ScienceCBSE Class 10 · Metals and Non-metals

Ionic Compounds: Formation and Properties

Updated 14 October 2026 · Fact-checked

Ionic compounds form when a metal atom transfers electrons to a non-metal atom, making oppositely charged ions held by strong electrostatic attraction. Sodium chloride and magnesium chloride are examples. Ionic compounds are hard, brittle solids with high melting and boiling points, dissolve in water but not kerosene, and conduct electricity when molten or dissolved.

Understand Ionic Compounds: Formation and Properties

Why atoms react. Noble gases such as neon (2, 8) and argon (2, 8, 8) have completely filled outer shells and show little chemical activity. Other atoms react in a way that gives them a filled outer shell, usually eight electrons (an octet).

Forming ions. Sodium (atomic number 11) has the arrangement 2, 8, 1. If it loses its one outer electron, the L shell with eight electrons becomes its outermost shell. It still has 11 protons but only 10 electrons, so it carries one unit of positive charge: the sodium cation, $N a^{+}$ (2, 8). Chlorine (atomic number 17) has 2, 8, 7 and needs one electron to complete its octet. Gaining it gives 17 protons and 18 electrons, one unit of negative charge: the chloride anion, $C l^{-}$ (2, 8, 8). The electron lost by sodium is taken by chlorine, a give-and-take that suits both.

The oppositely charged ions attract each other strongly and stay together as sodium chloride. Note that sodium chloride does not exist as separate molecules; it is a huge aggregate of $N a^{+}$ and $C l^{-}$ ions. Magnesium (2, 8, 2) gives away two electrons to become $M g^{2 +}$ ; each of two chlorine atoms takes one, so magnesium chloride is $M g C l_{2}$ , with ions $M g^{2 +}$ and $C l^{-}$ . Compounds formed by this transfer of electrons from a metal to a non-metal are called ionic or electrovalent compounds.

Electron-dot structures. Show only the outer electrons as dots or crosses around the symbol: sodium has one dot, magnesium two, oxygen six, chlorine seven. To show formation, draw arrows for the electrons moving from the metal to the non-metal, then write the ions with their charges and full octets in brackets. For sodium oxide, two sodium atoms each give one electron to one oxygen atom, forming $2 N a^{+}$ and $O^{2 -}$ . For magnesium oxide, one magnesium atom gives two electrons to one oxygen atom, forming $M g^{2 +}$ and $O^{2 -}$ .

Properties and their reasons. Each property follows from the strong attraction between ions.

Physical nature: solid and somewhat hard, because the ions attract strongly; generally brittle, breaking into pieces under pressure.
High melting and boiling points: a lot of energy is needed to overcome the strong inter-ionic attraction. NCERT lists NaCl melting at 1074 K and boiling at 1686 K, and CaO melting at 2850 K.
Solubility: generally soluble in water, insoluble in kerosene and petrol.
Conduction: electricity flows through a liquid only if charged particles can move. In water or when melted, the ions move to the oppositely charged electrodes, so the compound conducts. In the solid the ions are locked in a rigid structure and cannot move, so solid ionic compounds do not conduct. Heat in the molten state overcomes the attraction enough for ions to move freely.

Ionic Compounds: Formation and Properties: formulas and key facts

Formation of a sodium ion: $N a (2, 8, 1) \to N a^{+} (2, 8) + e^{-}$; Metal atoms lose electrons to form cations.
Formation of a chloride ion: $C l (2, 8, 7) + e^{-} \to C l^{-} (2, 8, 8)$; Non-metal atoms gain electrons to form anions.
Magnesium ion: $M g (2, 8, 2) \to M g^{2 +} (2, 8) + 2 e^{-}$; Two chloride ions balance one $M g^{2 +}$ : $M g C l_{2}$ .
Ions in common ionic compounds: $N a C l : N a^{+}, C l^{-}; M g C l_{2} : M g^{2 +}, C l^{-}; N a_{2} O : N a^{+}, O^{2 -}; M g O : M g^{2 +}, O^{2 -}$; Total positive charge equals total negative charge.
Melting points from NCERT Table 3.4: $N a C l 1074 K; L i C l 887 K; C a C l_{2} 1045 K; C a O 2850 K; M g C l_{2} 981 K$; All high, because of strong inter-ionic attraction.

How to solve Ionic Compounds: Formation and Properties questions

For 'show the formation of' and 'give reasons' questions on ionic compounds, follow these steps.

