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YOUR STUDY SPACE · CBSE

Atomic Foundations of Matter — Class 9 Science

Exploration · Chapter 9

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Chapter summary

This chapter explains how atoms combine into molecules and compounds and how to describe them quantitatively. Two basic laws are established by experiment: the law of conservation of mass (matter is neither created nor destroyed in a chemical reaction, so total mass of reactants equals total mass of products, proposed by Lavoisier) and the law of constant (definite) proportions (a compound always contains its elements in a fixed ratio by mass whatever its source, e.g. water is always hydrogen to oxygen 1:8, proposed by Proust). These laws underlie Dalton’s atomic theory, whose postulates state that matter is made of indivisible atoms that combine in simple whole-number ratios. A molecule is an electrically neutral group of two or more atoms that can exist independently. Atoms bond to become stable by completing their octet: covalent bonds form by sharing electrons (single, double or triple bonds, as in , , , ), while ionic bonds form by transfer of electrons creating cations and anions held by electrostatic attraction (as in NaCl, which forms a 3-D crystal lattice). The chapter teaches naming of covalent and ionic compounds and writing chemical formulae by criss-crossing valencies. Ionic compounds are usually water-soluble, high-melting and conduct electricity when dissolved or molten, while covalent compounds are usually low-melting and non-conducting. Finally it shows how to calculate molecular mass (for covalent compounds) and formula unit mass (for ionic compounds) by adding atomic masses.
Laws of conservation of mass and constant proportionsDalton’s atomic theoryCovalent bonding (sharing of electrons)Ionic bonding and properties of compoundsChemical formulae, molecular and formula unit mass

Key ideas & terms

Law of conservation of mass
In a chemical reaction matter is neither created nor destroyed, so the total mass of the products equals that of the reactants.
Law of constant proportions
A given compound always contains the same elements combined in a fixed ratio by mass, regardless of its source.
Covalent bond
A bond formed when atoms share one or more pairs of electrons to complete their octets, e.g. in water and chlorine molecules.
Ionic bond
The electrostatic force of attraction between oppositely charged ions formed by the transfer of electrons, e.g. in sodium chloride.
Molecular mass
The sum of the atomic masses of all atoms in a molecule of a covalent compound, e.g. water .
Formula unit mass
The sum of the atomic masses of the ions in the simplest whole-number ratio of an ionic compound, e.g. .

Extra questions & answers

State the law of conservation of mass.

In any chemical reaction, mass can neither be created nor destroyed, so the total mass of the reactants equals the total mass of the products.

Why did the balloon experiment with vinegar and baking soda show no mass change only when sealed?

Carbon dioxide gas is released in the reaction; in an open flask it escapes and the mass appears to fall, but when the gas is trapped in a balloon the total mass stays constant, confirming the law.

State the law of constant proportions with an example.

A compound always has its elements in a fixed mass ratio whatever its source; for instance, pure water from any source always contains hydrogen and oxygen in the mass ratio 1:8.

What is a molecule?

A molecule is an electrically neutral particle made of two or more atoms chemically bonded together that can exist independently and shows the properties of the substance.

How does a covalent bond form? Give an example.

It forms when atoms share pairs of valence electrons to complete their octets; for example, two hydrogen atoms share one electron each to form an molecule.

How does an ionic bond form? Give an example.

It forms when one atom transfers electrons to another, producing a cation and an anion that are held together by electrostatic attraction; for example sodium gives an electron to chlorine to form NaCl.

Why do ionic compounds conduct electricity in solution but not as solids?

In the solid the ions are fixed in the lattice, but when dissolved in water or melted the ions become free to move and carry charge, so the solution or melt conducts electricity.

How do you write the formula of an ionic compound by criss-crossing valencies?

Write the cation then the anion, place their valencies as subscripts crossed over to each other, and simplify to the lowest whole-number ratio, e.g. and give .

Calculate the molecular mass of carbon dioxide (). (C = 12 u, O = 16 u)

Molecular mass .

What is formula unit mass and why is it used for ionic compounds?

