Structure of Atom – Text Book Solutions

Q. 1. What are canal rays?

Ans. The positively charged radiations discovered by E. Goldstein in a gas discharge are called canal rays.

Q. 2. If an atom contains one electron and one proton, will it carry any charge or not?

Ans. No, the atom will not carry any charge. It will be neutral.

Q. 3. On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus of an atom?

Ans. According to Rutherford’s model, proton is present in the nucleus of an atom.

Q. 5. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

Ans According to J.J. Thomson’s model, an atom is a sphere in which positive charge is uniformly distributed. The electrons are embedded into it in such a way to give most stable electrostatic arrangement. This model explains the neutrality of the atom because equal number of electrons and positively charged particles are embedded together.

Q. 6. What do you think would be the observation if the a—particle scattering experiment is carried out using a foil of a metal other than gold?

Ans. Rutherford used a very thin foil of gold (0.00006 in thick) because it is the most malleable metal. The a-particles were much smaller than the gold atoms in the foil. But if any other metal would have been used, there I would not have been much size difference between u-particles and metal atoms and therefore there would not have been any penetration.

Q. 7. Name the three sub-atomic particles of an atom.

Ans. The three sub atomic particles of an atom are electron, proton and neutron.

Q. 8. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Ans. Number of neutrons = Atomic mass — Number of protons

Therefore, the number of neutrons in the atom = 4—2 = 2

Q. 9. Write the distribution of electrons in carbon and sodium atoms.

Ans. (a) Electronic distribution in carbon atom
Atomic number of carbon 6
Number of electrons = 6
Distribution of electrons: K = 2, L= 4

(b) Distribution of electrons in sodium atom
Atomic number of sodium atom = 11
Number of electrons in sodium atom = 11
Distribution of electrons in sodium atom: K = 2, L = 8, M = 1

Q. 10. If K and L shell of an atom are full, then what would be the total number of electrons in the atom?

Ans.. The maximum capacity of K shell is 2 electrons and L shell can accommodate maximum 8 electrons in it. Therefore, there will be ten electrons in the atom.

Q. 11. How will you find the valency of chlorine, sulphur and magnesium?

Ans. (i) To determine the valency of chlorine atom
Atomic number of chlorine = 17
Number of electrons in chlorine atom = 17
Distribution of electrons in chlorine atom: K 2, L = 8, M = 7
Valency of chlorine atom = 8— 7 = Valency of chlorine atom is 1.
(ii) To determine the valency of sulphur atom
Atomic number of sulphur atom = 16
Number of electrons in sulphur atom = 16
Distribution of electrons in sulphur atom: K=2, L= 8, M = 6
Valency of sulphur atom = 8—6 = 2

Valency of sulphur atom is 2.

(iii) To determine the valency of magnesium atom
Atomic number of magnesium = 12
Number of electrons present in magnesium atom = 12
Distribution of electrons in magnesium atom: K= 2, L = 8, M = 2
Valency of magnesium atom is 2 since there are only two electrons in the outermost shell.

Q. 12. If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is the atomic number of the atom and(ii) what is the charge on the atom?

Ans. (1) Atomic number will be 8.
(ii) Charge would be zero because number of the positive charges (protons) and negative charges (electrons) is equal.


What is the relation between the two species?

Composition of the nuclei of two atomic species ‘X’ and ‘Y’ are given as under
                       X    Y
Protons         6      6
Electrons      6      8
Give the mass numbers of X and Y. What is the relation between the two species?
Mass number of X = 6 + 6 = 12
Mass number of Y 6 + 8 = 14
Atomic number of  X = 6
Atomic number of  Y = 6
X and Y are isotopes, since both the elements contain equal number of atomic number and different mass number.

Atoms and Molecules – Long Answer Questions

Q1. What are the postulates of Dalton’s atomic theory of matter?

Ans. According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms. Important points of Dalton’s atomic theory are,

-> All matter is made of very tiny particles called atoms.
-> Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
-> 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 kinds of atoms are constant in a given compound.

Q2. Define and explain atomic mass of an element.

Ans. The atomic mass of an element is the relative mass of its atom as compared with the mass of a particular atom of carbon-12 (\({ C }^{ 12 }\)) isotope taken as 12 units. Thus the atomic mass of an element indicates the number of times one atom of an element is heavier than 1/12th of a carbon-12 (\({ C }^{ 12 }\)) isotope atom. For example, the atomic mass of oxygen is 16 which indicates that an atom of oxygen is 16 times heavier than 1/12th of a carbon-12 (\({ C }^{ 12 }\)) isotope atom.

Q3. Define:

(i). Atomicity    (ii). Ion    (iii). Molecular mass    (iv). Relative formula mass.


(i). Atomicity: Number of atoms in a single molecule of an element is known as atomicity. This can also be defined as the number of atoms constituting a molecule.

