Atoms and Molecules | NCERT Class 9 Science Chapter 3 | Notes and pdf | In-Text Questions & Answers | Exercises Questions & Answers
Q1. What is atom?
Atom of an element is the smallest, indivisible and tiny particle which takes part in chemical reaction.
Q2. What is symbol of an atom?
Symbol of an atom of an element expresses it’s identity. IUPAC (International Union of Pure and Applied Chemistry), an international scientific organization, specifically frames the rules for names and symbol of element.
In every cases ⇒ total positive charge of cation = total negative charge of anion
Q3. How do you write symbol of different elements?
See the following common rules to write the symbol of atom of an element –
A. First letter in uppercase from English name ⇒
| Element | Symbol | Element | Symbol | Element | Symbol |
|---|---|---|---|---|---|
|
Hydrogen |
H |
Nitrogen |
N |
Oxygen |
O |
|
Sulphur |
S |
Carbon |
C |
Uranium |
U |
|
Iodine |
I |
Fluorine |
F |
Boron |
B |
|
Phosphorus |
P |
Vanadium |
V |
Yttrium |
Y |
B. First letter in uppercase and second letter in lowercase from English name ⇒
| Element | Symbol | Element | Symbol | Element | Symbol |
|---|---|---|---|---|---|
|
Argon |
Ar |
Barium |
Ba |
Bromine |
Br |
|
Calcium |
Ca |
Cobalt |
Co |
Neon |
Ne |
|
Silicon |
Si |
Aluminium |
Al |
Lithium |
Li |
|
Beryllium |
Be |
Helium |
He |
Krypton |
Kr |
|
Scandium |
Sc |
Titanium |
Ti |
Nickel |
Ni |
|
Gallium |
Ga |
Selenium |
Se |
Rhodium |
Rh |
C. First letter in uppercase and any other letter in lowercase from English name ⇒
| Element | Symbol | Element | Symbol | Element | Symbol |
|---|---|---|---|---|---|
|
Magnesium |
Mg |
Chlorine |
Cl |
Zinc |
Zn |
|
Manganese |
Mn |
Rubidium |
Rb |
Chromium |
Cr |
|
Arsenic |
As |
Zirconium |
Zr |
Cadmium |
Cd |
|
Neodymium |
Nd |
Niobium |
Nb |
Technetium |
Tc |
D. From Latin, German & Greek name ⇒
| Element | Other Name | Symbol | Element | Other Name | Symbol |
|---|---|---|---|---|---|
|
Sodium |
Natrium |
Na |
Gold |
Aurum |
Au |
|
Iron |
Ferrum |
Fe |
Mercury |
Hydragyrum |
Hg |
|
Copper |
Cuprum |
Cu |
Silver |
Argentum |
Ag |
|
Potassium |
Kalium |
K |
Lead |
Plumbum |
Pb |
|
Tin |
Stannum |
Sn |
Antimony |
Stibium |
Sb |
Q4. What is the law of constant or definite proportion?
Hydrogen and Oxygen form water (H2O). Their mass ratio is 1:8. If we decompose 9 gram water, we will get 1 gram hydrogen and 8 gram oxygen. At any condition this mass ratio in water is constant. ‘Elements in a chemical substance are always present in definite proportions by mass’ is known as laws of definite proportions which is stated by Proust. Carbon di-oxide (CO2) contains Carbon and Oxygen in mass ratio of 3:8. At any condition this mass ratio is same. Salt i.e. Sodium Chloride (NaCl) contains Sodium and Chlorine in mass ratio of 23:35.5. This mass ratio is unaltered at any condition. So all matters formed in chemical reaction have definite or constant mass ratio of their constituents at any condition.
Q5. What is the law of conservation of mass?
When 1 gram hydrogen and 8 gram oxygen take part in chemical reaction, we will get 9 gram water. Similarly 3 gram Carbon and 8 gram Oxygen will form 11 gram Carbon-di-oxide (CO2) in chemical reaction. Thus we get that mass of newly formed matter in chemical reaction is the summation of masses of it’s components. So mass can neither be created or destroyed in chemical reaction. This is known as law of conservation of mass.
Q6. What is Dalton’s atomic theory?
Firstly Indian and Greek philosophers gave idea of divisibility of matter in pre-Christ era. If division of matter is continued, we will find indivisible tiny particle at a stage. Further division of said tiny particle is not possible. This tiny particle is named as Atom by Greek philosopher Democritus and Paramanu by Indian Maharishi Kanad.
John Dalton, a British Chemist, at first explained the concept or theory of atom based on laws of definite proportion and conservation of mass. Dalton’s theory postulates that –
- All matter is made of indivisible tiny particle i.e. atom which takes part in chemical reaction.
