Atoms and Molecules – CBSE Notes for Class 9 Science

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Facts that Matter

• Law of Chemical Combination
Given by Lavoisier and Joseph L. Proust as follows:
(i) Law of conservation of mass: Mass can neither be created nor destroyed in a chemical reaction. e.g., A + B —> C + D Reactants —> Products Mass of reactants = Mass of products
(ii) Law of constant proportion: 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.
These laws lacked explanation. Hence, John Dalton gave his theory about the matter. He said that the smallest particle of matter is called ‘atom’.
• Dalton’s Atomic Theory
1. Every matter is made up of very small or tiny particles called atoms.
2. Atoms are not divisible and cannot be created or destroyed in a chemical reaction.
3. All atoms of a given element are same in size, mass and chemical properties.
4. Atoms of different elements are different in size, mass and chemical properties.
5. Atoms combine in the ratio of small whole number to form compounds.
6. The relative number and kinds of atoms are constant in a given compound.
• Atom
Atoms are the smallest particles of an element which can take part in a chemical reaction.
Size of an atom: Atomic radius is measured in nanometres.
Symbols of atoms:
(a) Symbols for some elements as proposed by Dalton:
(b) Symbols of some common elements:
• Molecule
It is the smallest particle of an element or a compound which can exist independently.
• Molecules of an element constitutes same type of atoms.
• Molecules may be monoatomic, di-atomic or polyatomic.
• Molecules of compounds join together in definite proportions and constitutes different type of atoms.
• Atomicity
The number of atoms constituting a Molecule is known as its atomicity.
• Ions
The charged particles (atoms) are called ions, they are formed by attaining positive charge or negative charge on it.
Negatively charged ion is called anion (Cl).
Positively charge ion is called cation (Na+).
• Valency
The combining capacity of an element is known as its valency. Valency is used to find out how atom of an element will combine with the atom of another element to form a chemical compound.
(Every atom wants to become stable, to do so it may loose, gain or share electrons.)
(i) If an atom consists of 1, 2 or 3 electrons in its valence shell then its valency is 1, 2 or 3 respectively,
(ii) If an atom consists of 5, 6 or 7 electrons in the outermost shell, then it will gain 3, 2 or 1 electron respectively and its valency will be 3, 2 or 1 respectively.
(iii) If an atom has 4 electrons in the outermost shell than it will share this electron and hence its valency will be 4.
(iv) If an atom has 8 electrons in the outermost electron and hence its valency will be 0 .
•Some elements show more than one valency,hence termed as variable valency.
• Chemical Formulae
Rules: (i) The valencies or charges on the ion must balance.
(ii) A metal and non-metal compound should show the name or symbol of the
metal first.
e.g., Na+ Cl–    —–> NaCl
(ii) If a compound consist of polyatomic ions. The ion is enclosed in a bracket before writing the number to indicate the ratio.
e.g., [SO4]2-  —-> polyatomic radical
H1+  SO42-    â€”–>  H2SO4
• Molecular Mass
It is the sum of the atomic masses of all the atoms in a molecule of the substance. It is expressed in atomic mass unit (u).
• Formula Unit Mass
It is the sum of the atomic masses of all atoms in a formula unit of a compound. The constituent particles are ions.
• Mole Concept
Definition of mole: It is defined as 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.
1 mole = 6.022 x 1023 in number
Molar mass = mass of 1 mole –> is always expressed in grams, and is also known as gram atomic mass.
lu of hydrogen has –> 1 atom of hydrogen lg of hydrogen has —> 1 mole of hydrogen
= 6.022 x 1023  atoms of hydrogen