Class nine Science Atoms and molecules Extra Ques


Main Points of the chapter

(i) Law of conservation of mass: It states that mass can neither be created nor be destroyed in a chemical reaction, i.e., the total mass of reactants must be equal to the total mass of products.

Law of conservation of mass is also known as “Law of the indestructibility of matter.”

(ii) Law of constant proportion: It states, that in a pure chemical substance, the elements are always present in a definite proportion by mass. It is also known as the law of definite proportions.

Atoms: It is the smallest particle of an element that maintains its chemical identity throughout all chemical and physical changes.

Daltons’ Atomic theory

  • Every matter is made of very small particles called the atoms.
  •  Atoms are indivisible particles which cannot be created nor be destroyed in a chemical reaction.
  • Atoms of a given element are identical in mass as well as in chemical properties.
  •  Atoms of different elements have different masses and chemical properties.
  • Atoms combine in the ratio of small whole numbers forming compounds.
  • The relative number, as well as kinds of atoms, are constant in a given compound.

Drawbacks of Dalton’s Atomic Theory :

  • According to modern theory, the atom is not the ultimate indivisible particle of matter. Atom is divisible, i.e., they are themselves made-up of particles (protons, electrons, neutrons etc.).
  • In the case of isotopes of an element, the assumption that the atoms of the same element have the same mass does not hold good.

Atomic Symbols: A symbol signifies a shorthand representation of an atom of an element. The symbol of any element is based on the English name or Latin name and is represented by using only its first letter or the first letter and another letter.

  • The first letter is written in capital and the second letter in small.
  • The second letter can be any letter in the name of the element, which is internationally accepted.

elements named after places: Scandium (Sc), found in the earlier name of Scandinavia, Europium (Eu) – after the continent Europe, Polonium (Po ) – named after the curies after their home town in Poland. Named after Planets: Selenium (Se) – ‘Seles’ Greek name for the Moon, Plutonium (Pu) – Neptunium (Np), Uranium (U), Mercury (Hg) was named after ‘a planet but derives its symbol Hg from the Latin worde Hydragyrum’ meaning liquid silver.

 Named after Scientists : Curium (Cm) after Pierre and Marie curie, Fermium (Fm) after Enrico Fermi, Einsteinium (Es) after Albert Einstein, Mendelevium (Md) after Dimition Mendeleev.

Molecules of Elements: The molecules of an element are contributed by the same type of atoms. Molecules of many elements are made up of one atom of that element,e.g., argon (Ar), helium (He) etc. The molecules of most of the non-metals are made up of more than one atoms. For example, a molecule of oxygen consists of two atoms of oxygen. Ozone consists of three atoms of oxygen.

Main Points of the chapter

Class nine Science Atoms and molecules Extra Ques

Atomicity: The number of atoms present in a molecule of an element is called its atomicity.

Molecules of Compounds: Atoms of different elements join together in definite proportions to form molecules of compounds.

Chemical Formula: Chemical formulae of a compound is a symbolic representation of its composition.

The chemical formulae of a molecular compound are written as follows

1. Write the symbols of the side of the constituent element by side, in such a way that the cation is on the left and the anion is on the right.

2. Write their valency numbers over the symbols and criss-cross the valency number to write as subscripts to the symbols.

For example Formula of Hydrogen Chloride

Thus, the formula of the compound will be H1 Cl1 or HCl.

Formulae of hydrogen sulphide

The formula of the compound will be H2S1 or H2S.

The formula of calcium oxide

Formula: Ca2O2 or CaO

Molecular Mass: The molecular mass of a substance is the sum of the atomic masses of all atoms in a molecule of the substance.

For example, Molecular mass of water is 18.

H2O = 2 × H × 1 × O

= 2 × 1 × 1 × 16 = 18 amu .

Class nine Science Atoms and molecules Extra Questions

Atomic mass unit (amu or μ): The mass of 1/12 part of C – 12 is equivalent to one atomic mass unit. Previously, it was denoted by symbol amu but now, these days it is denoted by the symbol.

Mole concept: 1 mole of a compound has a mass equal to its relative molecular mass expressed in grams.

1 mole = 6.022 × 1023 number = Relative mass in grams.

The mole (or mole) is the SI unit of amount of a substance. One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams. The number of particles present in 1 mole of any substance is fixed, equal to 6.022 × 1023. This is a constant called the Avogadro’s number (No) or Avogadro’s constant (NA = 6.02 × 1023).