NCERT Solutions for Class 9 Science Exploration Chapter 9

Atomic foundations of matter explain how atoms combine in fixed ratios to form molecules, ions, and compounds.
Chemical laws, bonding, formulae, molecular mass, and formula unit mass help describe the composition and behaviour of substances.

NCERT Solutions for Class 9 Science Exploration Chapter 9 help students solve Atomic Foundations of Matter from the 2026-27 Class 9 Science textbook. This chapter explains the Law of Conservation of Mass, Law of Constant Proportions, Dalton’s Atomic Theory, covalent bonding, ionic bonding, chemical formulae, properties of ionic and covalent compounds, molecular mass, and formula unit mass. These NCERT Class 9 Science Solutions cover Pause and Ponder questions and all Class 9 Science Chapter 9 exercise solutions in a direct, step-wise format.

Key Takeaways

• Mass conservation: Mass is neither created nor destroyed during a chemical reaction.
• Fixed composition: A compound always contains the same elements in a fixed ratio by mass.
• Bonding: Covalent bonds form by sharing electrons, while ionic bonds form by electron transfer.
• Formula writing: Chemical formulae are written using symbols, valencies, charges, and simplest whole-number ratios.

NCERT Solutions for Class 9 Science Exploration Chapter 9 Structure 2026

NCERT Solutions for Class 9 Science Exploration Chapter 9 In-Text Questions

Class 9 Science Exploration Chapter 9 uses examples such as salt solution, vinegar and baking soda, water, sodium chloride, oxygen, nitrogen, magnesium chloride, and copper sulfate to explain atomic foundations of matter.

Q1. Does burning ethanol in an open beaker violate the Law of Conservation of Mass?

Question: A student burns $10 , g$ of ethanol in an open beaker. After the reaction, no residue is left in the beaker. Does this mean the Law of Conservation of Mass is violated? Explain.

Answer: No, the Law of Conservation of Mass is not violated.

Explanation:
When ethanol burns, it reacts with oxygen and forms gases such as carbon dioxide and water vapour. These gases escape into the air because the beaker is open.

The mass seems to disappear only because the products are not collected.

If the reaction is carried out in a closed container, the total mass of reactants and products will remain the same.

Q2. How much water forms when hydrogen reacts with oxygen?

Question: When $20 , g$ of hydrogen reacts completely with $160 , g$ of oxygen, how much water is formed according to the Law of Conservation of Mass?

Answer: $180 , g$ of water is formed.

Explanation:
According to the Law of Conservation of Mass:

$Mass , of , reactants = Mass , of , products$

Total mass of reactants:

$20 , g + 160 , g = 180 , g$

So, mass of water formed:

$180 , g$

Q3. Find the mass of oxygen in a compound containing sulfur and oxygen.

Question: A compound consists of 40% sulfur and 60% oxygen by mass. In a sample of the same compound containing $20 , g$ of sulfur, what mass of oxygen must be present?

Answer: The mass of oxygen present is $30 , g$.

Explanation:
Sulfur and oxygen are present in the ratio:

$40 : 60 = 2 : 3$

If sulfur is $20 , g$, then oxygen is:

$20 \times \frac{3}{2} = 30 , g$

So, $30 , g$ of oxygen must be present.

Q4. How much oxygen combines with carbon in carbon monoxide?

Question: Carbon monoxide contains carbon and oxygen in the mass ratio $3 : 4$. How much oxygen will combine with $9 , g$ of carbon?

Answer: $12 , g$ of oxygen will combine with $9 , g$ of carbon.

Explanation:
Carbon : Oxygen = $3 : 4$

If carbon = $9 , g$, then oxygen:

$= 9 \times \frac{4}{3}$

$= 12 , g$

Q5. Why does the Law of Definite Proportions apply to compounds but not mixtures?

Answer: The Law of Definite Proportions applies to compounds because elements in a compound combine in a fixed ratio by mass.

Explanation:
In a compound, atoms combine chemically in definite proportions. For example, pure water always contains hydrogen and oxygen in the mass ratio $1 : 8$.

