ATOMS AND MOLECULES NCERT 9 SAMPLE PAPER

Q1. Write the chemical formulae of the following compounds:- (i)Ammonium sulphate (ii)Magnesium carbonate

Solution

(i)             Ammonium sulphate – (NH4)2SO4 

(ii)           Magnesium carbonate – MgCO3

Q2. What are the rules for writing the chemical formulae?

Solution

Rules for writing the chemical formulae Step 1 :  Write the symbol of a basic radical (element with a positive valency) on the left hand side and that of the acidic radical (element with a negative valency) on the right hand side. Step 2 : Write the valency number/charge of each of the respective ions at the bottom of its symbol. Step 3 : Interchange the valency number. Ignore the (+) and (-) sign. Step 4 :  Write the interchanged number. Step 5 :  Write the compound’s formula. Step 6: Cross the reduced valencies. If 1 appears, ignore it.  And if a group of atoms receives a valency number more than 1, enclose it within brackets.  

Q3. Oxygen always makes up 8/9 of the mass of pure sample of water. This corresponds to which law? What is the combining ratio of Hydrogen by mass in water?

Solution

This statement is in accordance with the Law of definite proportion. Hydrogen always makes up 1/9 of the mass of pure sample of water.

Q4. Explain the mechanism behind writing the formula of the following compounds: 1. Magnesium oxide 2. Hydrogen chloride 3. Carbon dioxide

Solution

1. Magnesium oxide: It is an ionic compound. While writing the formula of an ionic compound, the metal element is written on the left hand side and the non-metal element is written on the right hand side. Hence, the formula will be MgO. 2. Hydrogen chloride: It is a molecular compound. While writing the formula of a molecular compound, the less electronegative non-metal is written on the left hand side whereas the more electronegative non-metal is written on the right hand side. Hence, the formula will be HCl. 3. Carbon dioxide: It is a molecular compound. While writing the formula of a molecular compound, the less electronegative non-metal is written on the left hand side whereas the more electronegative non-metal is written on the right hand side. It has a prefix ‘di’ in the name means that the compound has 2 oxygen atoms. Hence, the formula will be CO2.

Q5. Explain giving examples what is a molecule of a compound.

Solution

The molecule of a compound contains two or more different types of atoms chemically combined together. For example: carbon dioxide (CO2) is a compound whose molecule consists of a carbon atom (C) and 2 oxygen atoms (O).

Q6. (a)What are polyatomic ions? Give one example each of polyatomic cation and anion. (b) What do you understand from the statement “relative atomic mass of sulphur is 32”? (c) What is meant by the term ‘chemical formula’?

Solution

(a) A group of atoms carrying a charge is known as polyatomic ion, example of polyatomic cation: NH4+ and polyatomic anion: NO3.

(b) It means an atom of sulphur is 32 times heavier than 1/12th the mass of 1 Carbon-12 atom.

(c) Chemical formula represents the composition of a molecule of the substance in terms of the symbols of the elements present in the molecule.

Q7. Define the following along with one example each: 1. Monatomic molecule2. Diatomic molecule3. Triatomic molecule

Solution

Type of molecule Definition Example Monoatomic molecule It consists of only one atom. Neon (Ne) Diatomic molecule It consists of two atoms which may be same or different. Hydrogen (H2) Triatomic molecule It consists of three atoms which may be same or different. Ozone (O3)

Q8. What are Ions (or radicals)? Explain in details its types with the help of an example.    

