Class IX, NCERT (CBSE) | Chapter 3, Atoms and Molecules | Study Materials, Notes and Important Sample Questions

 Class IX, Ncert Cbse Chemistry (Science) Study Materials
(All presented in the Question/Answer format for your convenience)
Q.1: What are the ‘laws of chemical combination’?
Answer: There are two important laws of chemical combination established by Lavoisier, are as follows:
(i) Law of Conservation of Mass
The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
(ii) Law of Constant Proportions
The Law of Constant Proportions which is 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.     
Q.2: Explain the ‘Law of Conservation of Mass’.
Answer: See answer of Q. 1 above.
Q.3: What is the Law of Constant Proportion ?
Answer: See answer of Q. 1 above.
Q.4: What are the postulates of Dalton’s Atomic Theory of matter ?
Answer: According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms. The different postulates of Dalton’s atomic theory are stated as follows:
(1) All matter is made of very tiny particles called atoms.
(2) Atoms are indivisible particles, which can not be created or destroyed in a chemical reaction.
(3) Atoms of a given element are identical in mass and chemical properties.
(4) Atoms of different elements have different masses and chemical properties.
(5) Atoms combine in the ratio of small whole numbers to form compounds.
(6) The relative number and kinds of atoms are constant in a given compound.    
Q.5: What is the full form of IUPAC ? What is the present accepted norm of IUPAC for naming symbols of elements ?   
Answer: 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.    
Q.6: Write the names of symbols of five elements where the symbols are taken from their name in English.  
Answer: The symbols of the following elements have been taken from their name in English: Calcium (Ca), Oxygen (O), Zinc (Zn), Magnesium (Mg) etc.
Q.7: Write the names of symbols of five elements where the symbols are taken from their name in a language other than English.
Answer: 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.
Q.8: Define and explain atomic mass of an element.
Answer: 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 (12C) 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 1/12 th of a Carbon-12 (12C) isotope atom. For example, the atomic mass of oxygen is 16 which indicates that an atom of oxygen is 16 times heavier than 1/12 th of a Carbon-12 (12C) isotope atom.     
Q.9: Distinguish between a ‘molecule’ and a ‘compound’.
Molecule - a molecule is usually formed when at least two atoms of the same or different kinds combine. If two or three atoms of the same kind of element combine, then it is referred to as a molecule of an element or simply Molecule. For example, molecule of oxygen(O2) is formed by the combination of two oxygen atoms. However, there are exceptions such as argon (Ar), helium (He) which are made up of only one atom of that element.
Compound - Molecule formed by union of two or more atoms of different atoms is called a molecule of compound or simply Compound. For example, a molecule or a compound of carbon dioxide is formed by the union of one carbon atom and two oxygen atoms. However,  the noble gases consists of molecules of single atoms only.     
Q.10wer: Define: (a) Atomicity (b) Ion (c) Molecular Mass (d) Relative Formula Mass
(a) 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.
(b) 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.
(c) Molecular Mass - The 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 the atomic masses of all the atoms present in one molecule of the substance. For example, one molecule of water (H2O) 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.
(d) 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.       
Q.11: What is valency ? Mention use of valency.
Answer: Valency is the combining capacity of an element.
Valency 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 (HCl). Oxygen has a valency –2, so one atom of oxygen combines with two atoms of hydrogen to form water molecule (H2O).      
Q.12: What are the rules of writing a chemical formula? or, How is a molecular formula of a compound written ?
Answer: 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
      Ax+  By–
(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.
      Ay Bx    
(iv) If a radical is multi-atomic, use a small bracket around it before writing the valency number to indicate the ratio.
(v) Eliminate the common factor, if any, from the numbers used in subscripts. For example, calcium phosphate, a compound of calcium ion with valency 2+ and phosphate ion with valency 3 is written as Ca3(PO4)2.  
Q.13: Give the chemical name, 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
Washing Soda
Blue Vitriol
Green Vitriol
Oil of Vitriol
Soda ash
Lime water
Sodium bicarbonate
Sodium carbonate
Copper sulphate
Ferrous sulphate
Calcium sulphate
Sulphuric acid
Sodium carbonate
Calcium carbonate
Calcium hydroxide
CaSO4.2 H2O

Q.14: Name the smallest particle that has all the properties of an element.
Answer: Atom.
Q.15: How many ions are present in one mole of ions ?
Ans: 6.023 x 1023
Further study on this chapter:

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