ATOM AND MOLECULE, class 9 notes

 

ATOM AND MOLECULE

 

The idea of divisibility of matter was considered around 500 BC india.

 

Maharishi Kanad (Indian philosopher) postulated that if we go on dividing matter (padarth) in smaller particle so that further division will not be possible. This smallest particle is known as pramadu .

 

Pakudha Katyayama (Indian philosopher): said that these particles normally exist in a combined form and form different forms of matter.

 

Democritus and Leucippus (Greek philosophers) according to them if we divide matter, a stage will come when particles obtained cannot be divided further. Democritus called these indivisible particles atoms (meaning indivisible).

 

·         Antoine L. Lavoisier laid the foundation of chemical sciences

 

Law of Chemical Combination
two laws is Given by Lavoisier and Joseph L. Proust as follows:

 

1.  Law of conservation of mass: Mass can neither be created nor destroyed in a chemical reaction.

e.g., A + B → C + D

Mass of reactants = Mass of products

2. Law of constant proportion: This law was stated by Proust as “In a chemical substance the elements are always present in definite proportions by mass”..

E.g., in water, the ratio of the mass of hydrogen to the mass of oxygen is always 1 : 8 respectively.

Dalton’s Atomic Theory :-

In 1808 John Dalton provided the basic theory about the nature of matter. His theory was based on the laws of chemical combination.

 

According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.

 

Postulates of Dalton’s atomic theory:-

·         All matter is made up of very tiny particles called ‘Atoms’. 

·         Atom are indivisible particles, which can’t be created or destroyed in a chemical reaction. (Proves ‘Law of Conservation of Mass’) 

·         Atoms of an element have identical mass and chemical properties. 

·         Atoms of different elements have different mass and chemical properties. 

·         Atoms combine in the ratio of small whole numbers to form compounds. (proves ‘Law of Constant Proportion’) 

·         The relative number and kinds of atoms are constant in a given compound.

Atom: - According to modern atomic theory, an atom is the smallest particle of an element which takes part in chemical reaction such that during the chemical reaction.

 Atoms are very small and hence can’t be seen even through very powerful microscope. 

 Measurement of Atomic radius: -   1 nm = 10⁹m

·         Atomic radius of smallest atom in hydrogen is 0.37 x 10^-¹⁰m or 0.037 nm. 

Dalton was the first scientist to use the symbols for elements

 

Berzilius suggested that pictorial symbols are difficult for typing in publications so the symbols of elements be made from one or two letters of the name of the element.

 

In the beginning, the names of elements were derived from the name of the place where they were found for the first time. For example, the name copper was taken from Cyprus.

 

IUPAC (International Union of Pure and Applied Chemistry) approves names of elements.

The abbreviations used for lengthy names of elements are termed as their symbols.

·         The symbol of an element is formed by writing only the first letter

•          Many of the symbols are the first one or two letters of the element’s name in English. the first letter is always capital and the second is always small. (i) chlorine, Cl, (ii) zinc, Zn
•      the symbols have been taken from the names of elements in Latin, German or Greek. For example, the symbol of iron is Fe from its Latin name ferrum, sodium is Na from natrium, potassium is K from kalium

 

Atomic Mass :- The mass of an atom of an element is called its atomic mass.  Dalton’s atomic theory proposed was that of the atomic mass.

 

In 1961, IUPAC have accepted ‘atomic mass unit’ (u) to express atomic and molecular mass of elements and compounds. Now written as ‘u’ – unified mass

 

One atomic mass unit (amu)  is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12.

 

1 amu or u = 1/12 × Mass of an atom of C₁₂

·         Relative Atomic mass of an element may be defined as the average mass of an atom of the element as compared with the mass of an atom of carbon (C-12 isotope) taken as 12 amu.

·         Relative atomic mass= 

·         Gram Atomic Mass: The atomic mass of an element expressed in grams is known as gram atomic mass.

 

·         1u = 1.66 × 10²⁷kg

 

Various atomic mass units, scientists initially took 1/16 of the mass of an atom of naturally occurring oxygen as the unit.

 Reasons of using oxygen as reference:

•          Oxygen reacted with a large number of elements and formed compounds.
•          This atomic mass unit gave masses of most of the elements as whole numbers.

 

 How do atoms exist?

 Atoms of most of the elements are very reactive and do not exist in Free State. 

Only the atoms of noble gases (such as He, Ne, Ar, Kr, Xe and Rn) are chemically uncreative and can exist in the free state as single atom. 

Atoms of all other elements combine together to form molecules or ions.

