Notes-Class 9-Science-Chapter-4-Measurement of Matter-Maharashtra Board

Topics to be learn :

• Laws of chemical combination
• Atom — Shape, mass, valency
• Molecular mass and the concept of mole
• Radicals

 Dalton’s atomic theory:

• Dalton proposed that matter is made up of very small particles. These small particles are called atoms.
• An atom is a hard and solid ball.
• Being the smallest unit of matter, it is indivisible.

 Formation of compounds: The compounds are formed by a chemical reaction of different elements.

 Q.What are the molecular formulae of salt, slaked lime, water, lime, lime stone?

 Law of conservation of matter : There is no rise or drop in the weight of the matter during a chemical reaction. In a chemical reaction the total weight of the reactants is the same as the total weight of the products formed due to the chemical reactions and this is called the law of conservation of matter.

Law of constant proportion :The proportion by weight of the constituent elements in the various samples of a compound is fixed.

• Irrespective of the source, water always contains the same elements. i.e. hydrogen and oxygen combine together in the fixed ratio of 1 : 8 by weight to form water.
• Similarly, 44 g carbon dioxide always contains 12 g carbon and 32 g oxygen together in a fixed ratio of 12 : 32 i.e. 3 : 8 by weight.

 Atom : Atoms are very very tiny. The fundamental particles in atom are the protons, neutrons and electrons. The proton is positively charged particle. The electron is negatively charged particle. The neutron is electrically neutral.

• The size of an atom is determined by its radius. The atomic radius of an isolated atom is the distance between the nucleus of an atom and its outermost orbit. It is expressed in nano-metres.

• In 1897, Sir J .J . Thomson studied the cathode rays in detail and determined the ratio of charge to the mass of the cathode rays particles. From this experiments it was discovered that the atoms have an internal structure.

Due to atoms are very very tiny, modern instruments like the electron microscope, field ion microscope, scanning tunnelling microscope have the capacity to show enlarged images of the atom.

Atomic size: The atomic size depends on the number of electron orbits in the atom. The greater the number of orbits the larger the size.

Example : An atom of K is bigger than an atom of Na.

If two atoms have the same outermost orbit, then the atom having the larger number of electrons in the outermost orbit is smaller than the one having fewer electrons in the same outermost orbit.

Example- An atom of Mg is smaller than an atom of Na.

Atomic mass number : The sum of the number of protons and neutrons in the nucleus of the atom is called the atomic mass number (p + n).

Unified Mass has been accepted as the unit of atomic mass. It is called Dalton. Its symbol is u.

1 u = 1.66053904 x 10^-27 kg.

Symbols of elements : The abbreviation of the name of an element is either the first letter or the first and second/ another specific letter in its name. It is called the symbol of an element.

Molecule: Molecule is made of two or more atoms of the same element or different elements.

Some elements are in a monoatomic molecular state and some are in a polyatomic molecular state.

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

It is expressed in the unit Dalton (u).

Example : Molecular Mass of Magnesium Chloride (MgCl₂)

Molecular mass = Sum of constituent atomic masses -

Molecular mass of MgCl₂ = (Atomic mass of Mg) x 1 + (Atomic mass of Cl) x 2

= (24) x 1 + (35.5) x 2 = 24 + 71 = 95

Molecular mass of MgCl₂ = 95 u.

(1) It is not possible to weigh one molecule using a weighing balance. (2) The number of molecules cannot be the same in equal weights of different substances. (3) If we want equal number of molecules of different substances, it will not work if we take the equal weights of those substances.

Mole : A mole is that quantity of a substance whose mass in grams is equal in magnitude to the molecular mass of that substance in Daltons.

• The molecular mass of oxygen is 32u = 32g of oxygen is 1 mole of oxygen
• The atomic mass of carbon is 12 u = 12g carbon is 1 mole of carbon.

Number of moles of a substance (n) = \(\frac{\text{mass of substance in grams}}{\text{Molecular massof substance}}\)

Avogadro's number (Avogadro constant): The number of molecules in one mole of any substance is constant. A mole of any substance stands for 6.022 x 10²³ molecules. Therefore this number is called Avogadro’s number. It is

denoted by N_A.

Example: A mole of water, i.e. 18 g of water contains 6.022 x 10²³ molecules of water.

 Valency : The capacity of an element to combine is called its valency. The valency of an element is indicated by a specific number. The number of electrons that an atom of an element gives away or takes up while forming an ionic bond, is called the valency of that element.

The number of electrons that an atom of an element gives or takes to make the outermost orbit stable determines the valency of that element.

 Q.The valency of carbon is 4. Give Reason

Q.Neon is a chemically inert element. Give Reason

Cations & anions :Positively charged ions are called cations, while negatively charged ions are called anions.

• Example, MgCl₂ contains Mg⁺⁺ and Cl^- as cation and anion respectively.

Valence electrons : The electrons present in the outermost orbit of an atom are called valence electrons.

 Variable valency : Under different conditions the atoms of some elements give away or take up different numbers of electrons. In such cases those elements exhibit more than one valency. This property of elements is called variable valency.

Iron (Fe) exhibits the variable valencies 2 and 3. Therefore, iron forms two compounds with chlorine FeC12 and FeCl3.

Q. Explain how the element sodium is monovalent.

Radicals: A cation (positively charged ion) and anion (negatively charged ion) are two constituents of compounds that take part independently in chemical reactions, and are therefore, called radicals.

(i) Simple radicals : A radical formed from a single atom (mono atomic) is called a simple radical. It is positively or negatively charged simple radical.

Positively charged simple radicals : K⁺, Ca²⁺, Al³⁺.

Negatively charged simple radicals : Cl^- , O^2-, N^3-.

(ii) Composite radicals :A radical formed from a group of atoms is called a composite radical. It is positively charged or negatively charged composite radical.