Mixtures do not have a fixed composition. Salt and water can be mixed in different amounts, so the ratio is not fixed.

Therefore, the law holds true for compounds but not for mixtures.

Q6. Do copper oxide samples prepared in ratios 4:1 and 8:2 justify the Law of Constant Proportions?

Answer: Yes, both results justify the Law of Constant Proportions.

Explanation:
The two ratios are:

$4 : 1$

$8 : 2 = 4 : 1$

Both ratios are the same after simplification.

This shows that copper and oxygen combine in a fixed ratio by mass to form the same oxide of copper.

Q7. Assertion and Reason on water formation and Dalton’s Atomic Theory

Question:
Assertion (A): $2 , g$ of hydrogen combines with $16 , g$ of oxygen to form $18 , g$ of water.
Reason (R): According to Dalton’s Atomic Theory, atoms combine in a simple whole number ratio by mass to form compounds.

Answer: The correct option is (iii) A is true, but R is false.

Explanation:
The assertion is true because $2 , g$ hydrogen and $16 , g$ oxygen combine to form $18 , g$ water.

The reason is false because Dalton’s Atomic Theory says atoms combine in simple whole-number ratios, not simple whole-number ratios by mass.

Mass ratios can be fixed, but they are not necessarily simple whole numbers.

Q8. Draw the structure of nitrogen molecule.

Question: Nitrogen has five valence electrons. Draw the structure of the nitrogen molecule $(N_2)$.

Answer: A nitrogen molecule has a triple covalent bond between two nitrogen atoms.

Explanation:
Each nitrogen atom has 5 valence electrons. It needs 3 more electrons to complete its octet.

Two nitrogen atoms share three pairs of electrons.

The structure is:

$N \equiv N$

This triple bond makes the nitrogen molecule stable.

Q9. Explain formation of fluorine molecule.

Question: The atomic number of fluorine is 9. Explain the formation of fluorine molecule $(F_2)$.

Answer: Two fluorine atoms form a fluorine molecule by sharing one pair of electrons.

Explanation:
Fluorine has atomic number 9.

Electronic configuration:

$2, 7$

Each fluorine atom has 7 valence electrons and needs 1 more electron to complete its octet.

Two fluorine atoms share one electron each and form a single covalent bond:

$F - F$

Q10. Show formation of carbon dioxide molecule.

Question: Show the formation of carbon dioxide $(CO_2)$.

Answer: Carbon dioxide is formed when carbon shares two pairs of electrons with each oxygen atom.

Explanation:
Carbon has 4 valence electrons. Oxygen has 6 valence electrons.

Carbon needs 4 electrons to complete its octet. Each oxygen needs 2 electrons.

Carbon forms two double covalent bonds with two oxygen atoms.

Structure:

$O = C = O$

Q11. Show formation of hydrogen sulfide molecule.

Question: Show the formation of hydrogen sulfide $(H_2S)$.

Answer: Hydrogen sulfide is formed when sulfur shares one electron each with two hydrogen atoms.

Explanation:
Hydrogen has 1 electron and needs 1 more to complete its duplet.

Sulfur has 6 valence electrons and needs 2 more to complete its octet.

Sulfur shares one electron with each hydrogen atom.

Structure:

$H - S - H$

Q12. Show formation of ammonia molecule.

Question: Show the formation of ammonia $(NH_3)$.

Answer: Ammonia is formed when nitrogen shares one electron each with three hydrogen atoms.

Explanation:
Nitrogen has 5 valence electrons and needs 3 more electrons to complete its octet.

Each hydrogen atom needs 1 electron to complete its duplet.

Nitrogen shares electrons with three hydrogen atoms.

Structure:

  H

   |

H - N - H

Q13. Why does neon neither transfer nor share electrons?

Answer: Neon neither transfers nor shares electrons because its outermost shell is already complete.

Explanation:
Neon has atomic number 10.

Electronic configuration:

$2, 8$

Its valence shell has an octet. Therefore, it is stable and largely unreactive.

