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 – 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

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