Class 9 Exam  >  Class 9 Notes  >  Science Class 9  >  Short and Long Answer Questions: Atoms and Molecules

Class 9 Science Chapter 3 Question Answers - Atoms and Molecules

Very Short Answer Type Questions

Q1. Define the law of conservation of mass. 

In a chemical reaction mass can neither be created nor destroyed.
E.g., 2Na + Cl2 ——–> 2NaCl
2 x 23 + 2 x 35.5 ——> 2(23 + 35.5)


Q2. Explain the law of constant proportion.

In a chemical substance, the elements are always present in definite proportions by mass.
Example: In nitric oxide, the ratio of the mass of nitrogen to the mass of oxygen N: O is always 1:1.Class 9 Science Chapter 3 Question Answers - Atoms and Molecules


Q3. Who coined the term atom?

John Dalton coined the term atom.


Q4. Define atom.

The smallest particle of matter, which can take part in a chemical reaction is called an atom.


Q5. Define molecule.

The smallest particle of an element or compound which can exist independently is called a molecule.


Q6. Define atomicity.

The number of atoms constituting a molecule is known as its atomicity.


Q7. What is an atomic mass unit?

The sum of the atomic masses of all the atoms in a molecule of the substance is expressed in the atomic mass unit. E.g., H20 = 1 x 2 + 16 = 18 amu


Q8. How do atoms exist?

Atoms exist in the form of atom, molecule or ions.


Q9. Give the chemical formula for ammonium sulphate.

Ammonium sulphate
NH4+ SO42-
Chemical formula —-> (NH4)2S04.


Q10. What is Avogadro’s constant?

The Avogadro’s constant (6.022 x 1023) is defined as the number of atoms that are present in exactly 12 g of carbon-12.


Q11. Find the molecular mass of H20.

Molecular mass of H20
= (2 x 1) + (16)
= 2 + 16 = 18 u


Q12. Give the atomicity of phosphorous and nitrogen.

The atomicity of phosphorus is P4 i.e., 4.
The atomicity of nitrogen is N2 i.e., 2. 


Q13. What is an ion?

Charged atom is called an ion. The ion can be positively charged called cation or negatively charged called anion. 


Q14. Give one example of cation and anion.

Cation => Na+
Anion => Cl


Q15. Give one difference between cation and anion.

Cations are positively charged ion.
Anions are negatively charged ion.




Short Answer Type Questions

Q1. Give the unit to measure the size of an atom and give the size of a hydrogen atom.

The unit to measure the size of an atom is nanometer, and the size of a hydrogen atom is 10-10m.


Q2. What is IUPAC, give its one function?

IUPAC is International Union for Pure and Applied Chemistry. It approves the names of elements.


Q3. Give the Latin name for sodium, potassium, gold and mercury.

Sodium —> Natrium, Gold —> Aurum
Potassium —> Kalium, Mercury —> Hydrargyrum


Q4. What is the ratio by mass of combining elements in H2O, CO2 and NH3?

H2O ratio by mass of combining elements 2: 16 —>1: 8 (H: O)
CO2 ratio by mass of combining elements 12: 32—> 3: 08 (C: O)
NH3 ratio by mass of combining elements 14 : 3—>14 : 3 (N: H)


Q5. Define valency and give the valency for the following elements:
Magnesium, Aluminium, Chlorine and Copper. 

Valency: The combining capacity of an element is called its valency. Valency of the following elements:
Magnesium – 2
Aluminium – 3
Chlorine – 1
Copper – 2 


Q6. What is a polyatomic ion? Give one example.

A group of atoms carrying a charge is known as a polyatomic ion.
E.g., Ammonium – NH4+
Nitrate – N03 


Q7. Write down the formula for:
Copper nitrate, calcium sulphate and aluminium hydroxide.

Chemical formula:
Copper nitrate —> Cu(N03)2
Calcium sulphate —> CaS04 

Aluminium hydroxide —> Al(OH)3


Q8. What is formula unit mass? How is it different from molecular mass?

The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. The constituent particles of formula unit mass are ions and the constituent particles of molecular mass are atoms.


Q9. What are the rules for writing the symbol of an element?