- Atom can not be created or destroyed in chemical reaction.
- Mass and chemical properties of atoms of a particular element remain unaltered.
- Mass and chemical properties of atoms of different elements are different.
- Compound contains whole number of atom.
- Relative number and kinds of atoms of a specific compound are constant.
Q7. What is atomic mass or weight?
How small a single atom is? What is the weight of an atom? An atom is too small in size to view under microscope. One cubic centimetre volume can houses 3X1019 nos. hydrogen atom. One hydrogen atom weighs 1.67X10-27 Kg. Just imagine how tough it is to measure the weight of single atom of an element. Due to this relative mass concept is introduced and a universally accepted system was adopted in 1961. One atomic mass unit is equal to (1/12) times of weight of single atom of Carbon-12 isotope. As per IUPC recommendation, one atomic mass is written as “u”. Later we will learn average atomic mass in ‘Structure of the Atom‘ chapter. Therefore –
1 u = weight of single atom of carbon-12 isotope / 12
One single atom of hydrogen weighs equal to (1/12) times of weight of single atom of Carbon-12 isotope. So atomic mass unit of hydrogen is 1 u. Atomic mass of some elements are listed below-
| Element | Atomic Mass (u) | Element | Atomic Mass (u) |
|---|---|---|---|
|
Hydrogen |
1 |
Nitrogen |
14 |
|
Oxygen |
16 |
Sodium |
23 |
|
Carbon |
12 |
Magnesium |
24 |
|
Sulphur |
32 |
Calcium |
40 |
|
Chlorine |
35.5 |
Uranium |
238 |
|
Aluminium |
26.98 |
Copper |
63.54 |
Q8. What do you mean by ‘atomic mass of Oxygen is 16’ ?
Atomic weight of Oxygen is 16. It means that an atom of oxygen is 16 times heavier than (1/12) times of a Carbon-12 atom.
Q9. What is molecule and what are the types of molecule?
A molecule is the smallest particle of a matter which can exist independently showing all properties of that matter. Actually molecule contains single or multiple atom from single or multiple elements. Atoms of molecule are chemically bonded and attractive force between atoms exists. Generally we divide molecule into two groups – i) Elementary molecule, & ii) Compound molecule.
Elementary molecule : It is molecule of elements.
Compound molecule: It is molecule of compound and is formed by the atoms of various elements in chemical reaction in definite proportions. For example – Water (H2O), Carbon dioxide (CO2) etc.
Q10. What is atomicity of an element?
Number of atoms present in single molecule of an element is called atomicity.
- When single atom forms a molecule, then it is mono-atomic molecule. For example – Helium (He), Argon (Ar) etc.
- When double atom forms a molecule, then it is diatomic molecule. For example – Oxygen, Nitrogen, Hydrogen, Chlorine etc.
- When three atom forms a molecule, then it is tri-atomic molecule. For example – Ozone etc.
- When four atom forms a molecule, then it is tetra-atomic molecule. For example – Phosphorus etc.
- When multiple (>four) atom forms a molecule, then it is poly-atomic molecule. For example –Sulphur etc.
Q11. How do you write molecular formula or symbol of an element?
See the following rules to write molecular formula of an element –
1. Some elements which generally exists in gaseous state are expressed by writing atomicity as subscript after the symbol of element –
| Name of the Element | Molecular Formula of the Element |
|---|---|
|
Oxygen |
O2 |
|
Nitrogen |
N2 |
|
Chlorine |
Cl2 |
|
Hydrogen |
H2 |
2. Molecule of metal is written as their atomic symbol –
| Name of the Element | Molecular Formula of the Element |
|---|---|
|
Copper |
Cu |
|
Iron |
Fe |
|
Potassium |
K |
|
Sodium |
Na |
|
Zinc |
Zn |
|
Mercury |
Hg |
|
Calcium |
Ca |
3. Molecule of non-metal which generally exists in solid state is written as their atomic symbol –
| Name of the Element | Molecular Formula of the Element |
|---|---|
|
Phosphorus |
P |
|
Carbon |
C |
|
Sulphur |
S |
|
Boron |
B |
|
Silicon |
Si |
Q12. How do you write molecular formula or symbol of inert gases?
Inert gases are expressed in atomic symbol Helium (He) , Neon (Ne) , Argon (Ar) etc. due to their existence in atomic state.
Q13. How do you write molecular formula or symbol of a compound?
See the following rules to write molecular or chemical formula of compound :-
A. When compound contains metal and non-metal, then symbol of metal is written first and symbol of non-metal is written after metal. For example, Salt (NaCl) ⇒ Na (metal) + Cl (non-metal) ⇒ NaCl.
