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(a) A Triatomic molecule 

Element : Ozone O 

Molecule : O₃ 

Atomicitv : 3 

(b) A polyatomic molecule 

Element : Phosphorus p 

Molecule : p₄ 

Atomicity : 4

The elements which form water are 

(i) Hydrogen and oxygen. 

Justification : Water has entirely different properties (i.e. is a liquid, extinguishes fire) from the elements it is made up of i.e. Hydrogen a gas catches fire oxygen a gas supporter of combustion. 

1. Energy is needed to form water on combining O₂ with H₂. 

2. We can not seperate the constituents of water by simple physical means.

(a) Metals : Iron, silver, gold. 

(b) Non-metals : Carbon, sulphur, oxygen. 

(c) Metalloids : Antimony, silicon, boron. 

(d) Noble gases : Helium, argon, neon.

For arranging all the elements in a systematic and simple manner. The arrangement of elements was done in the form of a table called Periodic Table in which elements are arranged in increasing order of their atomic numbers. Elements in the Periodic Table are arranged in Horizontal rows called Periods and vertical columns called Groups. Metallic elements are placed on the left non-metallic on the right and noble gases on the extreme right of the Periodic table.

Element is a pure substance. 

1. It is the basic unit of matter and cannot be broken down into two or more simpler substances by any means. 

2. It is mainly classified into metals, non-metals, metalloids and noble gases. 

Compound is a pure substance. 

1. It is formed by combination of two or more elements. 

2. The elements are combined together in a fixed ratio. 

3. It can be broken down into its elements by chemical means. 

Mixture is an impure substance. 

1. It is formed by combination of two or more elements, compounds or both. 

2. The substances are mechanically mixed together in any ratio.

Metals are ductile, i.e., they can be drawn or stretched into thin wires. They are malleable, i.e., they can be beaten into thin sheets.

A radical is an atom or group of atom of same or different elements that behaves in the manner of positive or negative ions. 

(a) No₃ – nitrate 

(b) OH – hydroxide 

(c) So₄ – sulphate 

(d) Co_{3 –} carbonate

(a) (i) Chlorine – greenish yellow ,

(ii) Nitrogen dioxide – reddish brown. 

(b) Ammonia. 

(c) Carbon monoxide

(a) K — Valency-1 (C) 

(b) Zn — Valency-2 (B) 

(c) A1 — Valency-3 (A) 

(d) Na— Valency-1 (C) 

(e) Ca — Valency-2 (B)

Importance of symbols and Formulae : 

Symbols and formulae of substance gives a lot of information like. 

1. Types of elements present in the compound. E.g. (H₂0 is made of two elements hydrogen and oxygen). 

2. Number of each kind of atoms in one molecule. E.g. (water has 2 atoms of hydrogen combined with 1 atom of oxygen.) 

3. Mass of one molecule of the compound. E.g. [H₂O has mass (1 × 2) + 16 = 18 g].

1. Symbols increases scientific communication across the world. 

2. Symbols helps to make equations and data shorter and concise. 

3. Symbols are helpful for Scientists as it would take time and paper to do their job. They had to write out the full name of every element instead of its symbol.

Element: Mercury, sulphur, gold, coal, oxygen. 

Compound : Sugar, water, sand, alcohol

A formula gives us the following information about a compound. 

1. Types of elements present in the compound. 

2. Number of each kind of atoms in one molecule of the compound. 

3. Mass of one molecule of the compound.

Example: 

A molecule of carbon dioxide gas is represented by CO₂ It indicates that a carbon dioxide molecule is formed by the combination of two elements i.e. carbon and oxygen. The number of carbon atom is one and that of oxygen atom is two. The mass of one molecule of carbon dioxide can be calculated by adding the mass of one atom of carbon and two atoms of oxygen.

(a) Iron : To make machines tools and building material. 

(b) Brass : To make water taps and utensils. 

