Group I and VII ions and ionic bonding
Track electron transfer and distinguish the formation of ions from the attraction that bonds them.
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Write neutral shell arrangements for the first 20 elements.
- Know that opposite electrical charges attract.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to an atom's charge when it gains one electron?
What you will learn
- Describe cation and anion formation for Group I and VII examples.
- Represent transferred electrons and describe ionic melting and conductivity properties.
Transfer produces ions; attraction is the bond
A Group I atom loses its one outer electron to form a +1 cation. A Group VII atom gains one electron to form a −1 anion. Sodium changes from 2,8,1 to Na⁺ with 2,8; chlorine changes from 2,8,7 to Cl⁻ with 2,8,8. The nuclei remain unchanged. The ionic bond is the strong electrostatic attraction between the opposite charges after transfer.
For a dot-and-cross diagram, use one symbol for electrons originally from sodium and another for chlorine's electrons. Draw the resulting ions in separate brackets, label their charges and show chloride's eight outer electrons with one transferred electron distinguished. Do not draw a shared pair linking the ions: that would suggest a covalent bond.
Describe the bulk properties
Typical ionic compounds have high melting and boiling points. They conduct electricity when molten or dissolved in water, but conduct poorly when solid. A dissolved compound is described as aqueous; not every ionic compound dissolves readily. Melting and dissolving are different changes, although both can make ions mobile.
A sodium chloride formula gives a 1:1 ratio of sodium ions to chloride ions, balancing +1 and −1. It does not describe isolated NaCl molecules in the solid. The Extended lesson develops the repeating lattice model and the structure-based explanation of these properties.
Sources for this lesson
Pause and explain
What is the ionic bond in sodium chloride?
Worked example
Show how lithium (2,1) and fluorine (2,7) form the ions in lithium fluoride.
Show the worked solution
- Lithium loses one electron, becoming Li⁺ with arrangement 2.
- Fluorine gains that electron, becoming F⁻ with arrangement 2,8.
- The oppositely charged ions attract and combine in a 1:1 ratio.
Answer Li⁺ and F⁻; one electron transferred from lithium to fluorine.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw outer-shell dot-and-cross ions for sodium chloride, starting with Na (2,8,1) and Cl (2,8,7). Describe what must appear.
Give me a hint
Track sodium's outer electron into chloride's outer shell.
Compare my reasoning
- Show separate bracketed Na⁺ and Cl⁻ ions.
- Show eight electrons around Cl⁻: seven with chlorine's symbol and one with sodium's symbol.
- Label charges and identify electron transfer followed by attraction; no shared pair joins the ions.
Bracketed +1 and −1 ions; chloride has eight outer electrons including the transferred electron.
Look for these in your work
- Charges and outer-shell count are correct.
- Transferred electrons are visibly distinguished.
Potassium forms K⁺ and bromine forms Br⁻. Give the electron-transfer direction, ion types and formula ratio.
Give me a hint
Group I loses one; Group VII gains one.
Compare my reasoning
- One electron transfers from each potassium atom to a bromine atom.
- K⁺ is a cation and Br⁻ an anion.
- Equal +1 and −1 charges balance in a 1:1 ratio, giving KBr.
Potassium to bromine; cation and anion; 1:1 in KBr.
Look for these in your work
- Electron transfer has the correct direction.
- Charges explain the formula ratio.
An ionic solid does not conduct, but its molten sample does. State the pattern and say whether melting creates new ions.
Give me a hint
Separate ion existence from ion mobility.
Compare my reasoning
- The ionic compound conducts poorly as a solid and conducts when molten.
- The ions are already present in the solid.
- Melting allows those charged particles to move; it does not create their charge for the first time.
The solid's existing ions become mobile after melting.
Look for these in your work
- Both states' behaviour is described.
- The answer does not claim ions only appear after melting.
Common mistakes
- Describing an ionic bond as a shared pair of electrons.
- Leaving brackets and charges off a diagram of the resulting ions.
What can you explain now?
A potassium atom loses one electron. Name its ion type and charge.
Compare with the explanation
A cation with charge +1
Losing a negatively charged electron leaves one more proton than electron; the nucleus does not change.
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Make it stick
Draw Li⁺ and F⁻ from their neutral arrangements. Explain the difference between making an ion and forming the ionic bond.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.