ChemistryICSE Class 10 · Chemical Bonding

Electrovalent Bonding: Electron Dot Structures and Properties (ICSE Class 10)

Updated 17 October 2026 · Fact-checked

An electrovalent (ionic) bond is the electrostatic force of attraction between oppositely charged ions formed when one atom (usually a metal) transfers electrons to another (usually a non-metal). Sodium chloride, magnesium chloride and calcium oxide are the standard examples. Electrovalent compounds are hard, brittle crystalline solids with high melting and boiling points, dissolve in water, and conduct electricity only when molten or in solution.

Understand Electrovalent (Ionic) Bonding

1. How the bond forms. Atoms combine to reach the stable electron arrangement of the nearest noble gas: a duplet (2 electrons) for the first shell or an octet (8 electrons) for the outer shell. A metal atom with 1 to 3 outer electrons finds it easier to lose them and become a positive ion (cation). A non-metal atom with 5 to 7 outer electrons finds it easier to gain electrons and become a negative ion (anion). The two ions then attract each other: this attraction is the electrovalent bond. The number of electrons lost or gained by an atom is its electrovalency.

2. Electron dot structures asked in the syllabus.

Sodium chloride, NaCl. Na is 2,8,1 and Cl is 2,8,7. Na gives its one outer electron to Cl: $N a \to N a^{+} + e^{-}$ (now 2,8) and $C l + e^{-} \to C l^{-}$ (now 2,8,8). One Na⁺ and one Cl⁻ are joined: $N a^{+} [: \ddot{C l} :]^{-}$ (draw the eight dots around Cl⁻ inside square brackets, with the charge outside).
Magnesium chloride, MgCl₂. Mg is 2,8,2. It loses two electrons to give Mg²⁺ (2,8). Each Cl atom takes one electron, so two Cl⁻ ions are needed: $M g^{2 +}$ with two $[: \ddot{C l} :]^{-}$ .
Calcium oxide, CaO. Ca is 2,8,8,2 and O is 2,6. Ca gives two electrons to O: Ca²⁺ is 2,8,8 and O²⁻ is 2,8. The charges balance one to one, so the formula is CaO.

3. Writing a formula from charges. The total positive charge must equal the total negative charge. Cross the charges: Al³⁺ with O²⁻ gives Al₂O₃ (2 x 3 = 3 x 2 = 6).

4. Characteristic properties and why.

State: crystalline solids at room temperature. The ions sit in a regular lattice, held by strong attraction in all directions; they are hard but brittle (a blow pushes like charges next to each other, and the crystal cracks).
Melting and boiling points: high, because a lot of energy is needed to overcome the strong forces between the ions.
Conductivity (heat and electricity): a solid does not conduct, as its ions are fixed in place. In the molten state and in aqueous solution the ions are free to move and carry the current. This is why such compounds are electrolytes and can be decomposed by electrolysis.
Dissociation: they split into ions in water and in the molten state, for example $N a C l \to N a^{+} + C l^{-}$ .
Solubility: generally soluble in water (a polar solvent) and insoluble in organic solvents such as kerosene, ether and benzene.

Electrovalent compounds form ions that are already present in the crystal, so reactions between them in solution are fast. Compare with covalent compounds on the next topic page.

Electrovalent (Ionic) Bonding: formulas and key facts

Formation of NaCl: $N a \to N a^{+} + e^{-}$ ; $C l + e^{-} \to C l^{-}$ ; $N a^{+} + C l^{-} \to N a C l$
Formation of MgCl₂: $M g \to M g^{2 +} + 2 e^{-}$ ; $2 C l + 2 e^{-} \to 2 C l^{-}$ ; $M g^{2 +} + 2 C l^{-} \to M g C l_{2}$
Formation of CaO: $C a \to C a^{2 +} + 2 e^{-}$ ; $O + 2 e^{-} \to O^{2 -}$ ; $C a^{2 +} + O^{2 -} \to C a O$
Electron arrangements: $N a 2, 8, 1∣ M g 2, 8, 2∣ C a 2, 8, 8, 2∣ C l 2, 8, 7∣ O 2, 6∣ A l 2, 8, 3$
Key properties: Hard brittle crystals
High melting and boiling points
Conduct only molten or in solution
Soluble in water
Insoluble in organic solvents

How to solve Electrovalent (Ionic) Bonding questions

To draw or explain any electrovalent compound, follow the same six steps.