(c) Coal: Coal is used as fuel also used in thermal power plant to produce electricity.

(a) 2H is two atoms of hydrogen. H₂ is one molecule of hydrogen gas. 

(b) H₂0 represents one molecule of water. 3H₂0 represents 3 molecules of water.

(a) 1. Diamond 

2. Graphite 

3. Coal

(b) Diamond is used as gem. 

(c) 1. Copper. 

2. Aluminium as these are good conductors of electricity. 

(d) 1. Gold. 

2. Silver as these are shining, lustrous, and ductile. 

(e) Plastic is used as insulator as it is bad conductor of electricity.

(a) Elements : An element is defined as a pure substance made up of only one kind of atoms that cannot be converted into anything simpler than itself by any physical or chemical process. 

(b) Compounds : Compounds are pure substances composed of two or more elements in definite proportion by mass and has a definite set of properties. Compound is made up of only one kind of molecules.

Pure substances : “A substance of a definite composition which has consistent properties throughout, is called a pure substance” 

Types of pure substances: Pure substances are of two types 

(i) Elements, 

(ii) Compounds.

(a) False 

Correct : A compound is made up of two or more elements is a fixed proportion by mass.

(b) True

(c) True

(d) False 

Correct : Elements are made up of atoms.

(e) False 

Correct: All elements are made up of a limited number of basic substances

(f) True

(g) False 

Correct : Atoms combine to form molecule.

(h) False 

Correct : Atoms combine to form molecule.

(i) True

An element is a pure substance made up of identical atoms.

A : Non-metallic element 

B :  Metallic element 

C : Noble gas. 

D : Metalloid

Symbols : The short form or abbreviated name of the element (or radicals) 

The reason for representing the following elements by their symbols are as follow : 

(a) Hydrogen by ‘H’ – First letter of the name of element 

(b) Helium by He – First two letters of the name of Noble gases. 

(c) Copper by ‘Cu’ – Deriving symbols from their Latin names

Short forms of Dr. – Doctor, H.M. – Headmaster, AC – Air conditioner, Adv – Advance, C.M. – Chief Minister, DC – Direct current

1. A compound is a pure substance that contains two or more elements. Compounds are chemical combination of elements with properties that are different from the elements that formed them.

2. Substance is a compound only if its molecules are made up of atoms of different types e.g. Water, one molecule of water is made of two atoms of hydrogen and one atom of oxygen.

Atoms are basic building blocks of matter. Desk, chair, air are made up of atoms.

1. Churning 

2. Sedimentation and decantation 

3. hand picking 

4. filtration 

5. distillation 

6. separating funnel 

7. Centrifugation 

8. Chromatography

1. Distillation: It is a process of heating a solution containing soluble solids to form vapours of the liquid and then cooling the vapours to get the liquid back. e.g. A mixture of common salt and water is taken in distillation flask and heated. Steam rises up and comes out into condenser.

2. Sublimation: It is a process in which some solids on heating are transformed directly to vapour without passing through the liquid state, (i) It is used to separate a mixture of solids. The vapours are cooled separately, (ii) Used to separate ammonium chloride, iodine, camphor and sulphur from any mixture.

3. Magnetic separation: A mixture with iron fillings as one of the components can be separated using magnet to attract the iron particles away from the mixture.

4. Sedimentation: (i) It is a process of separating an insoluble solid from a liquid in which it is suspended, by allowing it to settle to the bottom of the container, e.g. Muddy water contains heavier particles like sand and soil, (ii) Leave this muddy water undisturbed for some time, (iii) The heavier soil and sand particles settle down and the clear upper water is poured out by decantation.

5. Filtration: It is used for separating insoluble solids from a liquid, e.g. mixture of chalk and water is poured through a filter paper in a funnel while the water gets collected in the beaker below, chalk is retained in the filter paper.

6. Evaporation: It is used for recovering dissolved solid substances from solutions by evaporation e.g. sugar can be recovered from sugar-water.