IUPAC —> International Union of Pure and Applied Chemistry approves name of elements.
Symbols are the first one or two letters of the element’s name in English. The first letter of a symbol is always written as a capital letter (upper case) and the second letter as a small letter (lower case).
e.g., Hydrogen —> H Helium —> He
Some symbols are taken from the names of elements in Latin, German or Greek.
e.g., Symbol of iron is Fe, its Latin name is Ferrum.
The symbol of sodium is Na, its Latin name is Natrium. 


Q10. Explain relative atomic mass and relative molecular mass.

Relative atomic mass: It can be defined as the number of times one atom of a given element is heavier than 1/12 th of the mass of an atom of carbon-12. Relative Molecular Mass: It is defined as the number of times one molecule
of a substance or given element is heavier than 1/12 th of the mass of one atom of carbon-12. 


Q11. The formula of carbon dioxide is CO2. What information do you get from this formula? 

(i) CO2 represents carbon-dioxide.
(ii) CO2 is one molecule of carbon dioxide.
(iii) CO2 is one mole of carbon dioxide i.e., it contains 6.022 x 1023 molecules of carbon dioxide.
(iv) CO2 contains 1 atom of carbon and two atoms of oxygen.
(v) CO2 represents 44 g of molar mass. 


Q12. State 3 points of difference between an atom and an ion.

AtomIon
An atom has no charge.An ion has either positive or negative charge.
Number of electrons = number of protons.Number of electrons ≠ number of protons.
Atom is reactiveIon is stable


Q13. Calculate the formula unit mass of NaCl and CaCl2.
(Na = 23, Cl = 35.5, Ca = 40)

Formula unit mass of NaCl = 23 + 35.5
= 58.5 u
Formula unit mass of CaCl2 = 40 + (2 x 35.5)
= 40 + 71 = 111 u 


Q14. The ratio by mass for hydrogen and oxygen in water is given as 1 : 8 respectively. Calculate the ratio by number of atoms for a water molecule.

The ratio by the number of atoms for a water molecule are:

ElementRatio by massAtomic massMass ratio/Atomic mass Simplest ratio
H111/1 = 12
O8168/16 = 1/21

Thus, the ratio by the number of atoms for water is H: O = 2: 1.


Q15. Write down the chemical formula for the following compounds:
(a) Aluminium carbonate
(b) Calcium sulphide
(c) Zinc carbonate
(d) Copper phosphate
(e) Magnesium bicarbonate
(f) Aluminium hydroxide.

The chemical formula are:
Class 9 Science Chapter 3 Question Answers - Atoms and Molecules
Class 9 Science Chapter 3 Question Answers - Atoms and Molecules


Q16. Give the atomicity of the following compounds:
(a) Ca(OH)2 (d) H2S04
(b) Mg(HC03)2 (e) Al2(S04)3
(c) Cu20. (f) MgCl2

The atomicity of the molecules are:
Class 9 Science Chapter 3 Question Answers - Atoms and Molecules


Q17. Explain the difference between 20, 02 and 03.

2O —> It represents 2 atoms of oxygen (cannot exist independently).
O2 —> It represents one molecule of oxygen (made up of 2 atoms) that can exist freely.
O3 —> It represents one molecule of ozone (made up of 3 atoms) it can exist independently.


Long Answer Type Questions

Q1. (a) How do atoms exist?
(b) What is atomicity?
(c) What are polyatomic ions?

(a) Atoms of some elements are not able to exist independently. For such elements, atoms form molecules and ions. In the case of metals and inert gases, atoms can exist independently.
Class 9 Science Chapter 3 Question Answers - Atoms and Molecules
(b) The number of atoms constituting a molecule is known as its atomicity.
E.g., O3 —> atomicity is 3
O2 —> atomicity is 2
(c) Polyatomic ions: When more than two atoms combine together and act like an atom with a charge on it is called a polyatomic ion.
E.g., OH, N03, NH4+


Q2. Calculate
(a) the mass of one atom of oxygen
(b) the mass of one molecule of oxygen
(c) the mass of one mole of oxygen gas
(d) the mass of one ion of oxygen
(e) the number of atoms in 1 mole of oxygen molecule

