ATOMS

Q 1: What are polyatomic ions? Give examples?

Ans: A polyatomic ion is a group of atoms carrying a charge (positive or negative). 

(OH−) hydroxide ion 

carbonate ion,

sulphate ion.

Q 2: Give the names of the elements present in the following compounds:

(a) Quick lime

(b) Hydrogen bromide

(c) Baking powder

(d) Potassium sulphate

Ans:

Compound	Chemical formula	Elements present
Quick lime	CaO	Calcium, oxygen
Hydrogen bromide	HBr	Hydrogen, bromine
Baking powder	NaHCO3	Sodium, hydrogen, carbon, oxygen
Potassium sulphate	K2SO4	Potassium, sulphur, oxygen

Q 3: Calculate the molar mass of the following substances:

(a) Ethyne, C₂H₂

(b) Sulphur molecule, S₈

(c) Phosphorus molecule, P₄ (atomic mass of phosphorus = 31)

(d) Hydrochloric acid, HCl

(e) Nitric acid, HNO₃

Ans:

(a) Molar mass of ethyne,                     C₂H₂ = 2 × 12 + 2 × 1 = 28 g

(b) Molar mass of sulphur molecule,         S₈ = 8 × 32 = 256 g

(c) Molar mass of phosphorus molecule,  P₄ = 4 × 31 = 124 g

(d) Molar mass of hydrochloric acid,        HCl = 1 + 35.5 = 36.5 g

(e) Molar mass of nitric acid,                HNO₃ = 1 + 14 + 3 × 16 = 63 g

Q 4: Define atomic mass unit.

Ans: Mass unit equal to exactly one-twelfth 1/12^th the mass of one atom of carbon-12 is called one atomic mass unit. It is written as ‘u’.

IX CLASS ATOMIC STRUCTURE ASSIGNMENT -1

1. All sample of carbon dioxide contains carbon and oxygen in the mass ratio of 3:8.This is in agreement of law of

(a)conservation of mass                   (b) constant proportions

(c) multiple  proportion                     (d) gaseous volume

2. The indivisibility  of atom was proposed by

(a) Rutherford                                     (b) Dalton

(c) Bohr                                                (d) Einstein

3. The value of the Avogadro constant  is

(a) 6.022 x 10^{– 22}                   (b) 6.022 x 10^{– 24}

(c) 6.022 x 10 ^{– 23}                  (d) None of these 

4. The volume of 1 mole of gas at standard temperature and pressure is

(a) 11.2 litres                                        (b) 22.4litres

(c) 100 litres                                         (d) none of these

5. The atomic mass of sodium is 23. The number of moles in 46 g of sodium is

(1) 1                                                      (b) 2       

(c) 2.3                                                   (d) 4.6

6. Empirical formula of a compound of molecular mass 120 is CH₂O. The molecular formula of the compound is

(a) CH₄0₂                                              (b) C₄H₈0₄

(c)C₃H₆ O₃                                            (d) none of these

7. Which of the following is a tetra atomic molecule.

(a) O₂                                                    (b) O₃     

(c) NO₂                                                 (d) SO₃

8. The number of atoms present in a molecule of a substance is called

(a) molecularity                                   (b) atomicity

(l)  valency                                          (d) reactivity

9. The valency of nitrogen in ammonia (NH₃) is

(a)  2                                                     (b)  0      

(c)  3                                                     (d)  4

10. The molecular formula of ethanoic acid is C₂H₄O₂ . Its empirical formula is

(a) C₄H₈O₂                                            (b) CH₂O

(c) CHO                                                (d) CHO₂

Answers.

1	b	6	b
2	b	7	b
3	c	8	b
4	b	9	c
5	b	10	b

Mark the statements True (T) or False (F).

1.        A molecule is made up of atoms.

2.        Atoms of different elements have the same mass.

3.        The symbol of silver is S. 

4.        1 amu =  1/12 th  the mass of one C–12 atom.

5.        The molecules of different substances have the same mass.

6.        All elements having valency 2 are called divalent.

7.        The formula of sodium carbonate is Na₂CO₃.

8.        Noble gas molecules are diatomic.

9.        The molecular formula of a compound is determined by the valency of the elements present in the compound.

Answer the following in one word or a maximum of one sentence.

1.Who was the first to propose the concept of compound atom?

2.What is meant by the gram–atomic mass of an element?

3.The atomic masses of sodium and chlorine are 23 and 35.5 respectively. What is the formula mass of       sodium chloride?

4.The atomic mass of Ca is 40 and the molecular mass of CaCO₃ is 100. What  is the percentage of Ca        in CaCO₃ ?

5.What is a symbol?

6.What is an atom?

7.Write the formula of calcium nitrate.

8.The molecular formula of sulphur is S. What do you understand by this?

9.Define valency of an element.

10.What is an ion?

11.Write the chemical formula of the following:    (a) Calcium chloride (b) Aluminium nitrate

12.Give the names of elements present in ammonium chloride.

13.What is the mass of 1 mole of oxygen atoms?