B. When compound contains two or more non-metal, normally Hydrogen (H) is written first. If it contains Oxygen, then Oxygen is written in last. For example – HCl (Hydrochloric Acid), HNO3 (Nitric Acid), H2SO4 (Sulphuric Acid) etc.
C. When compound contains two non-metal, then symbol of non-metal which exists in solid state is written first. For example – Carbon dioxide (CO2) etc.
D. Non-metals forming compound join with each other according to their combining capacity i.e.valency.
Q14. What is valency and what is Hydrogen based valency?
Valency is the combining capacity of an element to join with other elements.
Hydrogen based valency: Combining capacity of Hydrogen with other elements is very less. There are no other compounds except Hydrazoic acid (HN3) where multiple atom of any other elements join with single atom of Hydrogen. That’s why valency of Hydrogen is taken as one (01). So we may say that “no. of Hydrogen atoms react with single atom of any element” is thevalency of that element. Single atom of Chlorine (Cl) reacts with one no. Hydrogen(H) atom and forms Hydrochloric Acid(HCl). So valency of Chlorine is one (01). Single atom of Oxygen (O) reacts with two nos. Hydrogen(H) atom and forms water (H2O). So valency of Oxygen (O) is two(02).
Q15. What is Chlorine based valency?
If Hydrogen(H) does not react with any element, then how the valency of that element will be found? Here concept of Chlorine based valency arises. Chlorine (Cl) reacts with maximum elements and it’s valency is one(01). So we may say that “no. of Chlorine (Cl) atoms react with single atom of any element” is the valency of that element. Single atom of Calcium (Ca) reacts with two nos. Chlorine (Cl) atom & forms CaCl2. So valency of Calcium (Ca) is two(02). Single atom of Sodium (Na) reacts with one no. Chlorine (Cl) atom & forms salt NaCl. So valency of Sodium (Na) is two(01).
So we may summarily say that “no. of Hydrogen or Chlorine atoms react with single atom of any element” is the valency of that element.
Q16. Write down valency of various element.
Valency of some elements are given below ⇒
| Valency | Name of the Element |
|---|---|
|
|
Helium (He), Neon (Ne), Argon (Ar) Krypton (Kr), Xenon (Xe), Radon (Rn) etc. |
|
1 |
Iodine (I), Bromine (Br), Chlorine (Cl), Hydrogen (H) etc. |
|
2 |
Potassium (K), Oxygen (O), Calcium (Ca), Iron (Fe), Sulfur (S) etc. |
|
3 |
Aluminium (Al), Phosphorus (P), Gold (Au), Nitrogen (N) etc. |
|
4 |
Carbon (C), Silicon (Si), Lead (Pb), Tin (Sb) etc. |
|
5 |
Antimony (Sb), Phosphorus (P), Arsenic (As) etc. |
|
6 |
Chromium (Cr), Sulphur (S) etc. |
|
7 |
Manganese (Mn) etc. |
|
8 |
Osmium (Os) etc. |
Q17. Why some elements have zero valency?
Elements such as Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), Radon (Rn) etc. can not take part in chemical reaction with other elements and can not form compound. For this reason, these elements called inert gases and they have zero valency.
Q18. State some elements of multiple valency.
See the following examples of some elements having multiple valency.
| Valency | Name of the Element |
|---|---|
|
3 , 5 |
Phosphorus (P) |
|
2 , 4 , 6 |
Sulphur |
|
2 , 4 |
Lead (Pb) |
|
2 , 3 |
Iron (Fe) |
|
1 , 2 , 3 , 4 , 5 |
Nitrogen (N) |
|
1 , 2 |
Copper (Cu) |
Q19. What is compound radical and it’s valency?
Compound radical is a group of atom of different elements and it behaves like single atom in chemical reaction. Following are some examples given with their valency.
| Valency | Name of the Compound Radical |
|---|---|
|
1 |
Nitrite (NO2) |
|
1 |
Nitrate (NO3) |
|
1 |
Hydroxide (OH) |
|
1 |
Hydrogen/Bi-carbonate (HCO3) |
|
1 |
Ammonium (NH4) |
|
2 |
Carbonate (CO3) |
|
2 |
Sulphite (SO3) |
|
2 |
Sulphate (SO4) |
|
3 |
Phosphate (PO4) |
Q20. What are ion, cation and anion?
Atom consists positively charged proton and negatively charged electron. Quantity of positive charge and negative charge are equal and neutralise each other which makes the atom electrically neutral. If we take out one negatively charged electron from an atom, atom will be electrically imbalanced as quantity of negative charge reduces and atom turns into ion (positively charged). Opposite happens when extra electron enters in atom i.e. quantity of negative charge overpowers positive charge and atom turns into ion (negatively charged). Positively charged ion is called cation and negatively charged ion is called anion.