Solution

Ions (or Radicals) An atom or a group of atoms can exist independently with charge(s). These are formed by the loss or gain of electron(s). They are called radicals or more commonly as ions. Types of Ions or Radicals Ions are either positively charged or negatively charged. Positively charged ions are called cations. Example: Sodium ion (Na+)  Negatively charged ions are called anions. Example: Chloride ion (Cl).  Sometimes, groups of atoms also give up or accept electrons forming positive or negative groups of ions. Such groups of atoms having a positive or negative charge are called radicals. The single charged atoms are called monoatomic ions, e.g. Potassium ion, K+.  The double charged atoms are called diatomic ions, e.g.  Magnesium ion, Mg2+. The triple charged atoms are called triatomic ions, e.g. Nitride ion, N3-. A group of atoms that have a net charge on them are called polyatomic ions, e.g. ammonium ion NH4+, hydroxide ion OH, sulphate ion SO42-.

Q9. What do the following abbreviations stands for?(a) 2O(b) O2

Solution

(a) Two atoms of oxygen.

(b) One molecule of oxygen.

Q10. What is variable valency? Name three elements exhibiting variable valency.    

Solution

Variable Valency Sometimes, the same element may exhibit one valency in one compound and another valency in other compound. This property is called variable valency. Example Element Symbol Valencies exhibited (variable valencies)   Copper Cu 1, 2 Cu+1, Cu+2 Silver Ag 1, 2 Ag+1, Ag+2 Gold Au 1, 3 Au+1, Au+3 Iron Fe 2, 3 Fe+2, Fe+3  

Q11. (i)What do the following symbols formulae stand for : (a)2O (b) O(c)O(d)H2O (ii) Give the chemical formulae of the following compounds. (a) Potassium Carbonate(b) Calcium chloride (iii) Calculate the formula unit mass of Al2(SO4)3 (Given atomic mass of Al – 27u, S – 32u, O – 16u)

Solution

(i) (a)20 – Two atoms of oxygen (b) O2 molecule – Diatomic oxygen (c) O3 molecule – Triatomic oxygen (d) H2O – Two atoms of hydrogen and 1 atom of oxygen forming one molecule of water   (ii)K2CO– Potassium carbonate     CaCl2– Calcium Chloride   (iii) Al2(SO4)3 Al – 27 x 2= 54 u S – 32 x 3 = 96 u O – 16 x 12=192 u   Formula unit mass = 54 + 96 + 192 = 342 u

Q12. What does the charge indicate?

Solution

The charge indicates the valency of an ion. The valencies of ions and radicals are useful in writing the chemical formulae of the compounds. Magnesium ion is written as Mg2+, where the 2+ charge indicates that its valency is +2. Sulphate ion is written as SO42-, where the 2- charge indicates that its valency is -2.

Q13. (i)Give an example each of a monoatomic and a polyatomic element. (ii)How many atoms are present in CaCl2 molecule and SO42- ion?

Solution

(i) Monoatomic element – Carbon     Polyatomic element – Sulphur (ii) CaCl2 : atomicity – 3      SO42- : atomicity – 5

Q14. What is Avogadro’s number?

Solution

The number 6.022 x 1023, which represents a mole, is known as Avogadro’s number.

Q15. Explain the atomicity shown by noble gases.

Solution

Noble gases like helium, neon, argon, kypton are chemically unreactive and exist in the free state as single atoms i.e. they have one atom each in their molecules for ex: He, Ne, Ar and Kr. So, the atomicity of noble gases is 1.

Q16. Write the chemical symbols of two elements: (a) Which are formed from the first letter of the element’s name. (b) Whose name has been taken from the name of the element in Latin.

Solution

(a)C – Carbon, N – Nitrogen. (b)Na – Natrium, Fe – Ferrum.

Q17. Write the Latin name and symbol of (i) Gold (ii) Lead.

Solution

Gold: Symbol – Au and Latin name – Aurum Lead: Symbol – Pb and Latin name – Plumbum

Q18. Classify the following as ‘molecule of element’ or ‘molecule of compound’: NH3, Br2, CO2, O2

Solution

Molecule of a compound: NH3, CO2 Molecule of an element: Br2, O2

Q19. What is Dalton’s Atomic Theory? Write the main postulates of Dalton’s Atomic Theory.