1Write the electron arrangement of each atom (for example Mg 2,8,2 and Cl 2,8,7).
2Decide which atom loses electrons (the metal) and which gains (the non-metal), and how many.
3Write each ion with its new arrangement and charge, for example Mg²⁺ (2,8) and Cl⁻ (2,8,8).
4Balance the total charge to find how many of each ion are needed (one Mg²⁺ for two Cl⁻).
5Draw the dot structure with the anion in square brackets, all outer electrons shown, and the charge written outside the bracket.
6For property questions, link each property to ions in a lattice (fixed in the solid, free when molten or dissolved).

Electrovalent (Ionic) Bonding 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.

Why salt water carries current but dry salt does not

Suppose a student connects a bulb, a battery and two carbon rods. First the rods touch dry table salt crystals, and the bulb stays off. Then the same rods are dipped in a salt solution and the bulb glows.

Explain the difference using the structure of sodium chloride.

Try it first, then show the solutionSolution

In dry solid NaCl the Na⁺ and Cl⁻ ions are locked in the lattice and cannot move, so no current flows.
In water the lattice breaks and the ions move freely towards the oppositely charged rods, carrying current and lighting the bulb.

Answer: Solid NaCl has fixed ions and does not conduct; its solution has mobile ions and does.

Memory hook: Ions must be free to flow.

Tricks to remember Electrovalent (Ionic) Bonding

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

LEO the lion says GER
Loss of Electrons is Oxidation (metal becomes a cation); Gain of Electrons is Reduction (non-metal becomes an anion).
Use it when: Deciding which atom forms which ion.
Metals give, non-metals take
Group 1, 2 and 3 metals give away 1, 2 or 3 electrons; Group 15, 16 and 17 non-metals take 3, 2 or 1.
Use it when: Predicting the formula quickly.
Brackets and charge
Anions go in square brackets with the charge outside. The metal cation is shown without dots (or in brackets with its charge) because its original outer electrons have been transferred; its new outer shell is the complete octet underneath.
Use it when: Drawing marking-scheme perfect dot diagrams.

Electrovalent (Ionic) Bonding: solved examples

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

Example 1: Definition · 1 mark · Very short answerBoard level

Define electrovalent bond. Which type of atoms usually form it?

Try it first, then show the solutionSolution

It is the electrostatic attraction between oppositely charged ions formed by transfer of electrons.
It forms between a metal and a non-metal.

Answer: The force of attraction between oppositely charged ions formed by electron transfer, usually between a metal and a non-metal.

Example 2: Electron dot structure of MgCl₂ · 3 marks · Short answerBoard level

Show the formation of magnesium chloride from magnesium (atomic number 12) and chlorine (atomic number 17) using electron dot diagrams.

Try it first, then show the solutionSolution

Mg is 2,8,2 and Cl is 2,8,7.
Mg loses 2 electrons to give Mg²⁺ (2,8); each of two Cl atoms gains one electron to give Cl⁻ (2,8,8).
Draw Mg²⁺ with two $[: \ddot{C l} :]^{-}$ ions, each Cl⁻ shown with eight dots in brackets.
Equation: $M g + 2 C l \to M g^{2 +} + 2 C l^{-} \to M g C l_{2}$ .

Answer: One Mg atom gives one electron to each of two Cl atoms; the compound contains Mg²⁺ and 2 Cl⁻ ions.