Q21. Write down symbol of some elements, compound radical or poly-atomic ion.
Basically metal ions are cation and non-metal ions are anion. Also compound radical or poly-atomic ion may be cation or anion.
Cation is written as: [Symbol of Element] valency+
For example : Sodium – Na+, Potassium – K+, Silver – Ag+, Copper ( valency ‘1’ ) – Cu+, Iron ( valency ‘2’ ) – Fe2+, Iron ( valency ‘3’ ) – Fe3+, Calcium – Ca2+, Magnesium – Mg2+, Zinc – Zn2+, Copper ( valency ‘2’ ) – Cu2+,Ammonium – NH4+ , Hydrogen – H+ etc.
Anion is written as: [Symbol of Element] valency-
For example : Chlorine – Cl–, Oxide – O2-, Sulfide – S2-, Nitride – N3-, Iodine – I–, Bromine – Br–, Sulphite – SO32-, Sulphate – SO42-, Phosphate – PO43-, Nitrate – NO3-, Hydroxide – OH– , Hydride – H– etc.
Q22. How do you write molecular formula of a compound by using valency of the elements?
See the following common rules to write compound’s chemical formula. These are –
1. Compound contains two elements A & B. Valency of A is x & valency of B is y. Then chemical formula will be AyBx–
Symbol ⇒ A B
Valency ⇒ x y
Formula ⇒ AyBx
For example :
Water (H2O)
Symbol ⇒ H O
Valency ⇒ 1 2
Formula ⇒ H2O1 = H2O
valency “1” generally is not written in subscript
Magnesium chloride (MgCl2)
Symbol ⇒ Mg Cl
Valency ⇒ 2 1
Formula ⇒ Mg1Cl2 = MgCl2
valency “1” geneally is not written in subscript
2. Compound contains two elements A & B. Valency of A is x & valency of B is y. Now if there is a common factor ‘n’ of x & y. Then chemical formula will be A(y/n)B(x/n) .
Symbol ⇒ A B
Valency ⇒ x y
Formula⇒ Ay/nBx/n
For example :
Carbon dioxide (CO2)
Symbol ⇒ C O
Valency ⇒ 4 2
Formula ⇒ C2O4 = C(2/2)O(4/2)= CO2
In this case, common factor is 2
Sulfur dioxide (SO2)
Symbol ⇒ S O
Valency ⇒ 4 2
Formula ⇒ S2O4 = S(2/2)O(4/2)= SO2
In this case, common factor is 2
3. Compound containing polyatomic ion/compound radical will be written as Ay(BC)x where A is normal element with valency x & BC is polyatomic ion/compound radical with valency y.
Symbol ⇒ A BC
Valency ⇒ x y
Formula ⇒ Ay(BC)x
For example :
Sodium sulphate [ Na2(SO4) ]
Symbol ⇒ Na SO4
Valency ⇒ 1 2
Formula ⇒ Na2(SO4)1 = Na2(SO4)
Ammonium carbonate [ (NH4)2CO3 ]
Symbol ⇒ NH4 CO3
Valency ⇒ 1 2
Formula ⇒ (NH4)2 (CO3)1 = (NH4)2CO3
Q23. How do you write molecular formula by using cation and anion?