Solution

Dalton’s Atomic Theory Dalton’s theory is the basic theory about the nature of matter. According to his theory, all matter, whether a solid, liquid or gas or an element, compound or mixture, is composed of small particles called atoms. The Postulates of Dalton’s Atomic Theory All matter is made up of very tiny particles called atoms. Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction. The atoms of a given element are identical in mass and chemical properties. Atoms of different elements have different masses and chemical properties. Atoms combine in the ratio of small whole numbers to form compounds. The relative number and the types of atoms are constant in a given compound.

Q20. (i) Name the body which approves the nomenclature of elements and compounds. (ii) The symbol of sodium is written as Na and not as S. Give reasons.

Solution

(i) International Union of Pure and Applied Chemistry (IUPAC) approves the nomenclature of elements and compounds. (ii) The symbol of Na for sodium is derived from its Latin name Natrium. Hence, the symbol of sodium is written as Na and not as S.  

Q21. Give atomicity of following element.Neon, Nitrogen, Phosphorus, Sulphur

Solution

Name of element Atomicity Neon Monoatomic Nitrogen Diatomic Phosphorus Tetra-atomic Sulphur Poly-atomic

Q22. Define atomic mass unit

Solution

Definition of the atomic mass unit (amu or u): One atomic mass unit is a mass unit equal to exactly one twelfth begin mathsize 12px style open parentheses 1 over 12 t h close parentheses end style the mass of one atom of carbon–12.

Q23. Give an example of (i) triatomic (ii) Polyatomic molecule of elements.

Solution

(i)Triatomic molecules : O3 (ii)Polyatomic molecule : S8

Q24. Give symbols of the following elements. Copper, Sulphur, Sodium, Magnesium

Solution

Name of the element Symbol Copper Cu Sulphur S Sodium Na Magnesium Mg

Q25. Write the valency of metal and non-metal in CaCl2?

Solution

The valency of metal is 2 (Ca+2) and the valency of non-metal is 1(Cl-1).

Q26. What is the relationship between nanometer and metre. What will be the value of atomic radius of carbon in metres. (Atomic radius = 0.077 nm)

Solution

Nanometre is a very, very small unit of measuring length. It is written as nm.   1 nanometre = 1/10metre So, 1 nm = 10-9 m Atomic radius of C = 0.077 nm So, atomic radius of C in m = 0.077 x 10-9 nm.

Q27. (a) How do atoms exist? Explain with examples. (b) Why are we not able to see individual iodine molecules with our eyes but are able to see an iodine crystal?

Solution

(a) Atom exists in two ways: 1. In the form of molecules – Molecules can be formed either by the combination of atoms of the same element or of different elements. For ex: O2 is formed by combination of two Oxygen atoms. 2. In the form of ions – Ions are formed by the loss or gain of electrons by an atom. for ex: common salt or sodium chloride compound is formed by the collection of millions of sodium ions (Na+) and chloride ions (Cl). (b) We cannot see the individual iodine molecules with our eyes because they are very, very small but we can see iodine crystal because it is a collection of millions of iodine molecules held together.

Q28. Write the chemical formulae of: (i)Calcium sulphate (ii)Magnesium oxide (iii)Potassium chloride (iv)Sodium hydroxide (v)Calcium oxide (vi)Hydrogen sulphide

Solution

(i) Calcium sulphate – CaSO4 (ii) Magnesium oxide – MgO (iii) Potassium chloride – KCl (iv) Sodium hydroxide – NaOH (v) Calcium oxide – CaO (vi) Hydrogen sulphide – H2S

Q29. The chemical formula of oxide of an element ‘X’ is X2O5. Write the chemical formula of its chloride.

Solution

X2OSo, the valency of X = +5 Therefore, formula of chloride will be XCl5

Q30. Classify the following molecules as tetra-atomic and penta-atomic molecules:H2O2, CHCl3

Solution

Tetra-atomic molecules: H2O2Penta-atomic molecules: CHCl3

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