1Write the electronic configuration of each atom (for example Na 2, 8, 1; O 2, 6).
2Decide how many electrons the metal loses and the non-metal gains to reach an octet (Na loses 1, Mg loses 2, O gains 2, Cl gains 1).
3Choose the number of atoms so electrons lost = electrons gained (two Na for one O; one Mg for two Cl).
4Draw electron-dot structures of the atoms with arrows showing the transfer.
5Write the ions with charges and the final formula; name the cation and anion.
6For property questions, give the property and its reason in terms of ions: strong attraction, rigid structure, or ions free to move.

Ionic Compounds: Formation and Properties in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Conduction by ionic compounds

Suppose you are helping in the kitchen and spill some salty water around a plug socket. Your younger brother says it is safe because 'salt is a solid and solids like salt don't conduct electricity'.

Explain whether he is right, using what you know about ions in solid and dissolved sodium chloride.

Try it first, then show the solutionSolution

Sodium chloride is an ionic compound made of $N a^{+}$ and $C l^{-}$ ions.
In solid salt the ions are held in a rigid structure and cannot move, so dry solid salt does not conduct electricity. That part of his statement is true.
When salt dissolves in water, the ions are free to move towards oppositely charged electrodes, so salt solution conducts electricity.
Spilled salty water near a socket can therefore carry current and is dangerous; it should be wiped up with the power switched off.

Answer: He is wrong for salty water. Solid salt does not conduct because its ions cannot move, but dissolved salt has free-moving ions and conducts.

Memory hook: Locked ions are silent; free ions carry current.

Tricks to remember Ionic Compounds: Formation and Properties

Short memory aids. Say each one aloud once, then use it on a question the same day.

Metals give, non-metals take
Metals lose electrons (positive ions); non-metals gain electrons (negative ions).
Use it when: Assigning charges in formation diagrams.
HBSC for properties
Hard and brittle, Boiling and melting points high, Soluble in water, Conduct only when molten or dissolved.
Use it when: Listing the four properties of ionic compounds.
Cross the charges
$M g^{2 +}$ with $C l^{-}$ : the 2 goes to Cl, giving $M g C l_{2}$ ; $N a^{+}$ with $O^{2 -}$ gives $N a_{2} O$ .
Use it when: Writing formulae of ionic compounds.

Ionic Compounds: Formation and Properties: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Formation of magnesium chloride · 3 marks · Short answerBoard level

Show the formation of magnesium chloride by the transfer of electrons. Name the cation and anion. (Atomic numbers: Mg 12, Cl 17)

Try it first, then show the solutionSolution

Configurations: Mg 2, 8, 2; Cl 2, 8, 7 (½ mark).
Mg loses two electrons: $M g \to M g^{2 +} + 2 e^{-}$ (2, 8) (1 mark).
Each of two Cl atoms gains one: $C l + e^{-} \to C l^{-}$ (2, 8, 8) (1 mark).
Ions attract to give $M g C l_{2}$ ; cation $M g^{2 +}$ , anions $C l^{-}$ (½ mark).

Answer: $M g^{2 +}$ and two $C l^{-}$ ions form $M g C l_{2}$ ; cation magnesium ion, anion chloride ion.

Example 2: Electron-dot structures of oxides · 5 marks · Long answerHarder board level

(a) Write the electron-dot structures of sodium, oxygen and magnesium. (b) Show the formation of $N a_{2} O$ and $M g O$ by transfer of electrons. (c) Name the ions present in each. (Atomic numbers: Na 11, O 8, Mg 12)

Try it first, then show the solutionSolution

(a) Outer electrons: Na has 1 dot (2, 8, 1); O has 6 dots (2, 6); Mg has 2 dots (2, 8, 2) (1½ marks).
(b) $N a_{2} O$ : two Na atoms each transfer one electron to one O atom; O gains two electrons to become $O^{2 -}$ (2, 8); each Na becomes $N a^{+}$ (2, 8) (1½ marks).
(b) $M g O$ : one Mg atom transfers two electrons to one O atom; $M g^{2 +}$ (2, 8) and $O^{2 -}$ (2, 8) form (1 mark).
(c) $N a_{2} O$ : $N a^{+}$ and $O^{2 -}$ ; $M g O$ : $M g^{2 +}$ and $O^{2 -}$ (1 mark).

Answer: Na· , O with 6 dots, Mg with 2 dots; $2 N a^{+} + O^{2 -}$ gives $N a_{2} O$ ; $M g^{2 +} + O^{2 -}$ gives MgO.