For realization of chemical formula by ion, always keep in mind that ” total amount of charge in cation = total amount of charge in anion “. For example,
Water (H2O): Cation = Hydrogen ion = H+, Anion = Oxygen ion = O2-. To neutralize negative charge of 2-, we need positive charge 2+ i.e. 02 nos. H+ ion. So 02 nos. Hydrogen ion will join with one no. Oxygen ion to form an atom of water i.e. H2O. Every formula maintains –
Total amount of positive charge in cation = Total amount of negative charge in anion
For example -
Carbon dioxide (CO2)
Symbol ⇒ C O
Charge ⇒ 4+ 2-
Formula ⇒ C2O4 = C(2/2)O(4/2)= CO2
Sodium sulphate [ Na2(SO4) ]
Symbol ⇒ Na SO4
Charge ⇒ 1+ 2-
Formula ⇒ Na2(SO4)1 = Na2(SO4)
Sulfur dioxide (SO2)
Symbol ⇒ S O
Charge ⇒ 4+ 2-
Formula ⇒ S2O4 = S(2/2)O(4/2)= SO2
Carbon tetrachloride (CCl4)
Symbol ⇒ C Cl
Charge ⇒ 4+ 1-
Formula ⇒ C1Cl4 = CCl4
In every cases ⇒ total positive charge of cation = total negative charge of anion
Aluminium oxide(Al2O3)
Symbol ⇒ Al O
Charge ⇒ 3+ 2-
Formula ⇒ Al2O3
Calcium oxide (CaO)
Symbol ⇒ Ca O
Charge ⇒ 2+ 2-
Formula ⇒ Ca2O2=CaO
Magnesium oxide (MgO)
Symbol⇒ Mg O
Charge ⇒ 2+ 2-
Formula ⇒ Mg2O2 = Mg(2/2)O(2/2)= MgO
Ammonia (NH3)
Symbol ⇒ N H
Charge ⇒ 3+ 1-
Formula ⇒ N1H3= NH3
In every cases ⇒ total positive charge of cation = total negative charge of anion
Aluminium Nitride (AlN)
Symbol ⇒ Al N
Charge ⇒ 3+ 3-
Formula ⇒ Al3N3=AlN
Calcium hydroxide[Ca(OH)2]
Symbol ⇒ Ca OH
Charge ⇒ 2+ 1-
Formula ⇒ Ca1(OH)2= Ca(OH)2
Sulfuric Acid [H2SO4]
Symbol ⇒ H SO4
Valency ⇒ 1+ 2-
Formula ⇒ H2(SO4)1= H2SO4
Sodium Nitrate(NaNO3)
Symbol ⇒ Na NO3
Charge ⇒ 1+ 1-
Formula ⇒ Na1(NO3)1= NaNO3
Sodium hydroxide( NaOH)
Symbol ⇒ Na OH
Charge ⇒ 1+ 1-
Formula ⇒ Na1(OH)1=NaOH
Sodium carbonate (Na2CO3)
Symbol ⇒ Na CO3
Valency ⇒ 1+ 2-
Formula ⇒ Na2(CO3)1= Na2CO3
In every cases ⇒ total positive charge of cation = total negative charge of anion
Ammonium sulphate [(NH4)2SO4]
Symbol ⇒ NH4 SO4
Valency ⇒ 1+ 2-
Formula ⇒ (NH4)2(SO4)= (NH4)2 SO4
Ammonium carbonate [(NH4)2CO3]
Symbol ⇒ NH4 CO3
Valency ⇒ 1+ 2-
Formula ⇒ (NH4)2(CO3)1 =(NH4)2CO3
Magnesium carbonate (MgCO3)
Symbol ⇒ Mg CO3
Valency ⇒ 2+ 2-
Formula ⇒ (Mg)2(CO3)2= MgCO3
Calcium phosphate [Ca3(PO4)2]
Symbol ⇒ Ca PO4
Valency ⇒ 3+ 2-
Formula ⇒ Ca3(PO4)2= Ca3(PO4)2
Q24. What is the difference between O, 2O, O2 and 2O2 ?
O ⇒ Atomic symbol of Oxygen.
2O ⇒ Two nos. atom of Oxygen.
O2 ⇒ One molecule of Oxygen. Also it is molecular formula of Oxygen.
2O2 ⇒Two nos. molecule of Oxygen.
Q25. What is molecular mass?
Molecular mass of a molecule of an element or compound is the summation of atomic masses of all the atoms that it contains. For examples –
Hydrogen (H2) ⇒ It contains 02 Nos. Hydrogen atom. Atomic mass of single Hydrogen is 1 u. So molecular mass of Hydrogen molecule (H2) is 2 X 1 u = 2u.
Oxygen (O2) ⇒ It contains 02 Nos. Oxygen atom. Atomic mass of single Oxygen is 16 u. So molecular mass of Oxygen molecule (O2) is 2 X 16 u = 32u.
Carbon dioxide (CO2) ⇒ It contains 01 No. carbon atom and 02 Nos. Oxygen atom . So molecular mass of Carbon di-oxide ( CO2 ) is : ( 1X12 + 2X16 )u = 44 u.
Similarly molecular mass of some compounds are as follows –
H2O ⇒ ( 2X1 + 1X16 )u = 18u. HNO3 ⇒ ( 1X1 + 1X14 + 3X16 )u = 63u.
HCl ⇒ ( 1X1 + 1X35.5 )u = 36.5 u , H2SO4 ⇒ ( 2X1 + 1X32 + 4X16 )u = 98u.
Q26. What is mole?
One moleof any substance is the quantity in grams of it’s molecular mass. For example – one mole H20 means 18 gram H2O and one mole CO2 means 44 gram CO2. Similarly onemole O2 means 32 gram O2.
Q27. What is Avogadro number?