(ii). Ion: An ion is a charged particle which can be positively or negatively charged. A negatively charged ion is called an anion and a positively charged ion is called a cation. Ions may consist of a single charged atom or a group of atoms that have a net charge on them known as polyatomic ion.

(iii). Molecular mass: Molecular mass of a substance is the relative mass of its molecule expressed in atomic mass unit (u). It is equal to the sum of atomic masses of all the atoms present in one molecule of the substance. For example, one molecule of water (\({ H }_{ 2 }O\)) contains two atoms of hydrogen and one atom of oxygen.

Molecular mass of water = 2 x atomic mass of hydrogen + 1 x atomic mass of oxygen

= 2 + 16 = 18 u

(iv). Relative formula mass (or formula mass): The formula mass of a substance is the sum of atomic masses of all atoms in a formula unit of a compound. Formula mass is used for substances whose constituent particles are ions.

Q4. What are the rules of writing a chemical formula?


    How is the molecular formula of a compound written?

Ans. In order to write formula of a compound comprised of cations and anions, the valencies of the ion must be known. Let us consider a compound composed of cation A with valency \({ x }^{ + }\) and anion B with valency \({ y }^{ – }\). Then the molecular formula can be written by following the rules given below:

(i). Write down the symbols of the cation and anion side by side.

\( A \)   \( B \)

(ii). Write their valencies at the top corners as

\({ A }^{ x+ }\)    \({ B }^{ y- }\)

(iii). The valencies or charges on the ions must balance after combining. So, interchange between the ions their valencies and these are placed on the lower side of each radical or used as subscripts.

\({ A }_{ y }\)    \({ B }_{ x }\)

(iv). If a radical is multi-atomic, use a small bracket around it before writing the valency number to indicate the ratio. For example, calcium phosphate, a compound of calcium ion with valency \({ x }^{ + }\) and phosphate ion with valency \({ y }^{ – }\) is written as \(C{ a }_{ 3 }{ (P{ O }_{ 4 }) }_{ 2 } \)

(v). Eliminate the common factor, if any, from the numbers used in subscripts.

Atoms and Molecules – Short Answer Questions

Q1. Write the names of symbols of five elements where the symbols are taken from their name in a language other than English.

Ans. The names of symbols of the following elements have been taken from their name in a language other than English: Silver (Ag), gold (Au), lead (Pb), sodium (Na), iron (Fe) etc.

Q2. What is valency? Mention a use of valency.

Ans. Valency is the combining capacity of an element. It can be used to find out how many atoms of an element will combine with the other element to form a chemical formula. For example, hydrogen has a valency + 1 and chlorine has a valency —1, so one atom of hydrogen combines with one atom of chlorine to form hydrochloric acid (HC1). Oxygen has a valency —2, so one atom of oxygen combines with two atoms of hydrogen to form water molecule (\( { H }_{ 2 }O \)).

Q3. How many atoms are present in 0.35 mol of pure copper metal?

Ans. 1 mol of copper metal = 6.022 x \({10}^{23}\) atoms of Cu

Therefore 0.35 mol of copper metal = 0.35 x 6.022 x \({10}^{23}\) atoms of Cu

= 2.1 x \({10}^{23}\) atoms of Cu

Q4. An atom of neon has a mass of 3.35 x \({10}^{-23}\) g. How many atoms of neon are there in 20 g of the gas?

Ans. Number of atoms

= \(\frac{Total   mass}{Mass  of  1  atom}\)

= \( \frac { 20 g }{ 3.35\times { 10 }^{ 23 } g } \)

= 5.97 x \({10}^{23}\)

Q5. How many atoms are there in

(i). 100 u of helium,

(ii). 100 g of helium? (atomic mass of helium is 4 u)


(i). Number of He atoms in 100 u = \(\frac{Total  mass  of  helium}{Mass  of  1  atom  of  helium}\)

= \(\frac{100 u}{4 u}\)

= 25

(ii). Number of He atoms in 1OO g = No. of moles of helium x Avogadro number

= \(\frac{Total  mass}{Mass  of  1  atom}\) x 6.022 x \({10}^{23}\)
= \(\frac{100 g}{4 g}\) x 6.022 x \({10}^{23}\)
= 25 x 6.022 x \({10}^{23}\)
= 1.5 x \({10}^{25}\)

Q6. Calculate the number of moles in 5.75 g of sodium. (atomic mass of sodium is 23)

Ans. 1 mol of sodium atoms = Gram atomic mass of sodium = 23 g

23 g of sodium = 1 mol of sodium

5.75 g of sodium = (5.75/23) mol of sodium = 0.25 mol

Q7. How many grams of each of the following elements must be taken to get 1 mol of the element?