Example 3: Conductivity in three states · 3 marks · Short answerHarder board level

A student tests solid sodium chloride, molten sodium chloride and an aqueous solution of sodium chloride in a circuit with a bulb. (a) In which cases does the bulb glow? (b) Explain each result. (c) Why does sodium chloride not dissolve in kerosene?

Try it first, then show the solutionSolution

(a) It glows with molten NaCl and with the aqueous solution, not with the solid (1 mark).
(b) Solid: ions fixed in a rigid structure cannot move. Molten: heat overcomes the attraction so ions move freely. Solution: ions are free to move to the oppositely charged electrodes (1 mark).
(c) Ionic compounds are generally insoluble in solvents like kerosene and petrol; the ions do not separate in kerosene (1 mark).

Answer: (a) Molten and aqueous. (b) Movement of ions is possible only in those two. (c) Ionic compounds are insoluble in kerosene.

Example 4: Reasoning about melting points · 3 marks · Short answerHOTS / competency

Using NCERT data, sodium chloride melts at 1074 K and calcium oxide at 2850 K. (a) Why are both values high? (b) Calcium oxide contains $C a^{2 +}$ and $O^{2 -}$ ions, while sodium chloride contains $N a^{+}$ and $C l^{-}$ . Suggest why calcium oxide has the higher melting point. (c) Why are ionic compounds brittle?

Try it first, then show the solutionSolution

(a) Ionic compounds have strong forces of attraction between ions; much energy is needed to break them, so melting points are high (1 mark).
(b) Ions with double charges attract each other more strongly than ions with single charges, so more energy is needed to separate $C a^{2 +}$ and $O^{2 -}$ (1 mark).
(c) The ions are held in a fixed arrangement; under pressure layers shift so that ions of the same charge come close and repel, and the crystal breaks into pieces (1 mark).

Answer: (a) Strong inter-ionic attraction. (b) Doubly charged ions attract more strongly. (c) Shifting layers bring like charges together, so the solid shatters.

Where students lose marks in Ionic Compounds: Formation and Properties

The slips examiners see most often, and what to write instead.

Mistake: Showing sharing instead of transfer
Students draw a shared pair between Na and Cl, like a covalent bond.
Wrong working
Na and Cl share one pair of electrons.
Right working
Na transfers one electron to Cl, giving $N a^{+}$ and $C l^{-}$ .
Do this: In ionic compounds draw an arrow from the metal electron to the non-metal and show charges on both ions.
Usually costs: Most of the marks in a 3-mark formation question.
Mistake: Missing charges or brackets on ions
The final step is rushed.
Wrong working
Mg + 2Cl → MgCl2 with no charges shown.
Right working
$M g^{2 +}$ and $2 C l^{-}$ (each with 8 outer electrons) give $M g C l_{2}$ .
Do this: Write each ion with its charge and its full octet.
Usually costs: 1 mark.
Mistake: Saying solid ionic compounds conduct
Students remember 'ionic compounds conduct' without the condition.
Wrong working
NaCl conducts electricity because it has ions.
Right working
NaCl conducts only when molten or dissolved, when its ions can move.
Do this: Always add the condition: in molten state or in aqueous solution.
Usually costs: 1 mark in assertion-reason and reasoning questions.
Mistake: Calling NaCl a molecule
The formula looks like a molecule.
Wrong working
A molecule of sodium chloride.
Right working
Sodium chloride is an aggregate of oppositely charged ions.
Do this: Use 'formula unit' or 'ions', not 'molecule', for ionic compounds.
Usually costs: ½ mark.

Exam tips

Write configurations under each atom and ion in formation diagrams; they earn marks on their own.
Use dots for one atom's electrons and crosses for the other's so the transfer is clear.
For property questions, each answer needs the property and the ion-based reason.
Quote the NCERT melting points only if asked; otherwise 'high melting and boiling points' with the reason is enough.
Name ions correctly: chloride, not chlorine ion; oxide, not oxygen ion.