It is found that one mole of any substance contains 6.022 X 1023 nos. particles i.e. molecules or atoms or ions. Number 6.022X1023 is known as Avogadro number. It is written as NO. One mole H20 i.e. 18 gram H20 contains 6.022 X 1023 nos. H20 molecule. One mole CO2 i.e. 44 gram CO2 contains 6.022 X 1023 nos. CO2 molecule.
In-Text Questions & Answers | NCERT Class 9 Science Chapter 3 | Atoms and Molecules
In-Text Questions & Answers | NCERT Class 9 Science | Atoms and Molecules | Page Nos. 32 – 33
1. In a reaction, 5.3 gram of sodium carbonate reacted with 6 gram of acetic acid. The products were 2.2 gram of carbon dioxide, 0.9 gram water & 8.2 gram of sodium acetate. Prove that this follows the law of conservation of mass.
In this case –
sodium carbonate + acetic acid = carbon dioxide + water +sodium acetate,
⇒ 5.3 gram + 6 gram = 2.2 gram + 0.9 gram + 8.2 gram, (after putting the quantities)
⇒ 11.3 gram = 11.3 gram,
Therefore, total mass before reaction = total mass after reaction.
So it follows the law of conservation of mass.
2. Hydrogen & oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 gram of hydrogen gas?
Water ⇒ Hydrogen : Oxygen ⇒ 1 : 8. It means that 1 gram hydrogen reacts with 8 gram oxygen. So, 3 gram hydrogen will react with (3 x8) = 24 gram oxygen.
3. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction.
4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
The relative number & kind of atoms in a given compound remain constant.
In-Text Questions & Answers | NCERT Class 9 Science | Atoms and Molecules | Page Nos. 35
1. Define atomic mass unit.
One atomic mass unit is equal to (1/12) times of weight of single atom of Carbon-12 isotope. One atomic mass is written as u. Therefore –
1 u = weight of single atom of carbon-12 isotope / 12
2. Why is it not possible to see an atom with naked eyes?
We can not see an atom with naked eyes because it is too small & does not exist independently.
In-Text Questions & Answers | NCERT Class 9 Science | Atoms and Molecules | Page Nos. 38-39
1. Write down the formulae of – i) sodium oxide, ii) aluminium chloride, iii) sodium sulphide, iv) magnesium hydroxide.
1) Sodium Oxide ⇒ Na2O, 2) Aluminium Chloride ⇒ AlCl3, 3) Sodium Sulphide ⇒ Na2S, 4) Magnesium Hydroxide ⇒ Mg(OH)2.
2. Write down the names of compounds represented by the following formulae: i) Al2(SO4)3, ii) CaCl2, iii) K2SO4, iv) KNO3, v) CaCO3.
i) Al2(SO4)3 ⇒ Aluminium sulphate, ii) CaCl2 ⇒ Calcium chloride, iii) K2SO4 ⇒ Potassium sulphate, iv) KNO3, ⇒ Potassium nitrate, v) CaCO3 ⇒ Calcium carbonate.
3. What is meant by the term chemical formula?
It expresses the symbolic representation of the composition of a compound. We can easily know the number & kinds of atoms of different elements that a compound contains. For example – H2O (water). From the chemical formula H2O, we come to know that two hydrogen atom & one oxygen atom are chemically bonded to form one molecule of H2O i.e. water.
4. How many atoms present in a – i) H2S molecule, & ii) PO43- ion?
i) H2S molecule = [02 nos. hydrogen atom + 01 no. sulphur atom] = 03 nos. atom.
ii) PO43- ion = [01 no. phosphorus + 04 nos. oxygen atom] = 05 nos. atom.
In-Text Questions & Answers | NCERT Class 9 Science | Atoms and Molecules | Page Nos. 40
1. Calculate the molecular masses of H2, O2, Cl2, CO2, C2H6, C2H4, NH3, CH3OH.
Atomic masses of H = 1 u, O = 16 u, Cl = 35.5 u, C = 12 u, N = 14.
H2 = (2×1) u = 2 u, O2 = (2×16) u = 32 u, Cl2 = (2×35.5) u = 71 u, CO2 = [(1×12) + (2×16)] u = 44 u,
C2H6 = [2×12 + 6×1] u = 30 u, C2H4 = [2×12 + 4×1] u = 28 u, NH3 = [1×14 + 3×1] u = 17 u,
CH3OH = [(1×12) + (1×3) + (1×16) + (1×1)] u = 32 u.
2. Calculate the formula unit masses of ZnO, Na2O, K2CO3. Given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, & O = 16 u.
Atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, & O = 16 u.
ZnO = [1×65 + 1×16] u = 81 u,
Na2O = [2×23 + 1×16] u = 62 u,
K2CO3 = [2×39 + 1×12 + 3×16] u = 138 u.