Since it does not need to gain, lose, or share electrons, it normally does not form bonds.

Q14. What kind of ion will oxygen form?

Answer: Oxygen forms an oxide ion, $O^{2-}$.

Explanation:
Oxygen has atomic number 8.

Electronic configuration:

$2, 6$

It needs 2 more electrons to complete its octet.

So, oxygen gains 2 electrons and forms:

$O^{2-}$

Q15. Complete the magnesium chloride bonding statement.

Question: Among magnesium and chlorine, magnesium atom can give two electrons to become $Mg^{2+}$. However, chlorine can take only one electron to become ______. Now, ______ ion of magnesium and ______ ions of chlorine combine to give magnesium chloride.

Answer:
Among magnesium and chlorine, magnesium atom can give two electrons to become $Mg^{2+}$. However, chlorine can take only one electron to become $Cl^-$. Now, one ion of magnesium and two ions of chlorine combine to give magnesium chloride.

Explanation:
Magnesium loses 2 electrons and forms $Mg^{2+}$.

Each chlorine atom gains 1 electron and forms $Cl^-$.

To balance charge:

$Mg^{2+} + 2Cl^- \rightarrow MgCl_2$

Q16. Show formation of potassium chloride and calcium chloride.

Answer: Potassium forms $KCl$, while calcium forms $CaCl_2$.

Explanation:

Formation of potassium chloride:

Potassium has electronic configuration $2, 8, 8, 1$. It loses one electron to form $K^+$.

Chlorine gains one electron to form $Cl^-$.

$K^+ + Cl^- \rightarrow KCl$

Formation of calcium chloride:

Calcium has electronic configuration $2, 8, 8, 2$. It loses two electrons to form $Ca^{2+}$.

Two chlorine atoms each gain one electron to form two $Cl^-$ ions.

$Ca^{2+} + 2Cl^- \rightarrow CaCl_2$

Q17. Illustrate how sodium sulfide is formed.

Answer: Sodium sulfide is formed when two sodium atoms each transfer one electron to one sulfur atom.

Explanation:
Sodium has electronic configuration $2, 8, 1$. It loses one electron to form $Na^+$.

Sulfur has electronic configuration $2, 8, 6$. It needs two electrons to complete its octet and forms $S^{2-}$.

So, two sodium ions combine with one sulfide ion:

$2Na^+ + S^{2-} \rightarrow Na_2S$

Q18. Name the following covalent compounds.

Answer:

Q19. Write the formula for the given compounds.

Answer:

Q20. Write formulae for compounds formed from ions.

Question: Write the formulae for compounds formed from:
(i) $Fe^{3+}$ and $OH^-$
(ii) $K^+$ and $CO_3^{2-}$

Answer:
(i) $Fe(OH)_3$
(ii) $K_2CO_3$

Explanation:
For $Fe^{3+}$ and $OH^-$, three hydroxide ions are needed to balance one ferric ion.

$Fe^{3+} + 3OH^- \rightarrow Fe(OH)_3$

For $K^+$ and $CO_3^{2-}$, two potassium ions are needed to balance one carbonate ion.

$2K^+ + CO_3^{2-} \rightarrow K_2CO_3$

Q21. Identify the bond in a compound that conducts electricity in solution.

Question: What type of chemical bond is present in a solid compound that does not conduct electricity in solid state but conducts electricity when dissolved in water?

Answer: The compound has an ionic bond.

Explanation:
Ionic compounds do not conduct electricity in solid state because ions are fixed in the crystal lattice.

When dissolved in water, the ions become free to move. These mobile ions conduct electricity.

Q22. Predict formula, bond type, and conductivity for metal M.

Question: Metal M has two electrons in its valence shell and reacts with oxygen. Predict its formula, type of bond, and electrical conductivity of its aqueous solution.

Answer: The formula is $MO$, the bond is ionic, and its aqueous solution conducts electricity if enough compound dissolves.

Explanation:
Metal M loses two electrons to form $M^{2+}$.

Oxygen gains two electrons to form $O^{2-}$.