It is the sum of the atomic masses in the simplest formula unit; it is used because ionic compounds form continuous crystal lattices rather than discrete molecules.

Think It Over

Water can be obtained from various sources. Are all these samples of water chemically identical?

Yes. Pure water is always the same compound . Regardless of source - river, borewell or ocean - after purification it contains hydrogen and oxygen in the fixed mass ratio and shows the same chemical properties. Impurities may differ, but the water itself is identical, as required by the Law of Constant Proportions.

Oxygen is sometimes represented as O and sometimes as . What is the difference between these symbols?

represents a single atom of oxygen. represents a molecule of oxygen made of two oxygen atoms joined by a double covalent bond. cannot exist freely under normal conditions; atmospheric oxygen exists as molecules.

Why does dissolved salt in water conduct electricity, but sugar does not?

Salt such as sodium chloride is an ionic compound. In water it dissociates into free mobile ions and , which carry charge and conduct electricity. Sugar is a covalent compound. It dissolves as neutral molecules and does not produce ions, so its solution does not conduct electricity.

Think as a Scientist

You are given a chemical reaction in which zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas. Zinc + Hydrochloric acid (dilute) Zinc chloride + Hydrogen Design and perform an experiment to test the hypothesis that mass is conserved during the chemical reaction. You may use a set-up different from the one shown in Activity 9.2.

Aim: To verify that total mass before and after the reaction remains constant. Procedure: Take a small conical flask with about dilute and a small piece of zinc metal. Place the flask and a balloon containing the zinc together on a digital balance and note the initial mass. Attach the balloon to the mouth of the flask without dropping the zinc, so the system is closed. Lift the balloon to drop zinc into the acid. Effervescence of inflates the balloon but gas cannot escape. Weigh the closed system again. Observation: Initial and final readings are same within experimental error. Conclusion: Mass is conserved. If the flask were left open, would escape and the apparent mass would decrease, illustrating the need for a closed system.

Pause and Ponder

1. 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.

No. The law is not violated. Ethanol burning is . In an open beaker the gaseous products and water vapour escape into the air, and oxygen from the air is a reactant that was not weighed. If the mass of oxygen used and all gaseous products are counted in a closed container, total mass of reactants equals total mass of products.

2. 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?

By conservation of mass, mass of products equals mass of reactants. Here . If reaction is complete and water is the only product, of is formed. This also corresponds to the fixed ratio by mass in water.

Pause and Ponder

3. 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 to satisfy the Law of Constant Proportions?

The mass ratio . For of sulfur, required oxygen is . Any pure sample must contain of oxygen per of sulfur.

4. Carbon monoxide (CO) contains carbon and oxygen in the mass ratio of 3:4. How much oxygen will combine with 9 g of carbon to form carbon monoxide?

Ratio . For carbon, oxygen needed is . So of oxygen combines with carbon to give of .

5. The Law of Definite Proportions holds true for compounds but not for mixtures. Give reason.

A compound is formed by chemical combination of elements in a fixed atomic ratio, so its composition by mass is always fixed. A mixture is a physical intermixing of substances in any proportion without chemical bonding, so its composition can be varied continuously. Hence the law applies only to compounds.

6. Students X and Y, both prepared an oxide of copper by combining copper and oxygen in the ratios of 4:1 and 8:2, respectively. Do their results justify the Law of Constant Proportions? Explain.

Yes. Both ratios simplify to the same ratio . Both students obtained copper and oxygen in the same fixed proportion by mass in the product, which is exactly what the Law of Constant Proportions predicts. The absolute masses differ but the ratio is constant.

Pause and Ponder

7. 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. Choose the correct option: (i) Both A and R are true, and R is the correct explanation of A. (ii) Both A and R are true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.

Assertion is true: water is with by mass, so . Dalton's theory states atoms combine in simple whole number ratio by number, not explicitly by mass, though this leads to a fixed mass ratio. The ratio by mass is a consequence of constant atomic masses. Therefore R as phrased is not strictly correct; atoms combine in simple whole number ratio by number. So A is true but R is false. Correct option is (iii).