Practice questions on Ionic Compounds: Formation and Properties

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of the following is an ionic compound?
(a) $C H_{4}$ (b) $C l_{2}$ (c) $M g C l_{2}$ (d) $H_{2} O$
Hint
Look for a metal combined with a non-metal.
Answer
(c) $M g C l_{2}$ , formed by electron transfer from magnesium to chlorine.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Solid sodium chloride does not conduct electricity.
Reason (R): Sodium chloride is made of molecules, not ions.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Sodium chloride exists as an aggregate of ions.
Answer
(c). A is true because the ions in the solid cannot move; R is false because NaCl is made of ions, not molecules.
Q3 · 3 marks · Short answerHarder board level
Element X has atomic number 19 and element Y has atomic number 17. (a) Write their electronic configurations. (b) Show the formation of the compound between them by electron transfer and write its formula. (c) Would the compound conduct electricity in the solid state? Give a reason.
Hint
X is potassium, Y is chlorine.
Answer
(a) X: 2, 8, 8, 1; Y: 2, 8, 7. (b) X loses one electron to form $X^{+}$ (2, 8, 8); Y gains it to form $Y^{-}$ (2, 8, 8); formula XY (KCl). (c) No; in the solid the ions are held in a rigid structure and cannot move.
Q4 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose a student is given four white solids, A to D, and records:
A: melts at a very high temperature, dissolves in water, solution conducts.
B: melts easily on a spatula, does not dissolve in water.
C: melts at a very high temperature, does not conduct as a solid, conducts when molten.
D: dissolves in kerosene but not in water.
(a) Which two are likely to be ionic? (1 mark)
(b) Give one reason for each choice. (1 mark)
(c) Explain why C conducts when molten but not when solid. (2 marks)
OR
(c) Explain why D is unlikely to be ionic using two observations. (2 marks)
Hint
Match each observation with the four properties of ionic compounds.
Answer
(a) A and C. (b) A: high melting point and its water solution conducts; C: high melting point and conducts when molten. (c) In the solid, ions are fixed in a rigid structure; on melting, heat overcomes the attraction and ions move freely, carrying current. OR (c) Ionic compounds are generally insoluble in kerosene and soluble in water; D shows the opposite on both counts.
Q5 · 3 marks · Short answerHarder board level
(a) Why do noble gases show little chemical activity? (b) Explain, with electronic configurations, why sodium forms $N a^{+}$ and not $N a^{2 +}$ . (c) How many electrons does each oxygen atom gain in forming MgO, and how does that compare with the number lost by magnesium?
Hint
Think of the octet that each atom reaches.
Answer
(a) Their outermost shells are completely filled. (b) Na is 2, 8, 1; losing one electron gives 2, 8, a stable octet; losing a second would break that octet. (c) Oxygen gains two electrons (2, 6 to 2, 8); magnesium loses two (2, 8, 2 to 2, 8). Electrons lost equal electrons gained.
Q6 · 3 marks · Short answerHOTS / competency
A compound Z has a melting point of about 1000 K, is a hard but brittle solid, and its solution in water conducts electricity. A student claims Z must be a molecular substance because 'all compounds are made of molecules'. Evaluate the claim, and suggest how Z's structure explains each of the three observations.
Hint
Compare with sodium chloride.
Answer
The claim is wrong. The properties match an ionic compound, which exists as an aggregate of oppositely charged ions, not molecules (like NaCl). High melting point: strong attraction between ions needs much energy to overcome. Hard but brittle: strong attraction makes it hard, but the rigid arrangement breaks under pressure. Conducting solution: in water the ions are free to move.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Another name for ionic compounds?Show answer
Electrovalent compounds.
2.Configuration of $N a^{+}$ ?Show answer
2, 8.
3.Why are ionic compounds hard?Show answer
Strong forces of attraction between positive and negative ions.
4.In which solvents are ionic compounds generally insoluble?Show answer
Kerosene and petrol.
5.Ions present in MgO?Show answer
$M g^{2 +}$ and $O^{2 -}$ .
6.Why don't solid ionic compounds conduct?Show answer
Their ions cannot move in the rigid solid structure.

Ionic Compounds: Formation and Properties: frequently asked questions

How are ionic compounds formed?

A metal atom transfers one or more electrons to a non-metal atom. The positive and negative ions formed attract each other strongly and stay together as an ionic compound.

Why do ionic compounds have high melting points?

The attraction between oppositely charged ions is strong, so a large amount of energy is needed to break it.

Do ionic compounds conduct electricity?

Only when molten or dissolved in water, when the ions can move. In the solid state the ions are fixed and cannot carry current.

What is an electron-dot structure?

A diagram that shows an element's symbol with its outer-shell electrons as dots or crosses around it.

Is sodium chloride made of molecules?

No. It exists as an aggregate of oppositely charged sodium and chloride ions.