Example 3: Why conduction changes with state · 3 marks · Short answerHarder board level

Calcium oxide does not conduct electricity as a solid but does so when molten. Explain, and state the process that happens when current passes through the molten compound.

Try it first, then show the solutionSolution

CaO has Ca²⁺ and O²⁻ ions held in fixed positions in the solid lattice, so no charge carrier is free.
On melting, the lattice breaks and the ions become mobile, so they carry charge.
Passing current through the melt decomposes it by electrolysis: Ca²⁺ moves to the cathode, O²⁻ to the anode.

Answer: Fixed ions in the solid cannot move; mobile ions in the melt conduct, and the compound is decomposed by electrolysis.

Example 4: Formula and electrons · 4 marks · Long answerHarder board level

Element X has atomic number 13 and element Y has atomic number 8. (a) Write the electron arrangements. (b) Name the ions formed. (c) Write the formula of the compound. (d) Why is it a high-melting solid?

Try it first, then show the solutionSolution

(a) X is 2,8,3; Y is 2,6.
(b) X loses 3 electrons to form X³⁺ (2,8); Y gains 2 electrons to form Y²⁻ (2,8).
(c) Cross the charges: X2Y3 (2 x 3 = 3 x 2 = 6), which is aluminium oxide.
(d) Strong electrostatic attraction between the ions in the lattice needs a great deal of energy to overcome.

Answer: (a) 2,8,3 and 2,6 (b) Al³⁺ and O²⁻ (c) Al₂O₃ (d) Strong ionic forces in the lattice.

Example 5: Reasoning on solubility · 4 marks · Long answerHOTS / competency

A white solid dissolves in water, does not dissolve in kerosene, melts at a high temperature and conducts only when molten or dissolved. State the type of bonding in it and justify each observation in one line.

Try it first, then show the solutionSolution

Bonding is electrovalent (ionic).
Dissolves in water, a polar solvent, which separates the ions; not in kerosene, a non-polar solvent.
High melting point: strong attraction between ions in the lattice.
Conducts only when molten or dissolved because the ions are free to move only then.

Answer: An electrovalent compound, with each property following from oppositely charged ions held in a lattice.

Where students lose marks in Electrovalent (Ionic) Bonding

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

Mistake: Showing electrons shared in an ionic structure
Candidates draw a shared pair between Na and Cl instead of transferring the electron, which is the covalent picture.
Wrong working
Na and Cl joined by a shared pair.
Right working
Na⁺ and [Cl]⁻ as separate ions, with the transferred electron now on Cl.
Do this: Draw two separate ions with charges and brackets.
Usually costs: 1 to 2 marks.
Mistake: Missing charges or brackets on the anion
The outer shell of Cl⁻ is drawn with 7 dots, or the charge is left out.
Do this: Show eight dots, put the ion in square brackets and write the charge outside.
Usually costs: 1 mark.
Mistake: Writing MgCl for magnesium chloride
The valencies are not balanced: Mg gives two electrons, so two chlorine atoms are needed.
Wrong working
MgCl
Right working
MgCl₂
Do this: Balance total positive and negative charge.
Usually costs: 1 mark.
Mistake: Saying solid ionic compounds conduct
Candidates write that ions are present and therefore the solid conducts; they forget that the ions are fixed.
Do this: Say: conduct only in the molten state or in solution because the ions are then free to move.
Usually costs: 1 mark.

Exam tips

Always write the electron arrangement before and after electron transfer; it earns method marks.
In dot diagrams, show the cation without outer dots (for example Mg²⁺) and the anion with eight dots in brackets.
For "give a reason" on conduction, mention both the state of the ions (fixed or free) and the word electrolyte.
Link ionic compounds in the molten or dissolved state to electrolysis, since the syllabus asks for this link.