The ions combine in a 1:1 ratio:

$M^{2+} + O^{2-} \rightarrow MO$

Since ions are formed by electron transfer, the bond is ionic.

An aqueous solution conducts electricity because it contains ions.

Q23. Find molecular mass of nitric acid.

Question: Find the molecular mass of nitric acid $(HNO_3)$. Atomic masses: H = 1 u, N = 14 u, O = 16 u.

Answer: The molecular mass of nitric acid is $63 , u$.

Explanation:

$HNO_3 = (1 \times 1) + (14 \times 1) + (16 \times 3)$

$= 1 + 14 + 48$

$= 63 , u$

Q24. Find molecular mass of methane.

Question: Find the molecular mass of methane $(CH_4)$. Atomic masses: C = 12 u, H = 1 u.

Answer: The molecular mass of methane is $16 , u$.

Explanation:

$CH_4 = (12 \times 1) + (1 \times 4)$

$= 12 + 4$

$= 16 , u$

Q25. Find formula unit mass of potassium chloride.

Question: Find the formula unit mass of potassium chloride $(KCl)$. Atomic masses: K = 39 u, Cl = 35.5 u.

Answer: The formula unit mass of potassium chloride is $74.5 , u$.

Explanation:

$KCl = 39 + 35.5$

$= 74.5 , u$

Q26. Find formula unit mass of magnesium hydroxide.

Question: Find the formula unit mass of magnesium hydroxide, $Mg(OH)_2$. Atomic masses: Mg = 24 u, O = 16 u, H = 1 u.

Answer: The formula unit mass of magnesium hydroxide is $58 , u$.

Explanation:

$Mg(OH)_2 = 24 + 2[(16) + (1)]$

$= 24 + 2(17)$

$= 24 + 34$

$= 58 , u$

NCERT Solutions for Class 9 Science Exploration Chapter 9 Exercise Questions

The Revise, Reflect, Refine section includes questions on valence electrons, ionic and covalent bonding, formula writing, conservation of mass, ions, formula unit mass, and charge balance.

Q1. Element A has one electron in its third shell. Element B has six electrons in its second shell.

Q1(i). How many electrons does A tend to give or take?

Answer: A tends to give one electron.

Explanation:
Element A has one electron in its third shell. It can become stable by losing this one valence electron.

Q1(ii). What kind of ion would A form?

Answer: A forms a positive ion or cation, $A^+$.

Explanation:
When an atom loses one electron, it has one extra positive charge.

Q1(iii). How many electrons does B tend to give or take?

Answer: B tends to take two electrons.

Explanation:
Element B has six electrons in its second shell. It needs two more electrons to complete its octet.

Q1(iv). What kind of ion would B form?

Answer: B forms a negative ion or anion, $B^{2-}$.

Explanation:
When an atom gains two electrons, it forms an ion with two negative charges.

Q1(v). What kind of bond would be formed between A and B?

Answer: An ionic bond would be formed.

Explanation:
A transfers electrons and B accepts electrons. Bonding by electron transfer forms an ionic bond.

Q1(vi). What is the formula of the compound formed?

Answer: The formula is $A_2B$.

Explanation:
Two $A^+$ ions are needed to balance one $B^{2-}$ ion.

$2A^+ + B^{2-} \rightarrow A_2B$

Q2. Element X has six electrons in its outer shell and forms a diatomic molecule.

Q2(i). Why does X form a diatomic molecule?

Answer: X forms a diatomic molecule because each atom needs two more electrons to complete its octet.

Explanation:
An atom with six valence electrons shares two electrons with another atom of the same element.

Q2(ii). What kind of bond would it form?

Answer: It forms a covalent bond.

Explanation:
The atoms share electrons, so the bond is covalent.

Q2(iii). Draw the structure of the molecule.

Answer: The molecule has a double bond.

Explanation:
Each atom shares two electrons with the other atom.

Structure:

$X = X$

This is similar to the oxygen molecule, $O_2$.

Q2(iv). Element Y has two electrons in its second shell. Draw the structure of the molecule X forms with Y.