Pause and Ponder

8. Nitrogen has five valence electrons. Draw the structure of the nitrogen molecule ().

Atomic number of nitrogen is 7, electronic configuration . Each N needs 3 electrons to complete the octet, so two N atoms share three pairs of electrons. Structure is with a triple covalent bond: each atom contributes 3 electrons, forming 3 shared pairs leaving one lone pair on each N. Dot diagram shows six shared electrons in the middle and two unshared electrons on each atom.

9. The atomic number of fluorine is 9. Explain the formation of the fluorine molecule ().

Fluorine configuration is , with 7 valence electrons, needing one more to attain an octet. Two fluorine atoms each share one electron to form one shared pair. The resulting molecule has a single covalent bond, represented as . Each F now has 8 electrons around it, 6 non-bonding and 2 shared.

Pause and Ponder

10. Show the formation of the following molecules: (i) Carbon dioxide () (ii) Hydrogen sulfide () (iii) Ammonia ()

(i) : Carbon needs 4 electrons, each oxygen needs 2. Carbon shares two electrons with each oxygen, forming two double bonds . (ii) : Sulfur needs 2 electrons, each hydrogen needs 1. Sulfur shares one electron with each of two H atoms, forming two single bonds , with two lone pairs remaining on S. (iii) : Nitrogen needs 3 electrons, each hydrogen needs 1. Nitrogen shares one electron with each of three H atoms, forming three single bonds, with one lone pair left on N, giving a pyramidal molecule.

11. Neon (atomic number 10) neither transfers nor shares its valence electrons. Explain.

Neon configuration is . Its outermost shell already has a complete octet of 8 electrons and is fully stable. It has no tendency to lose, gain or share electrons, so it does not form chemical bonds under normal conditions and exists as a monoatomic gas.

Pause and Ponder

12. What kind of ion will oxygen (O) form?

Oxygen has configuration with 6 valence electrons. It needs 2 electrons to complete its octet, so it gains 2 electrons to form an anion with charge, oxide ion .

13. Fill in the blanks. Among magnesium and chlorine, magnesium atom can give two electrons to become . However, chlorine can take only one electron to become ____________. Now, __________ ion of magnesium and __________ ions of chlorine combine to give magnesium chloride.

Chlorine can take only one electron to become . Now, one ion of magnesium and two ions of chlorine combine to give magnesium chloride .

14. Show the formation of cations of potassium (K) and calcium (Ca) atoms, and the formation of their corresponding chlorides using diagrams.

Potassium loses one electron: . Chlorine gains it: . Then , ionic bond by single electron transfer. Calcium loses two electrons: . Each chlorine gains one electron, so two atoms are needed: . Then . Dot diagrams show transfer of outer electrons from metal to non-metal.

15. Illustrate how sodium sulfide () is formed.

Sodium loses one electron to become . Sulfur needs two electrons to become . Two sodium atoms each donate one electron to one sulfur atom: , . Electrostatic attraction between and forms ionic compound .

Pause and Ponder

16. Name the following: (i) _______________________________ (ii) _______________________________ (iii) _______________________________ (iv) _______________________________

(i) = carbon dioxide (ii) = nitrogen dioxide (iii) = sulfur hexafluoride (iv) = phosphorus trichloride

17. Write the formula for the following: (i) Sodium hydrogencarbonate _____________________ (ii) Sulfur dioxide ____________________________________ (iii) Ferric chloride ___________________________________ (iv) Cuprous oxide ___________________________________

(i) Sodium hydrogencarbonate = (ii) Sulfur dioxide = (iii) Ferric chloride = (ferric is ) (iv) Cuprous oxide = (cuprous is , oxide is )

18. Write the formulae for the compounds formed from the following pairs of ions: (i) and (ii) and

(i) with criss-cross charges 3 and 1 to give . (ii) with criss-cross to give .

Pause and Ponder

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

Ionic bond. In the solid state ions are locked in a fixed lattice and cannot move, so no conduction. When dissolved in water the lattice breaks and ions become free to move, allowing electrical conduction.