(a) Mass of one atom of oxygen
1 mole of oxygen atom  = 16 gm = 6.022 x 1023 atoms.
∴ Mass of one atom of oxygen = 16/6.022 x 1023 = 2.65 x 1023
(b) Mass of one molecule of oxygen
1 molecule of oxygen = O2
= 2 x 16
= 32 u
(c) Mass of one oxygen gas
1 molecule of oxygen gas is O2 = 32 u
(d) Mass of one ion of oxygen
One mole of oxygen = 6.022 x 1023 atoms = 16g. 16  
Mass of one ion of oxygen =  16/6.022 x 1023 
= 2.65 x 1023
(e) Number of atoms in one mole of oxygen molecule 1 mole of oxygen molecule i.e.
0= 6.022 x 1023 molecules.
1 molecule of 02 = 2 atoms.
∴ Number of atoms in 1 mole of oxygen molecule = 6.022 x 1023 x 2 atoms
= 1.2044 x 1024 atoms


Q3. What is meant by atomic mass, the gram atomic mass of an element? Why is the mass have different expressions i.e., ‘u’ and ‘g’?

The atoms are very tiny and their individual mass cannot be calculated as it is negligible. Hence the mass of atoms is expressed in units with respect to a fixed standard. Initially, a hydrogen atom with mass 1 was taken as a standard unit by Dalton. Later, it was replaced by an oxygen atom (0=16). But due to the isotopes, the masses were found in fractions instead of the whole number. Hence, carbon (C=12) isotope was taken as a standard unit and was universally accepted.
The atomic mass unit is equal to one-twelfth (1/12) the mass of an atom of carbon-12, its unit is u.
Gramatomic mass: When the atomic mass of an element is expressed in grams, it is called the gram atomic mass of the element.
The mass of atoms, molecules are expressed in ‘u’ and the mass of moles i.e., the molar mass is expressed in g.

The document Class 9 Science Chapter 3 Question Answers - Atoms and Molecules is a part of the Class 9 Course Science Class 9.
All you need of Class 9 at this link: Class 9
80 videos|363 docs|97 tests

Up next

FAQs on Class 9 Science Chapter 3 Question Answers - Atoms and Molecules

1. What are atoms and molecules?
Ans. Atoms are the basic building blocks of matter, comprising of protons, neutrons, and electrons. Molecules, on the other hand, are formed when two or more atoms combine chemically.
2. How do atoms and molecules differ?
Ans. Atoms are individual particles, while molecules are made up of two or more atoms bonded together. Molecules can be either of the same element or different elements.
3. What is the difference between an element and a compound?
Ans. An element is a substance made up of only one type of atom, whereas a compound consists of two or more different elements chemically combined in a fixed ratio.
4. How are atoms represented in chemical equations?
Ans. Atoms are represented using chemical symbols, which are typically the first letter or first two letters of the element's name. The number of atoms is indicated by subscripts written after the chemical symbol.
5. Can atoms or molecules be created or destroyed?
Ans. According to the law of conservation of mass, atoms and molecules cannot be created or destroyed in a chemical reaction. They can only be rearranged to form different substances.
80 videos|363 docs|97 tests
Download as PDF

Up next

Explore Courses for Class 9 exam
Signup for Free!
Signup to see your scores go up within 7 days! Learn & Practice with 1000+ FREE Notes, Videos & Tests.
10M+ students study on EduRev
Download the FREE EduRev App
Track your progress, build streaks, highlight & save important lessons and more!
Related Searches

Class 9 Science Chapter 3 Question Answers - Atoms and Molecules

,

mock tests for examination

,

study material

,

Class 9 Science Chapter 3 Question Answers - Atoms and Molecules

,

Previous Year Questions with Solutions

,

Sample Paper

,

Exam

,

pdf

,

Semester Notes

,

Free

,

MCQs

,

past year papers

,

practice quizzes

,

Viva Questions

,

video lectures

,

Class 9 Science Chapter 3 Question Answers - Atoms and Molecules

,

Summary

,

shortcuts and tricks

,

ppt

,

Objective type Questions

,

Extra Questions

,

Important questions

;