-cross diagram is the standard way of showing where every outer-shell electron in a species originates and how it is arranged. Cambridge examiners award marks for specific conventions — correct electron count, correct origin of each electron, brackets and charges on ions — so the rules must be followed exactly, whatever type of bonding is shown.

Key ideas — Conventions and dot-and-cross diagrams for ionic bonding

Only outer-shell (valence) electrons are shown. Electrons originally belonging to one atom are drawn as dots, and electrons from the other atom as crosses, so the origin of every electron in a shared pair, or transferred electron, can be checked. For ionic bonding, the diagram shows complete transfer: in sodium chloride, the single outer dot electron of sodium is redrawn as a cross around chlorine's seven outer electrons, giving chlorine a full octet; each ion is drawn separately, in square brackets, with its charge shown outside the bracket, big[Nabig]^+ big[ddotClbig]^-. In magnesium oxide, both outer electrons of magnesium transfer to oxygen, giving mathrmMg^2+ and mathrmO^2-, again drawn as separate bracketed ions.

🧑‍🏫 Teacher's explanation

Think of ionic bonding like a gift exchange between two people. Instead of sharing a book (covalent bonding), one person gives their book entirely to another. Once the gift is given, the giver feels 'positive' because they lost something negative (an electron), and the receiver feels 'negative' because they gained something negative.

📖 Definitions

🔍 Worked example — Easy

Draw the dot-and-cross diagram for the formation of Sodium Chloride, NaCl.

🔍 Worked example — Standard

Draw the dot-and-cross diagram for Magnesium Oxide, MgO.

🔍 Worked example — Exam-style

Calculate the total number of electrons shown in a correct dot-and-cross diagram for Calcium Chloride, CaCl_2.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Dot-and-cross diagrams for covalent and coordinate bonding

For a covalent bond, a shared pair is drawn as one dot and one cross placed together between the two atoms, with any remaining outer electrons (lone pairs) shown around the atom they belong to. In ammonia, NH_3, three shared pairs join nitrogen to three hydrogens, and nitrogen keeps one lone pair.

🧑‍🏫 Teacher's explanation

Think of covalent bonding like a game of tug-of-war where neither side wins; instead, both atoms hold onto the same rope (the electron pair) to stay stable. In a dot-and-cross diagram, we use different symbols (dots and crosses) to keep track of which atom 'owns' which electron, even though they are shared.

📖 Definitions

🔍 Worked example — Easy

Draw the dot-and-cross diagram for a molecule of methane, CH_4. (Assume C is the central atom and use dots for C electrons and crosses for H electrons).

🔍 Worked example — Standard

Draw the dot-and-cross diagram for the ammonium ion, NH_4^+. Use dots for N electrons and crosses for H electrons.

🔍 Worked example — Exam-style

Draw the dot-and-cross diagram for the sulfate ion, SO_4^2-, showing the expanded octet of Sulfur.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Species with an expanded octet

For species that expand their octet, every bonding pair and lone pair present is drawn around the central atom, even where the total exceeds eight electrons, since Period 3 elements can use accessible 3d orbitals. In sulfur hexafluoride, SF_6, six shared pairs are drawn around sulfur, one to each fluorine, giving twelve electrons around the central sulfur atom. In phosphorus pentachloride, PCl_5, five shared pairs are drawn around phosphorus, giving ten electrons around the central atom; each surrounding chlorine atom keeps its usual three lone pairs and one bonding pair, retaining a normal octet itself.

🧑‍🏫 Teacher's explanation

Think of an atom's valence shell like a small, standard-sized storage box that usually only fits 8 items (the octet rule). Most atoms are happy with just 8. However, once you move down to Period 3 of the Periodic Table, the atoms become larger and gain access to a 'second floor'—the 3d orbitals. This extra space allows them to hold more than 8 electrons around their central nucleus.

📖 Definitions

🔍 Worked example — Easy

Draw the dot-and-cross diagram for SF_4 (Sulfur tetrafluoride). Sulfur is the central atom.

🔍 Worked example — Standard

Calculate the total number of valence electrons present in one molecule of PCl_5.

🔍 Worked example — Exam-style

In SF_6, how many electrons are drawn around the central Sulfur atom in a dot-and-cross diagram?

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Species with an odd number of electrons (free radicals)

A free radical is a species with an unpaired (odd) electron, formed typically by homolytic fission of a covalent bond. In a dot-and-cross diagram, the unpaired electron is shown singly, as a single dot or cross with no partner, usually marked with a small dot beside the atom's symbol to show it is not part of a pair. For example, when chlorine undergoes homolytic fission (as in the initiation step of free-radical substitution), each chlorine atom keeps one electron from the original bonding pair, giving two chlorine radicals, Cl^bullet, each with seven outer electrons — three lone pairs and one unpaired single electron — rather than the usual eight.

🧑‍🏫 Teacher's explanation

Imagine a dance floor where everyone is supposed to be dancing in pairs. Most stable molecules are like a perfectly choreographed ballroom dance where every person has a partner. However, sometimes a bond breaks so violently that one person is left standing alone without a partner. This 'lonely' person is the free radical.

Ch 7 · Dot-and-cross diagrams · AS & A Level Chemistry 9701

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