20. Metal M, with two electrons in its valence shell (M shell), reacts with oxygen to form a compound that is slightly soluble in water. Predict its: (i) formula (ii) type of bond (iii) electrical conductivity of its aqueous solution.

Metal M has configuration so it is magnesium-like, valency 2. (i) Formula with oxide is (e.g., ). (ii) It loses 2 electrons to oxygen, forming an ionic bond. (iii) Its aqueous solution, though slightly soluble, will contain mobile and or rather after reaction with water, so it will conduct electricity but weakly due to low solubility.

Pause and Ponder

21. Find the molecular mass of nitric acid (). Atomic mass — H = 1 u; N = 14 u; O = 16 u.

Molecular mass of .

22. Find the molecular mass of methane (). Atomic mass — C = 12 u; H = 1 u.

Molecular mass of .

Pause and Ponder

23. Find the formula unit mass of potassium chloride (KCl). Atomic mass — K = 39 u; Cl = 35.5 u.

Formula unit mass of .

24. Find the formula unit mass of magnesium hydroxide, . Atomic mass — Mg = 24 u; O = 16 u; H = 1 u.

Formula unit mass of .

Revise, Reflect, Refine

1. A particular element (A) has one electron in its third shell. There is another element (B) with six electrons in its second shell. (i) How many electrons does A tend to give or take to become stable? (ii) What kind of ion would it form? (iii) How many electrons does B tend to give or take to become stable? (iv) What kind of ion would it form? (v) If A and B were to combine, what kind of bond would be formed? (vi) What would be the formula for the compound thus formed?

A is (sodium-like), B is (oxygen-like). (i) A tends to give 1 electron to achieve octet. (ii) It forms a cation . (iii) B tends to gain 2 electrons. (iv) It forms an anion . (v) Transfer of electrons gives an ionic bond. (vi) To balance charges, two with one gives formula (e.g., ).

2. An element X has six electrons in its outer shell and forms a diatomic molecule. (i) Why would that be so? (ii) What kind of bond would it form? (iii) Draw the structure of the molecule it would form. (iv) A certain other element Y has two electrons in its second shell. Draw the structure of the molecule that X would form with Y.

X has 6 valence electrons (like oxygen), needs 2 more for octet. (i) By sharing 2 electrons with another X atom, both achieve octet, so it exists as . (ii) It forms a covalent bond, specifically a double bond with two shared pairs. (iii) Structure is with two lone pairs on each X, dot structure shows 4 shared electrons. (iv) Y has configuration (beryllium/magnesium-like) valency 2. It donates 2 electrons to X, but since X needs covalent sharing, actually Y as metal would transfer electrons forming ionic . Covalently if both non-metals, Y would share: For X needing 2 and Y needing 6 to fill, they would form with two pairs shared? More simply as ionic: ionic lattice. Diagram shows Y losing 2 electrons to X.

3. You want to design a new ionic compound, where the total positive charge is 6+ and the total negative charge is 6 –. Which of the following combinations gives the correct number of ions? (i) 2 and 3 (ii) 3 and 1 (iii) 2 and 3 (iv) 3 and 2

Total positive must equal 6 and total negative must equal 6. (i) but , not balanced. (ii) but . (iii) and , balanced. (iv) but . So only (iii) gives correct 6+ and 6- balanced compound .

4. Choose the correct statement(s) and correct the false statement(s). (i) Elements are made up of molecules and compounds are made up of atoms. (ii) The molecule of a compound is always made up of two or more atoms of the same kind. (iii) One molecule of nitrogen gas contains three nitrogen atoms. (iv) Water is made of two hydrogen atoms, covalently bonded with one oxygen atom.

(i) False. Elements are made up of atoms (same kind), compounds are made up of molecules containing different kinds of atoms. (ii) False. A compound molecule is made of two or more atoms of different kinds, e.g., , . (iii) False. One molecule of nitrogen gas is diatomic , containing two atoms; triple bonded. (iv) True. Water is , two H covalently single-bonded to one O.

