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.

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⁺ SO₄^2-
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.

Atom	Ion
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 reactive	Ion 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:

Element	Ratio by mass	Atomic mass	Mass ratio/Atomic mass	Simplest ratio
H	1	1	1/1 = 1	2
O	8	16	8/16 = 1/2	1

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:

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:

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.

(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.
0₂ = 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.