Answer: Element Y has a complete second shell, so it is stable and generally does not form a molecule with X.

Explanation:
If Y has two electrons in its second shell, its configuration is $2, 2$ only if total electrons are 4, but if the question means “only second shell is filled with 2 electrons,” Y would need more context.

For the usual Class 9 interpretation, an atom with a complete outer shell does not share or transfer electrons. If Y is beryllium with configuration $2, 2$, it can form a compound with X by electron transfer. If X is oxygen-like, then the likely formula is $YX$.

Q3. Which ion combinations give total positive charge $6+$ and total negative charge $6-$?

Answer: The correct combinations are (iii) and (iv).

Explanation:

Option (i):
$2Al^{3+} = 6+$, but $3Cl^- = 3-$
Charges do not balance.

Option (ii):
$3Mg^{2+} = 6+$, but $1PO_4^{3-} = 3-$
Charges do not balance.

Option (iii):
$2Fe^{3+} = 6+$ and $3O^{2-} = 6-$
Charges balance.

Option (iv):
$3Ca^{2+} = 6+$ and $2SO_4^{2-} = 4-$
Charges do not balance.

So, only (iii) gives total $6+$ and $6-$.

Q4. Choose correct statements and correct the false ones.

Answer:

Q5. Write chemical formulae for the compounds.

Answer:

Q6. Write formulae of compounds formed from ion pairs.

Answer:

Q7. Which diagram correctly represents $Cl^-$ ion?

Answer: The correct diagram should show 18 electrons arranged as $2, 8, 8$.

Explanation:
Chlorine has atomic number 17, so a neutral chlorine atom has 17 electrons.

When chlorine gains one electron, it forms $Cl^-$.

Total electrons in $Cl^-$:

$17 + 1 = 18$

Electronic configuration:

$2, 8, 8$

So, the correct figure is the one with 18 electrons arranged in three shells as 2, 8, 8.

Q8. Determine formula unit mass of the given substances.

Q8(i). Ammonium nitrate, $NH_4NO_3$

Answer: The formula unit mass is $80 , u$.

Explanation:
Atomic masses: N = 14 u, H = 1 u, O = 16 u

$NH_4NO_3 = (14 \times 2) + (1 \times 4) + (16 \times 3)$

$= 28 + 4 + 48$

$= 80 , u$

Q8(ii). Phosphoric acid, $H_3PO_4$

Answer: The formula mass is $98 , u$.

Explanation:
Atomic masses: H = 1 u, P = 31 u, O = 16 u

$H_3PO_4 = (1 \times 3) + 31 + (16 \times 4)$

$= 3 + 31 + 64$

$= 98 , u$

Q8(iii). Sodium hydrogencarbonate, $NaHCO_3$

Answer: The formula unit mass is $84 , u$.

Explanation:
Atomic masses: Na = 23 u, H = 1 u, C = 12 u, O = 16 u

$NaHCO_3 = 23 + 1 + 12 + (16 \times 3)$

$= 23 + 1 + 12 + 48$

$= 84 , u$

Q9. Write formulae for compounds formed by reaction of elements.

Answer:

Q10. Complete the cation-anion formula table.

Answer:

Q11. Verify the Law of Conservation of Mass.

Question: $5.3 , g$ sodium carbonate and $6.0 , g$ acetic acid react to produce $2.2 , g$ carbon dioxide, $0.9 , g$ water, and $8.2 , g$ sodium acetate.

Answer: The Law of Conservation of Mass is valid.

Explanation:
Total mass of reactants:

$5.3 + 6.0 = 11.3 , g$

Total mass of products:

$2.2 + 0.9 + 8.2 = 11.3 , g$

Since:

$Mass , of , reactants = Mass , of , products$

the Law of Conservation of Mass is obeyed.

Q12. Species has 11 protons, 12 neutrons, and 10 electrons.

Q12(i). What are atomic number and mass number?

Answer: Atomic number is 11 and mass number is 23.