        (i). Sodium             (ii). Chlorine         (iii). Copper

Ans. The mass of 1 mol of an element is its atomic mass expressed in grams. Since the atomic masses of sodium, chlorine and copper are 23 g, 35.5 g and 63.6 g respectively, we get

(i). 23 g             (ii). 35.5 g             (iii). 63.5 g

Atoms and Molecules – More Short Answer Questions

Q1. Explain the law of conservation of mass.

Ans. The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. For example, in the reaction \(C+{ O }_{ 2 }\longrightarrow C{ O }_{ 2 } \), total mass of carbon dioxide produced is equal to the total mass of carbon and oxygen gas taking part in the reaction.

Q2. What is the law of constant proportions?

Ans. The law of constant proportions, also known as the law of definite proportions, was established by Proust. According to this law, in a chemical substance the elements are always present in definite proportion by mass. All pure samples of a compound contain the same elements combined together in the same proportion by mass. For example, a sample of water would always contain hydrogen and oxygen in the ratio of 1:8 by mass irrespective of the source of water.

Q3. What is the full form of IUPAC? What is the present accepted norm of IUPAC for naming symbols of elements?


The full name of IUPAC is International Union of Pure and Applied Chemistry.

Many of the 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 is always a small letter (lower case). For example, hydrogen is written as H and aluminium is written as Al. Some symbols are formed from the first letter of the name and a letter, appearing later in the name. For example, chlorine as Cl and zinc as Zn. Some other symbols are taken from the names of elements in Latin, German or Greek. For example, the symbol of iron is Fe from Latin name ferrum.

Q4. Find the ratio by number of calcium and oxygen in the compound calcium oxide. (Given ratio by mass of calcium and oxygen in the compound is 5:2. Atomic mass of Ca = 40 u and O = 16 u)



Mass ratio

Atomic mass

Number of atoms
(Mass ratio/atomic mass)

Simplest ratio











Thus, the formula of calcium oxide should be CaO as the simplest ratio of the elements is 1:1.

Q5. Determine the mass of the following:

(i). 6.022 x  \({ 10 }^{ 23 } \) numbers of \({ O }_{ 2 }\) molecules

(ii). 6.022 x \({ 10 }^{ 23 } \)numbers of \({ O }_{ 2 }\) atoms (atomic mass of oxygen = 16 u)


(i) The number of moles = \(\frac { Number\quad of\quad molecules }{ Mass\quad Avagadro’s\quad constant } \)

= \(\frac { 6.022\times { 10 }^{ 23 } }{ 6.022\times { 10 }^{ 23 } } \) = 1

Now, Mass = Mole x Molecular mass = 1 x 32 = 32 g

(ii). The number of moles = \(\frac { Number\quad of\quad molecules }{ Mass\quad Avagadro’s\quad constant } \)

= \(\frac { 6.022\times { 10 }^{ 23 } }{ 6.022\times { 10 }^{ 23 } } \) = 1

Now, Mass = Mole x Molecular mass = 1 x 16 = 16 g

Atoms and Molecules – Very Short Answer Questions

Q1. Name the smallest particle that has all the properties of an element.

Ans. Atom.

Q2. How many ions are present in one mole of ions?

Ans. 6.022 x \({10}^{23}\).

Q3. Write the names of symbols of elements where the symbols are taken from their names in English.

Ans. The symbols of the following elements have been taken from their names in English: Calcium (Ca), oxygen (0), zinc (Zn), magnesium (Mg) etc.

Q4. What is an atom according to Dalton’s atomic theory?

Ans. According to Dalton’s atomic theory, an atom is the ultimate particle of matter which cannot be further divided into anything simpler than itself.

Q5. Carbon dioxide, collected from different sources contains carbon and oxygen in the same proportion. Which law of chemical combination governs this?

Ans. This is governed by law of constant proportions.

Q6. Give the chemical names and chemical formulae for the following compounds having common names:

Baking soda, washing soda, blue vitriol, green vitriol, gypsum, oil of vitriol or white vitriol, soda ash, marble, lime water


Common name

Chemical name

Chemical formula

Baking soda

Sodium bicarbonate

\(NaHC{ O }_{ 3 }\)

Washing soda

Sodium carbonate

\(N{ a }_{ 2 }C{ O }_{ 3 }.10{ H }_{ 2 }O\)

Blue vitriol

Copper sulphate

\(CuS{ O }_{ 4 }.5{ H }_{ 2 }O\)

Green vitriol

Ferrous sulphate

\(FeS{ O }_{ 4 }.7{ H }_{ 2 }O\)


Calcium sulphate

\(CaS{ O }_{ 4 }.2{ H }_{ 2 }O\)

Oil of vitriol

Sulphuric acid

\({ H }_{ 2 }S{ O }_{ 4 }\)

Soda ash

Sodium carbonate

\(N{ a }_{ 2 }C{ O }_{ 3 }\)


Calcium carbonate

\(CaC{ O }_{ 3 }\)

Lime water

Calcium hydroxide

\(Ca({ OH) }_{ 2 }\)