In-Text Questions & Answers | NCERT Class 9 Science | Atoms and Molecules | Page Nos. 42
1. If one mole of carbon atoms weighs 12 grams, what is the mass in grams of 1 atom of carbon?
In this case –
one mole of carbon atoms = 12 grams carbon. And we know, Avogadro number = 6.022X1023 .
So, weight of 6.022X1023 nos. carbon atoms = 12 gram.
Then, weight of 01 no. carbon atom = [12/6.022X1023 ] gram = 1.9926 x 10-23 gram.
2.Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?
In this case – atomic mass of Na = 23 u, Fe = 56 u. Avogadro number = 6.022X1023 .
Now, 23 gram Na contains 6.022X1023 nos. atom.
So, 100 gram Na contains = [(100/23) x 6.022X1023 ] = 2.6182X1024 nos. atom.
Similarly, 56 gram Fe contains 6.022X1023 nos. atom.
So, 100 gram Fe contains = [(100/56) x 6.022X1023 ] = 1.0753X1024 nos. atom.
Therefore, 100 gram Na contains more atoms than 100 gram Fe.
Exercises Questions & Answers | NCERT Class 9 Science | Atoms and Molecules | Page Nos. 43 - 44
1. A 0.24 gram sample of compound of oxygen & boron was found by analysis to contain 0.096 gram of boron & 0.14 gram of oxygen. Calculate the percentage composition of the compound by weight.
In this case, total sample ( boron+oxygen) = 0.24 gram.
Boron = 0.096 gram. So % of boron in sample = (0.096/0.24) x 100% = 40 %.
Oxygen = 0.14 gram. So % of Oxygen in sample = (0.144/0.24) x 100% = 60 %.
Therefore, ratio by mass : Boron : Oxygen = 0.096 : 0.144 = 96 : 144 = 4 : 6
2. When 3.0 gram of carbon is burnt in 8.00 gram of oxygen, 11.00 gram carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.0 gram of carbon is burnt in 50.00 gram of oxygen? Which law of chemical composition will govern your answer?
We apply laws of definite proportion in this case.
3 gram carbon + 8 gram oxygen = 11 gram carbon dioxide.
So, ratio by mass = carbon : oxygen = 3 : 8.
In this case, 3 gram carbon is burnt in 50 gram oxygen. But only 11 gram carbon dioxide is formed as there is only 3 gram carbon and as per laws of definite proportion, 3 gram carbon reacts with 8 gram oxygen.
3. What are polyatomic ions? Give examples.
Compound radical or poly-atom is a group of atom of different elements and it behaves like single atom in chemical reaction. It also turns into cation & anion by losing or adding electron respectively. For example – Ammonium – NH4+ ,Sulphite – SO32-, Sulphate – SO42-, Phosphate – PO43-, Nitrate – NO3-, Hydroxide – OH– etc.
4. Write chemical formulae of – a) Magnesium chloride , b) Calcium oxide, c) Copper nitrate, d) Aluminium chloride, e) Calcium carbonate.
a) Magnesium chloride [ MgCl2]
Symbol ⇒ Mg Cl
Valency ⇒ 2 1
Formula ⇒ Mg1Cl2= MgCl2
b) Calcium oxide (CaO)
Symbol ⇒ Ca O
Charge ⇒ 2+ 2-
Formula ⇒ Ca2O2=CaO
c) Copper Nitrate [ Cu(NO3)2]
Symbol ⇒ Cu NO3
Valency ⇒ 2 1
Formula ⇒ (Cu)1(NO3)2= Cu(NO3)2
d) Ammonium Chloride [ AlCl3 ]
Symbol ⇒ Al Cl
Valency ⇒ 3 1
Formula ⇒ (Al)1(Cl)3 = AlCl3
e) Calcium Carbonate [ CaCO3 ]
Symbol ⇒ Ca CO3
Valency ⇒ 2 2
Formula ⇒ Ca2(CO3)2= CaCO3
5. Write name of the elements present in the following compounds – a) Quick lime, b) Hydrogen bromide, c) Baking powder, d) Potassium sulphate.
Quick lime: Calcium (C) and Oxygen (O).
Hydrogen bromide: Hydrogen (H) and Bromine (Br).
Baking powder: Sodium (Na), Hydrogen (H), Carbon (C) and Oxygen (O).
Potassium sulphate: Potassium (K), Sulphur (S) and Oxygen (O).
6. Calculate the molar mass of the – a) Ethyne (C2H2), b) Sulphur molecule (S8), c) Phosphorus molecule (P4 atomic mass of Phosphorus is 31), d) Hydrochloric acide (HCl), e) Nitric Acid ( HNO3).
