Atoms and Molecules, Chemistry, Class IX

1. What is the law of conservation of mass?

Ans.: According to the law of conservation of mass, during a chemical reaction matter is neither created nor destroyed. Thus, total mass of the products is equal to the total mass of the reactants.

2. What is the law of constant proportions?

Ans.: A chemical compound always consists of the same elements combined together in the same ratio, independent of the method by which it is prepared, or the source from where it is obtained. For example, water is always found to contain only hydrogen and oxygen. The ratio is which hydrogen and oxygen are present in water is also found to be fixed at 1:8 by mass or 2:1 by volume independent of the source of water.

3. What are atoms?

Ans.: An atom is the smallest particle of an element that takes part in chemical reactions and maintains its chemical identity, throughout all chemical and physical changes.

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

Ans.: the major postulates of Dalton’s atomic theory are:

(i) all forms of matter are made up of very small particles called atoms.

(ii) atoms cannot be created, divided or destroyed as a result of a chemical change.

(iii) all atoms of an element are identical, and different from those of the other elements.

(iv) atoms of elements combine in the ratio of whole numbers to produce a large number of compound-atoms of a new substance.

According to Dalton’s atomic theory, an atom is the smallest, discrete and indivisible particle of matter.

Note: Modern researches have shown that the atom is not truly indivisible. The atom is made up of even smaller particles called electrons, protons and neutrons.

5. What are isotopes?

Ans.: Atoms of the same elements having different masses are called isotopes.

6. What is an atomic symbol?

Ans.: The symbol of an element is an abbreviation for the full name of the element. OR, the symbol of an element is a shorthand notation for its name. Symbol of an element consist of one or two letters derived from the Common or Latin or Greek name of the element. E.g. symbol of hydrogen is H, and tht of helium is He.

7. What is the significance of a symbol?

Ans.: The symbol of an element has both qualitative as well as quantitative significance. These are:

(i) the symbol stands for the name of the element.

(ii) the symbol stands for one atom of the element.

(iii) the symbol represents quantity of the element equal in mass to its atomic mass, or gram-atomic mass.

(iv) the symbol also represents mass of the element which contains one Avogadro’s number of atoms of that element.

6. What is the atomic mass unit?

Ans.: The mass equal to 1/12^th  of the mass of a carbon-12 atom is called one atomic mass unit. It is not a practical unit, because it is equivalent to a very small mass. 1 atomic mass unit is equal to 1.66 x 10^-27 kg.

7. What is relative atomic mass of an element?

Ans.: The relative atomics mass of an element is defined as the average relative mass of an atom of the element compared with an atom of carbon-12 taken as 12u. The relative atomic mass is a pure number, and hence it has no unit.

8. What is a molecule?

Ans.: A molecule is the smallest particle of an element or of a compound which can exist freely under ordinary conditions, and shows all the properties of that substance.

9. What are chemical formulae?

Ans.: Each chemical compound is known by a specific name. Writing the full name of a compound repeatedly is time-consuming and inconvenient. Therefore, in chemistry each substance is denoted by its chemical formula. There are two types of chemical formulae. These are:

(i) Molecular formula and (ii) empirical formula.

10. What is a molecular formula?

Ans.: The symbolic representation of a molecule of any substance representing the actual number of various atoms present in it is called its molecular formula. For example, the molecular formula of water is H₂O. Thus, one molecule of water contains two atoms of hydrogen and one atom of oxygen.

11. What is an empirical formula?

Ans.: Empirical formula of a substance is the simplest formula which gives the lowest whole-number ratio between the number of atoms of different elements present is that substance.

12. What is meant by the atomicity of a molecule?

Ans.: The number of atoms of all elements in a molecule of any substance is called its atomocity.

Note: Molecules are generally classified in terms of number of atoms present in it.

 E.g.  If molecules have two atoms then it is called diatomic molecules.  If molecules have three atoms then it is called triatomic molecules. If molecules have one atom them it is called monoatomic molecule.

13. What is an ion?

Ans.: The charged species obtained when an atom loses or gains electrons is called an ion. When an atom gains or loses one or more electrons, an ion is formed. Positively charged ion is called cation, and the negatively charged ion is called anion. Thus, a cation is formed when an atom loses one or more electrons and an anion is formed when an atom gains one or more electrons.

e.g.  When sodium atom loses an electron, sodium ion is formed. Sodium ion is positively charged.                  Na (g) –  e^-   =     Na⁺(g )

When chlorine atom gains one electron, a chloride ion is formed. Chloride ion is negatively charged.   Cl(g) –  e^-   =     Cl^-(g )

14. What are polyatomic ions? Give examples.

Ans.: A group of atoms carrying a net electrical charge on it is called polyatomic ions. E.g. Sulphate ion – SO₄^2-, Nitrate ion – NO₃^- etc.

15. What are valence electrons?

Ans.: The electrons present in the outermost shell are called valence electrons.

16. What is valency?

Ans.: The number of hydrogen or chlorine, or double the number of oxygen atoms which combine with one a tom of the element is termed its valency. E.g., in NaCl, the valency of Na is 1, because one sodium atom combines with one chlorine atom.

17. What are molecular compounds?

Ans.: The compounds formed by covalent bonding are called molecular compounds.

18. What is the molecular mass of a substance?

Ans.: The average mass of a molecule of a substance expressed in atomic mass units is termed as its molecular mass.

19. How is the molecular mass of a substance calculated?

Ans.: The molecular mass of any substance can be obtained by adding together the atomic masses of all the atoms present in one molecule of the substance. E.g., the molecular formula of water is H₂O. Therefore, Molecular mass of H₂O = 2 x Atomic mass of hydrogen + 1x Atomic mass of oxygen.

We know that, Atomic mass of hydrogen = 1u

And, Atomic mass of oxygen = 16 u

Therefore, Molecular mass of H₂O = 2 x 1u + 1 x 16u = 2u + 16u = 18u.

Note: The concept of formula mass is applicable only to ionic substances.

20. How to determine the mass percentage of an element in a compound?

Ans.: To determine the mass percentage of an element in any compound, we should know the mass of that element present in a certain known mass of the compound.

Therefore, Mass percentage of the element,

A = Mass of element A in the given mass of the compound/Total mass of the compound x 100

21. What is a mole?

Ans.: The mole is a concept of quantity in terms of number and mass. This concept relates the mass of any material to the number of atoms, ions or molecules in it.

OR

The mole is the amount of a substance which contains the same number of chemical units as there are atoms in exactly 12 gram of pure carbon-12. The unit of mole is given a symbol mol.

Note: Avogadro’s number: Avogadro’s number is commonly denoted by N_a or N₀. It is 6.023 x 10²³.

22. What is molar mass?

Ans.: Mass of one mole of any material is called its molar mass. The molar mass is measured in the units of gram per mole.

23. What is molar volume?

Ans.: One mole of any gaseous substance at 273K and 1 atmosphere pressure occupies a volume equal to 22.4 L or 22400mL. The volume occupied by one mole of any gaseous substance is called molar volume. So, molar volume of any gaseous substance at 273K and 1 atm pressure = 22.4 mol^-1 = 22400mL mol^-1.