14.Convert into mole:

      (a) 28 g of nitrogen atoms (b) 20 g of carbon dioxide

MCQ ATOMIC STRUCTURE CLASS IX [1]

1. All sample of carbon dioxide contains carbon and oxygen in the mass ratio of 3:8.This is in agreement of law of

(a)conservation of mass                   (b) constant proportions

(c) multiple  proportion                     (d) gaseous volume

2. The indivisibility  of atom was proposed by

(a) Rutherford                                     (b) Dalton

(c) Bohr                                                (d) Einstein

3. The value of the Avogadro constant  is

(a) 6.022 x 10^{– 22}                   (b) 6.022 x 10^{– 24}

(c) 6.022 x 10 ^{– 23}                  (d) None of these 

4. The volume of 1 mole of gas at standard temperature and pressure is

(a) 11.2 litres                                        (b) 22.4litres

(c) 100 litres                                         (d) none of these

5. The atomic mass of sodium is 23. The number of moles in 46 g of sodium is

(1) 1                                                      (b) 2       

(c) 2.3                                                   (d) 4.6

6. Empirical formula of a compound of molecular mass 120 is CH₂O. The molecular formula of the compound is

(a) CH₄0₂                                              (b) C₄H₈0₄

(c)C₃H₆ O₃                                            (d) none of these

7. Which of the following is a tetra atomic molecule.

(a) O₂                                                    (b) O₃     

(c) NO₂                                                 (d) SO₃

8. The number of atoms present in a molecule of a substance is called

(a) molecularity                                   (b) atomicity

(l)  valency                                          (d) reactivity

9. The valency of nitrogen in ammonia (NH₃) is

(a)  2                                                     (b)  0      

(c)  3                                                     (d)  4

10. The molecular formula of ethanoic acid is C₂H₄O₂ . Its empirical formula is

(a) C₄H₈O₂                                            (b) CH₂O

(c) CHO                                                (d) CHO₂

Answers.

1	b	6	b
2	b	7	b
3	c	8	b
4	b	9	c
5	b	10	b

ATOMICITY

Atomicity

The number of atoms contained in a molecule of a substance (element or compound) is called its atomicity.

Based upon atomicity we can classify molecules as follows.

1. Monoatomic molecules:  Noble gases like helium, neon, argon, etc., exist in atomic forms, i.e., they are monoatomic. For example, helium, neon and argon exist as He, Ne and Ar respectively.

2. Diatomic molecules: These molecules consist of two atoms. For example, hydrogen (H₂) Oxygen (O₂), Nitrogen (N₂) chlorine (Cl₂), Carbon monoxide (CO), hydrogen chloride (HCl) and sodium chloride (NaCl).

3. Triatomic molecules : These molecules consist of three atoms. For example, ozone (O₃) is a triatomic molecule. Similarly, carbon dioxide (CO₂), nitrogen dioxide (NO₂) and hydrogen sulphide (H₂S) are examples of triatomic molecules.

4. Tetraatomic molecules : These molecules consist of four atoms. For example, P₄ is a tetraatomic molecule of phosphorus.

5. Polyatomic molecules : Any molecule containing more than four atoms is called a polyatomic molecule. For example, sulphur (S₈) methane (CH₄), nitric acid (HNO₃), sulphuric acid (H₂SO₄), ethanol (C₂H₂OH). 

VALANCIES

Valency of positive ions. Depending upon the charge present on them a positive ions may be monovalent, bivalent, trivalent, tetravalent etc.. These are listed.

Hydrogen	H+	Barium	Ba2+	Aluminium	Al3+
Potassium	K+	Calcium	Ca2+	Chromium	Cr3+
Sodium	Na+	Magnesium	Mg2+	Iron	Fe3+
Silver	Ag+	Zinc	Zn2+
Copper	Cu+	Cobalt	Co2+
Gold	Au+	Copper	Cu2+
Ammonium	(NH4)+	Iron	Fe2+

Some elements have shown more than one valency. In such cases, Roman Numerals are used to denote the valencies. These are put in bracket e.g. copper (I) and copper (II) ; similarly, iron (II) and iron (III).

Valency of negative ions. Like positive ions, negative ions may also be monovalent, bivalent, trivalent, tetravalent etc. in nature.                              

Monovalent		Bivalent		Trivalent
Chloride	CI–	Sulphide	S2–	Nitride	N3–
Bromide	Br–	Oxide	O2–	Phosphide	P3–
Iodide	I –	Carbonate	(CO3) 2–	Phosphate	(PO4) 3–
Hydroxide	(OH) –	Sulphate	(SO4) 2–	Borate	(BO3) 3–
Nitrate	(NO3) –	Sulphite	(SO3) 2–	Arsenate	(AsO3)3–­
Nitrite	(NO2) –	Manganate	(MnO4) 2–
Bicarbonate	(HCO3) –	Oxalate	(C2O4) 2–­
Cyanide	(CN) –	Chromate	(CrO4) 2–­
Permanganate	(MnO4) –	Dichromate	(Cr2O7) 2–
Hydrogen sulphate	(HSO4) –

Symbols

Element	Symbol	Element	Symbol	Eelement	Symbol	Element	Symbol
Boron	B	Oxygen	O	Barium	Ba	Molybdenum	Mo
Carbon	C	Phosphorus	P	Beryllium	Be	Nickel	Ni
Fluorine	F	Sulphur	S	Bismuth	Bi	Neon	Ne
Hydrogen	H	Uranium	U	Bromine	Br	Strontium	Sr
Iodine	I	Vanadium	V	Calcium	Ca	Silicon	Si
Aluminium	Al	Lithium	Li	Chlorine	Cl	Nitrogen	N
Argon	Ar	Magnesium	Mg	Cobalt	Co	Platinum	Pt
Arsenic	As	Manganese	Mn	Chromium	Cr	Zinc	Zn