See Q7 especially to know atomic mass of various elements.
a) Ethyne (C2H2) : [ 12×2 + 1×2 ] gram = 26 gram.
b) Sulphur molecule (S8) : [ 32×8 ] gram = 256 gram.
c) Phosphorus molecule (P4) : [ 31×4 ] gram = 124 gram.
d) Hydrochloric acide (HCl) : [ 1×1 + 35.5×1 ] gram = 36.5 gram.
e) Nitric acid ( HNO3) : [ 1×1 + 14×1 + 16×3 ] gram = 63 gram.
7. What is the mass of – a) 1 mole of Nitrogen atoms, b) 4 moles of Aluminium atoms (Take 27 as atomic mass of Al), c)10 moles of Sodium sulphate (Na2SO3)?
Basically one mole of any substance is the quantity in grams of it’s molecular mass. And molecular mass of a molecule of an element or compound is the summation of atomic masses of all atoms it contains. Therefore -
a) 1 mole of Nitrogen atoms : [ 1×14 ] gram = 14 gram.
b) 4 moles of Aluminium atoms : [ 4 x 27 ] gram = 108 gram.
c) 10 moles of Sodium sulphate ( Na2 SO3) : 10 x [ 2×23 + 1×32 + 3×16 ] gram = 1260 gram.
[For a and b : 1 mole of atom is mentioned. So we have taken atomic mass. If the question is like – what is the mass of 1 mole of Nitrogen gas ? Nitrogen gas is N2. It contains two Nitrogen atom. So the answer would be – [ 2×14 ] gram i.e. 28 gram.
8. Convert into mole – a) 12 gram of oxygen gas, b) 20 gram of water, c) 22 gram of carbon dioxide.
a) 1 mole of oxygen gas = 1 mole O2 = (16×2) gram oxygen = 32 gram oxygen.
So 12 gram of oxygen gas = (12/32) mole of oxygen gas = 0.375 mole of oxygen gas.
b) 1 mole of water = 1 mole H2O = [1X2 + 16×1) gram water = 18 gram water.
So 20 gram of water = (20/18) mole of water = 1.11 mole of water.
c) 1 mole of carbon dioxide = 1 mole CO2 = (12X1 + 16×2) gram CO2 = 44 gram CO2 .
So 22 gram of carbon dioxide = (22/44) mole of carbon dioxide = 0.50 mole of carbon dioxide.
9. What is the mass of – a) 0.2 mole of oxygen atoms, & b) 0.5 mole of water molecules?
a) 1 mole of oxygen atom = 1 mole O = 16 gram oxygen atom.
So 0.2 mole of oxygen atom = (16×0.2) gram oxygen atom = 3.2 gram oxygen atom.
b) 1 mole of water molecule = 1 mole H2O = [1X2 + 16×1) gram water = 18 gram water.
So 0.5 mole of water molecule = (0.5×18) gram water molecule = 9 gram water molecule.
10. Calculate the number of molecules of sulphur(S8) present in 16 gram of solid sulphur.
Basically one mole of any substance contains 6.022 X 1023 nos. molecule.
In this case, 1 mole sulphur (S8) = (32×8) gram sulphur (S8) = 256 gram sulphur (S8).
So one mole of sulphur (S8) i.e. 256 gram sulphur (S8) contains 6.022 X 1023 nos. molecule.
So 16 gram sulphur (S8) contains = (16/256) x 6.022 X 1023 nos. atom = 3.76 X 1022 nos. molecule.
11. Calculate the number of aluminium ions present in 0.051 gram of aluminium oxide (Atomic mass of Al = 27 u).
In this case, 01 mole Al2O3 = [27×2 + 16×3] gram Al2O3 = 102 gram Al2O3. So 102 gram Al2O3 contains 6.022 X 1023 nos. molecule. So 0.051 gram Al2O3 contains [(0.051/102) x 6.022 X 1023] = 3.011 x 1020 nos. molecule .
Aluminium oxide (Al2O3)
Symbol ⇒ Al O
Charge ⇒ 3+ 2-
Therefore, we get that one Al2O3 molecule contains 2 nos. positive Al ion and 3 nos. negative O ion. 2 nos. positive Al ion = (2×3)+ = 6+ charge and 3 nos. negative O ion = (3×2)- = 6- charge. Thus both nullify each other. So 0.051 gram Al2O3 i.e. 3.011 X 1020 nos. molecule contains [2 x (3 X 1020)] = 6.022 x 1020 nos. Al ion.
Atoms and Molecules – To Be Continued – Atoms and Molecules