5. Write the chemical formulae for the following compounds. (i) Aluminium nitrate (ii) Calcium oxide (iii) Ferric oxide

(i) Aluminium nitrate: + . (ii) Calcium oxide: + . (iii) Ferric oxide: + after criss-cross and simplification.

6. Write the formulae of the compounds formed from the following pairs of ions. (i) and (ii) and (iii) and (iv) and

(i) (ii) (iii) (iv)

7. Which of the following, in Fig. 9.18, correctly represents ion (Atomic number of chlorine = 17). (i) (ii) (iii) (iv)

Neutral Cl has protons and electrons (configuration ). has gained one electron, so protons and electrons (configuration ). The correct diagram is the one showing a nucleus with +17 and 18 electrons arranged as 2 in first shell, 8 in second, 8 in valence shell.

8. Determine the formula unit mass of the following substances. (i) Ammonium nitrate (), used as a nitrogen fertiliser, which is essential for plant growth. (ii) Phosphoric acid (), used to make phosphate fertiliser and detergents. (iii) Sodium hydrogencarbonate (), used to relieve acidity and helps in digestion.

Using atomic masses N=14, H=1, O=16, Na=23, P=31. (i) . (ii) . (iii) .

9. Write the formulae for the compounds formed by the reaction of: (i) Magnesium and nitrogen (ii) Lithium and nitrogen (iii) Sodium and sulfur (iv) Aluminium and oxygen

(i) Magnesium + nitride . (ii) Lithium + . (iii) Sodium + sulfide . (iv) Aluminium + oxide .

10. Complete the Table 9.3 by writing the formulae of the compounds formed by the cations on the left and the anions at the top. is given as an example. Table 9.3

By criss-crossing charges: Row : , , . Row : , , . Row : , , . Row : , , .

11. 5.3 g of sodium carbonate and 6.0 g of acetic acid react to produce 2.2 g of carbon dioxide, 0.9 g of water, and 8.2 g of sodium acetate. Verify whether the law of conservation of mass is valid.

Total reactants = . Total products = . Mass of reactants equals mass of products, so Law of Conservation of Mass is obeyed.

12. If a species has 11 protons, 12 neutrons and 10 electrons then (i) what is its atomic number and mass number? (ii) is it neutral, a cation or an anion? Explain. (iii) write its electronic configuration. (iv) name the species.

(i) Atomic number = number of protons = 11. Mass number = protons + neutrons = . (ii) It has one fewer electron than protons, net charge +1, so it is a cation. (iii) With 10 electrons configuration is , like neon. (iv) Species is sodium ion (.)

The Journey Beyond

13. Two elements, A and B, have the following configurations — A: 2, 8, 5 B: 2, 8, 7 (i) Which element is more reactive? (ii) Will A and B form ionic or covalent bonds when they combine? Explain using electron transfer or sharing. (iii) Predict the formula of the compound they would form.

A is phosphorus-like (5 valence electrons), B is chlorine-like (7 valence electrons). (i) B is more reactive because it needs only one electron to achieve octet, while A needs three. (ii) Both are non-metals with high electronegativity and tendency to share rather than transfer, so they form covalent bonds by sharing electrons. (iii) A needs 3 electrons, B needs 1 each, so one A shares with three B atoms: formula (like ).

14. 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. Choose the correct option: (i) Both A and R are true, and R is the correct explanation of A. (ii) Both A and R are true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.

Assertion is true: ionic solid does not conduct when solid but does when molten due to mobile ions. Reason is false and reversed: in solid state ions are fixed, in molten state they are free to move. So correct option is (iii) A is true, but R is false.

15. The species , and have 13, 35 and 80 protons, respectively. How many electrons and neutrons do they have?

: neutral, protons 13, electrons 13, neutrons = 27-13 = 14. : protons 35, charge -1 means one extra electron: electrons 36, neutrons = 80-35 = 45. : protons 80, charge +2 means two fewer electrons: electrons 78, neutrons = 201-80 = 121.