Explanation:
Atomic number = number of protons = 11

Mass number:

$A = Protons + Neutrons$

$A = 11 + 12 = 23$

Q12(ii). Is it neutral, cation, or anion?

Answer: It is a cation.

Explanation:
The species has 11 protons but only 10 electrons.

There is one extra positive charge, so it is a cation.

Q12(iii). Write its electronic configuration.

Answer: Its electronic configuration is $2, 8$.

Explanation:
It has 10 electrons.

So, electron distribution is:

K-shell = 2

L-shell = 8

Q12(iv). Name the species.

Answer: The species is sodium ion, $Na^+$.

Explanation:
Atomic number 11 belongs to sodium. Since it has lost one electron, it is $Na^+$.

Q13. Elements A and B have configurations A: 2, 8, 5 and B: 2, 8, 7.

Q13(i). Which element is more reactive?

Answer: Element B is more reactive.

Explanation:
B has 7 valence electrons and needs only 1 electron to complete its octet.

A has 5 valence electrons and needs 3 electrons to complete its octet.

Since B needs fewer electrons to become stable, it is more reactive.

Q13(ii). Will A and B form ionic or covalent bonds?

Answer: A and B will form covalent bonds.

Explanation:
Both A and B are non-metal-like elements because they have more than four valence electrons.

They tend to gain or share electrons rather than lose them.

So, they form bonds by sharing electrons.

Q13(iii). Predict the formula of the compound.

Answer: The formula is likely $AB_3$.

Explanation:
A has valency 3 and B has valency 1.

So, one atom of A combines with three atoms of B.

Formula:

$AB_3$

Q14. Assertion and Reason on copper sulfate conductivity

Question:
Assertion (A): Copper sulfate conducts electricity in the molten state but not in the solid state.
Reason (R): Copper and sulfate ions are fixed in the lattice in molten state, while in solid state they can move freely.

Answer: The correct option is (iii) A is true, but R is false.

Explanation:
The assertion is true. Copper sulfate is an ionic compound. It conducts electricity in molten state because ions are free to move.

The reason is false because ions are fixed in the solid state and free to move in the molten state.

Q15. Find electrons and neutrons in $^{27}Al$, $^{80}Br^-$, and $^{201}Hg^{2+}$.

Answer:

Explanation:
For neutral aluminium:

$Electrons = Protons = 13$

$Neutrons = 27 - 13 = 14$

For $Br^-$:

$Electrons = 35 + 1 = 36$

$Neutrons = 80 - 35 = 45$

For $Hg^{2+}$:

$Electrons = 80 - 2 = 78$

$Neutrons = 201 - 80 = 121$

NCERT Solutions for Class 9 Science Exploration

Topics Covered in NCERT Solutions for Class 9 Science Exploration Chapter 9

Class 9 Science Exploration Chapter 9 covers chemical laws, atomic theory, bonding, formula writing, and mass calculations. The chapter connects atomic structure from Chapter 8 with how atoms form compounds.

• Law of conservation of mass Class 9
• Law of constant proportions Class 9
• Dalton atomic theory Class 9
• Postulates of Dalton’s Atomic Theory
• Molecules of elements and compounds
• Chemical bond
• Covalent bond Class 9
• Single, double, and triple bonds
• Naming covalent compounds
• Ionic bond Class 9
• Cations and anions
• Formation of sodium chloride
• Crystal lattice of ionic compounds
• Chemical formulae Class 9
• Formulae of covalent compounds
• Formulae of ionic compounds
• Properties of ionic and covalent compounds
• Molecular mass Class 9
• Formula unit mass Class 9

Important Concepts in NCERT Solutions for Class 9 Science Exploration Chapter 9

Atomic Foundations of Matter Class 9 explains how atoms combine and why compounds have fixed compositions. These concepts help students solve formula-writing, bonding, and mass-based questions.

Important Formulas in NCERT Solutions for Class 9 Science Exploration Chapter 9

Class 9 Science Chapter 9 solutions use formulas for mass conservation, fixed proportions, molecular mass, and formula unit mass.