CBSE Class 9 Science Atoms and Molecules Notes

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ATOMS AND MOLECULES

LAWS OF CHEMICAL COMBINATIONS

Before Dalton concept of atom was mere philosophical. Dalton explained about atom on the basis of Laws of Chemical Combinations.

There are three laws of chemical combination.

1. Law of Conservation of Mass

2. Law of Constant Proportions

3. Law of Multiple Proportions

LAW OF CONSERVATION OF MASS

Antoine L. Lavoisier, a French scientist, established the theory of Law of Conservation of Mass. The law of conservation of mass states, “Mass can neither be created nor destroyed in a chemical reaction”.

All matters in the universe exist in three states. There are two ways of classification of matter.

1. According to physical state as solid, liquid or gas.

2. According to its composition as element, compound or mixture.

According to this law mass of an isolated system will remain constant over time. This means when mass is enclosed in a system and none is allowed in or out, its quantity will never change. That is mass will be conserved, and hence this is called Law of Conservation of Mass. This means total mass of products is always equal to the total mass of reactants. As there is no loss of mass of substances, i.e. mass is conserved, that’s why Lavoisier called this the law of conservation of mass.

REACTANTS AND PRODUCTS:

In a chemical reaction the substances that combine or react are known as reactants and the new substance/substances formed are called product or products.

Example: When calcium oxide is dissolved in water calcium hydroxide is formed. The reaction involve in this can be written as:

Compounds are formed by the combination of two or more elements. In a compound the ratio of the atoms or element by mass remains always same irrespective of the source of compound. This means a certain compound always formed by the combination of atoms in same ratio by mass. If the ratio of mass of constituent atoms will be altered the new compound is formed.

Examples:-

Water is formed by the combination of hydrogen and oxygen. The ratio of masses of hydrogen and oxygen is always in 1:8 in water irrespective of source of water. Whether you collect the water from a well, river, pond or from anywhere the ratio of their constituent atoms by mass will always same.

Nitrogen dioxide is a compound, which is formed by the combination of nitrogen and oxygen. The ratio of nitrogen and oxygen by mass in nitrogen dioxide is in 7:16.
Nitrous oxide is a compound which is also formed by the combination of nitrogen and oxygen. The ratio of nitrogen and oxygen in nitrous oxide is in 28:16.
Nitric oxide is a compound, which is also formed by the combination of nitrogen and oxygen. The ratio of nitrogen and oxygen in nitric oxide is in 7:8.
From the above three examples it is clear that if the ratio of the atoms by mass is altered then the new compound is formed, such as in the case of nitrogen dioxide, nitrous oxide, nitric oxide. These three compounds are formed by the combination of same atoms but because of combination of the constituent atoms in different ratios by mass new compound is formed.

DALTON’S ATOMIC THEORY

John Dalton, a British Chemists and scientists gave the Atomic Theory in 1808. This theory is popularly known as Dalton’s Atomic Theory in the honour of John Dalton. He gave the theory on the basis of Laws of Chemical Combination and explains them properly. In his theory he explains about atom.

Main postulates of Dalton's atomic theory

1. Elements are made of extremely small particles called atoms.

2. Atoms of a given element are identical in size, mass, and other properties;

3. Atoms of different elements differ in size, mass, and other properties.

4. Atoms cannot be subdivided, created, or destroyed.

5. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

6. In chemical reactions, atoms are combined, separated, or rearranged.

INTEXT QUESTIONS PAGE NO. 32

Q1. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show
that these observations are in agreement with the law of conservation of mass. sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water

Answer: In the given reaction, sodium carbonate reacts with ethanoic acid to produce sodium ethanoate, carbon dioxide, and water.
sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water
Mass of sodium carbonate = 5.3 g (Given)
Mass of ethanoic acid = 6 g (Given)
Mass of sodium ethanoate = 8.2 g (Given)
Mass of carbon dioxide = 2.2 g (Given)
Mass of water = 0.9 g (Given)
Now, total mass before the reaction = (5.3 + 6) g = 11.3 g
And, total mass after the reaction = (8.2 + 2.2 + 0.9) g = 11.3 g
Therefore, Total mass before the reaction = Total mass after the reaction Hence, the given observations are in agreement with the law of conservation of mass.

Q2. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

Answer: It is given that the ratio of hydrogen and oxygen by mass to form water is 1:8. Then, the mass of oxygen gas required to react completely with 1 g of hydrogen gas is 8 g.
Therefore, the mass of oxygen gas required to react completely with 3 g of hydrogen gas is 8 × 3 g = 24 g.

Q3. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Answer: The postulate of Dalton’s atomic theory which is a result of the law of conservation of mass is: Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction.

Q4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Answer: The postulate of Dalton’s atomic theory which is a result of the law of conservation of mass is: Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction. ATOMS On the basis of Dalton’s Atomic On the basis of Dalton’s Atomic Theory atom can be defined as the smallest particles of matter are called atoms.

Characteristics of atoms:

• Atom is the smallest particle of matter.

• All elements are made of tiny particles called atom.

• Atoms are very small in size and cannot be seen through naked eyes.

• Atom does not exist in free-state in nature. But atom takes part in a chemical reaction.

• The properties of a matter depend upon the characteristics of atoms.

• Atoms are the building block of an element similar to a brick which combine together to make a building.

• The size of atoms is indicated by its radius.

• In ancient time atoms was considered indivisible.

SYMBOLS OF ATOMS OF ELEMENTS

Dalton was the first scientist to use the symbols for elements in a very specific sense. When heused a symbol for an element he also meant a definite quantity of that element, that is, one atom of that element. Berzilius suggested that the symbols of elements be made from one or two letters of the name of the element.

Many of the symbols are the first one or two letters of the element’s name in English. The firstletter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase).

For convenience elements are represented by unique symbols. For example: Hydrogen is represented by ‘H’. Oxygen is represented ‘O’. Nitrogen is represented by ‘N’. Iron is
represented by ‘Fe’. ments are represented by unique symbols. For example: 

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