Practice questions on Electrovalent (Ionic) Bonding

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
The electron arrangement of the ion formed when a calcium atom (atomic number 20) loses its outer electrons is:
(a) 2,8,8,2
(b) 2,8,8
(c) 2,8,6
(d) 2,8,10
Hint
Ca loses two electrons.
Answer
(b) 2,8,8.
Q2 · 1 mark · MCQBoard level
Which of these is NOT a property of an electrovalent compound?
(a) High melting point
(b) Conducts electricity in the solid state
(c) Soluble in water
(d) Brittle crystalline solid
Hint
Are the ions free in a solid?
Answer
(b) Conducts electricity in the solid state.
Q3 · 3 marks · Short answerHarder board level
Using electron dot diagrams, show the formation of calcium oxide. Give the electron arrangements of both ions.
Hint
Ca gives 2 electrons to O.
Answer
Ca (2,8,8,2) gives two electrons to O (2,6). Ca²⁺ is 2,8,8 and O²⁻ is 2,8, shown as $C a^{2 +}$ and $[: \ddot{O} :]^{2 -}$ with eight dots on O. Formula CaO.
Q4 · 2 marks · Short answerBoard level
Give two reasons why sodium chloride crystals are brittle and have a high melting point.
Hint
Think of ions in a lattice.
Answer
Strong attraction between ions in all directions needs much energy to break, so melting is high; a blow shifts layers so that like charges face each other and repel, so the crystal cracks.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Molten magnesium chloride is decomposed by passing electric current.
Reason (R): It contains Mg²⁺ and Cl⁻ ions that are free to move in the molten state.
(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
Why does the melt conduct?
Answer
(a).
Q6 · 4 marks · Long answerHarder board level
Element P has atomic number 12 and element Q has atomic number 9. (a) Write the electron arrangements of P and Q. (b) Name the type of bond and write the formula of the compound they form. (c) State two physical properties of the compound.
Hint
P gives 2 electrons; Q takes 1.
Answer
(a) P 2,8,2; Q 2,7. (b) Electrovalent; PQ2 (magnesium fluoride). (c) Crystalline solid with a high melting point; soluble in water and conducts when molten or dissolved.
Q7 · 2 marks · Short answerHarder board level
Why do electrovalent compounds in aqueous solution conduct electricity, while a solution of sugar does not? Name the type of particle that carries the current.
Hint
Sugar is covalent.
Answer
Electrovalent compounds dissociate into mobile ions in water and the ions carry current; sugar remains as neutral molecules and gives no ions.
Q8 · 3 marks · Short answerHOTS / competency
A white crystalline solid B conducts electricity in the molten state but not as a solid. It contains magnesium (Mg²⁺) ions and the same anion as sodium chloride. (a) What type of compound is B? (b) Name B and give its formula. (c) How many electrons does each Mg atom lose to form the cation of B?
Hint
B contains Mg²⁺ and the chloride ion.
Answer
(a) An electrovalent (ionic) compound. (b) Magnesium chloride, MgCl₂. (c) Two electrons.

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.What is electrovalency?Show answer
The number of electrons lost or gained by an atom to form an ion.
2.Electron arrangement of Cl⁻ ?Show answer
2,8,8.
3.Name the three ionic compounds whose dot structures are in the syllabus.Show answer
NaCl, MgCl₂ and CaO.
4.Why are ionic solids brittle?Show answer
A blow brings like charges together and the crystal splits.
5.In which two states do ionic compounds conduct?Show answer
Molten state and aqueous solution.

Electrovalent (Ionic) Bonding: frequently asked questions

What is an electrovalent bond in simple words?

It is the attraction between a positive ion and a negative ion that formed when one atom gave electrons to another.

Why do ionic compounds have high melting points?

Strong forces between the many ions in the lattice take a lot of heat energy to overcome.

Why do ionic compounds dissolve in water but not in kerosene?

Water is a polar solvent that separates the ions; kerosene is non-polar and cannot.

Is CaO an ionic compound?

Yes. It is made of Ca²⁺ and O²⁻ ions formed by transfer of two electrons from calcium to oxygen.