 Molecule :-  A molecule is a group of two or more atoms which are chemically bonded with each other which can exist in nature independently. A molecule shows all properties of that substance.

 Atoms of the same element or of different elements can join together to form molecules. Ex N₂, H₂

 E.g., ‘H₂O’ is the smallest particle of water which shows all the properties of water.

 Types of Molecule :-

 A molecule may have atom of same or different elements, depending upon this, molecule can be categorized into two categories:-

 

·         Homoatomic molecules or Molecules of Elements: The molecules of an element are constituted by the same type of atoms.

            Eg., O₂, N₂, O₃, S₈, P₄ etc.

·         Heteroatomic molecules or Molecules of compound: Atoms of different elements join together in definite proportions to form molecules of compounds.

Eg., H₂O , NO₂, SO₂ etc.

Atomicity: 

The number of atoms present in a molecule of an element or a compound is known as its atomicity.

For example,

·         atomicity of  argon (Ar) is 1 so called monoatomic,

·         atomicity of oxygen (O₂) is 2 so called diatomic

·         atomicity of ozone (O₃) is 3 so called triatomic,

·         Atomicity of sulphur(S₈) is 8 called polyatomic.

 

 

Ion:

It is an electrically charged atom or group of atoms. It is formed by the loss or gain of one or more electrons by an atom.

Ions are of two types:

(i) Cation: It is positively charged ion and is formed by the loss of one or more electrons from an atom

For example: sodium atom, loses one electron to form a sodium ion Na

Na – e⁻  →  Na⁺

(ii) Anion: It is a negatively charged ion and is formed by the gain of one or more electrons by an atom.

For example a chlorine atom gains one electron to form a chloride ion Cl⁻.

Cl + e⁻  →  Cl⁻

Valency:

It is defined by the combining power (or capacity) of an element.

Depending on their valency, elements can be classified as following:

(i) Monovalent cation: Having cationic valency of 1.

For example: Sodium ion (Na⁺ ). Potassium ion (K⁺ ), Hydrogen ion (H⁺ ).

Monovalent anion: Having anionic valency of -1.

For example: Chloride ion (Cl^-), Bromide ion (Br^-)

(ii) Divalent cation: Having cationic valency of 2.

For example: Magnesium ion (Mg²⁺), Ferrous ion (Fe²⁺).

Divalent anion: Having anionic valency of −2.

For example: Oxide ion (O²⁻), Sulphide ion (S²⁻).

(iii) Trivalent cations: Having cationic valency of 3.

For example: Aluminium ion (Al³⁺), Ferric ion (Fe³⁺).

Trivalent anion: Having anionic valency of -3.

For example: Nitride ion (N^-3), Phosphate ion (PO₄³⁻) etc.

 

Formula of Simple and Molecular Compounds:

 

Steps to construct the chemical formula of a compound:

 

(i)            While writing the chemical formulae for compounds, write the constituent elements with their valencies written down the respective elements.

(ii)          For a compound made up of a metal and a non-metal, the symbol of metal is written first.

(iii)         Then crossover the valencies of the combining atoms as shown in the following examples.

(iv)         In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.

Molecular Mass:

The number of times a molecule of a compound is heavier than the 1/12 of the mass of C-12 atom, is known as its molecular mass.

The molecular mass is equal to the sum of the atomic masses of all atoms present in one molecule of the substance.

For example, Molecular mass of H₂O= 2 × Mass of one H-atom + Mass of one O-atom = 2 ×1 + 16 = 18 u.

 

Formula unit mass:

It is the sum of the atomic masses of all atoms in a formula unit of a compound.

Formula unit mass is used for those substances whose constituent particles are ions.

For example, formula unit mass of ionic NaCl = 23 + 35.5 = 58.5 u.

Mole Concept:

 

→ Mole: A collection of 6.023 × 10²³ particles is named as one mole.

        1 mole = 6.023 × 10²³ particles = Mass of 1 mole particles in grams

→ The mass of 1 mole particles is equal to its mass in grams.

        1 mole atoms = gram atomic mass

        1 mole molecules = gram molecular mass

→ Avogadro’s constant or Avogadro’s number: 

The number of particles present in one mole (i.e. 6.023 x 10²³ particles) is called Avogadro’s number or Avogadro’s constant.

 

        

No of molecules / atoms in given mass of substance

= no of moles of that substance*  6.023 x 10²³   molecules / atoms

Que: If one mole of carbon atoms weigh 12 grams, what is the mass (in grams) of 1 atom of carbon?