Classified Notes
ATOMS AND MOLECULES
INTRODUCTION
The structure of matter has been a subject of speculation from very early times. According to greek philosopher
Democritus, if we go on dividing matter into smaller parts, a stage would be reached when particles obtained cannot be
divided further. He called these particles 'atoms' meaning indivisible.
Conclusion:
All matter is made up of small particles called atoms. Different kinds of atoms and molecules have different properties due
to which different kinds of matter also show different properties.
LAWS OF CHEMICAL COMBINATION
By studying the result of quantitative measurement of many reactions it was
observed that whenever substances react,
they follow certain laws. These laws are called the law of chemical
combination.
1. Law of conservation of mass.
2. Law of constant proportions.
3. Law of multiple proportions.
4. Law of conservation of mass ;
Law of Conservation of mass:
This law was given by the french chemist A.
Lavoisier in 1774. This law states that in every chemical reaction, the total mass before and after the reaction remains
constant.
"That is mass can neither be created nor destroyed in a chemical reaction". Lavoisier showed that when mercuric oxide
was heated, it produced free mercury and oxygen. The sum of masses of mercury and oxygen was found to be equal to
the mass of mercuric oxide.
Law of Definite Proportions:
This law was given by the french chemist A.
Levoisier and Joseph Proust. This law deals with the composition of chemical compounds.
This law is : A pure chemical compound always contains same elements combined together in same proportion by mass.
For example: Pure water obtained from different sources such as river, well etc. always contains hydrogen and oxygen
combined together in the ratio 1: 8 by mass similarly carbon dioxide can be obtained by different methods such as by
burning of carbon, by heating lime stone. It shows that samples of carbon dioxide obtained from different sources contain
carbon and oxygen in the ratio of 3: 8 by mass. Thus in water or CO2 this proportion of hydrogen and oxygen or
carbon and oxygen always remains constant.
Ex. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What weight of oxygen gas would be
required to completely react with 3 g of hydrogen gas.
, Classified Notes
Sol. Ratio in which hydrogen and oxygen
combine = 1:8
1 g of hydrogen cmbines with oxygen = 8 g
3 g of hydrogen will combine with oxygen = 8 x
3 = 24 g.
LAW OF MULTIPLE PROPORTIONS:
It was given by Dalton in 1808. According to it, when one element combines with the other element to form two or more
different compounds, the mass of one element, which combines with a constant mass of the other,
bear a simple ratio to one another. Example:
Carbon and oxygen when combine, can from two oxides that are CO (carbon monoxide),
CO2 (carbon dioxide).
In CO, 12g carbon combine with 16g of oxygen. In CO2, 12g carbon combine with 32g of oxygen. Thus, we can see the
mass of oxygen which combine with a constant mass of carbon (12g) bear simple ratio of 16:32 or 1:2.
DALTON'S ATOMIC THEORY
On the basis of laws of chemical combination
John Dalton proposed atomic theory in 1808.
The main postulates of the Dalton's atomic theory are :
1. All matter is made up of very tiny particles called atoms.
2. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction,
3. Atoms of a given element are identical in mass size and chemical properties.
4. Atoms of different element have different mass size and chemical properties.
5. Atoms combine in the ratio of small whole numbers to form compounds.
6. The relative number and kinds of atoms are constant in a given compound.
7. Atoms of same element can combine in more than one ratio to form more than one compound. For example,
hydrogen and oxygen combine to give water and hydrogen peroxide. in water, two atoms of hydrogen combine with one
atom of oxygen while in hydrogen peroxide, two atoms of hydrogen combine with two atoms of oxygen. for example:
The postulates of Dalton's atomic theory that "atoms can neither be created nor destroyed", was the result of law of
conservation of mass and the postulates of Dalton's atomic theory that "the element consist of atom having fixed mass",
and that the number and kind of atom in a given compound is fixed came from the law of constant proportions.
Drawbacks of Dalton's Atomic Theory:
Some of the drawbacks of the Dalton's atomic theory of matter are given below:
According to Dalton's atomic theory, atoms were thought to be indivisible. But it is now known that atoms can be
further divided into still smaller particle called electrons, protons and neutrons
. Dalton's atomic theory said that all the atoms of an element have exactly the same mass. But it is now known that
atoms of the same element can have slightly different masses, as in case of isotopes.
Dalton's atomic theory said that atoms of different elements have different masses.
But it is now known that even atoms of different elements can have the same mass as in case of isobars.
ATOMS AND MOLECULES
INTRODUCTION
The structure of matter has been a subject of speculation from very early times. According to greek philosopher
Democritus, if we go on dividing matter into smaller parts, a stage would be reached when particles obtained cannot be
divided further. He called these particles 'atoms' meaning indivisible.
Conclusion:
All matter is made up of small particles called atoms. Different kinds of atoms and molecules have different properties due
to which different kinds of matter also show different properties.
LAWS OF CHEMICAL COMBINATION
By studying the result of quantitative measurement of many reactions it was
observed that whenever substances react,
they follow certain laws. These laws are called the law of chemical
combination.
1. Law of conservation of mass.
2. Law of constant proportions.
3. Law of multiple proportions.
4. Law of conservation of mass ;
Law of Conservation of mass:
This law was given by the french chemist A.
Lavoisier in 1774. This law states that in every chemical reaction, the total mass before and after the reaction remains
constant.
"That is mass can neither be created nor destroyed in a chemical reaction". Lavoisier showed that when mercuric oxide
was heated, it produced free mercury and oxygen. The sum of masses of mercury and oxygen was found to be equal to
the mass of mercuric oxide.
Law of Definite Proportions:
This law was given by the french chemist A.
Levoisier and Joseph Proust. This law deals with the composition of chemical compounds.
This law is : A pure chemical compound always contains same elements combined together in same proportion by mass.
For example: Pure water obtained from different sources such as river, well etc. always contains hydrogen and oxygen
combined together in the ratio 1: 8 by mass similarly carbon dioxide can be obtained by different methods such as by
burning of carbon, by heating lime stone. It shows that samples of carbon dioxide obtained from different sources contain
carbon and oxygen in the ratio of 3: 8 by mass. Thus in water or CO2 this proportion of hydrogen and oxygen or
carbon and oxygen always remains constant.
Ex. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What weight of oxygen gas would be
required to completely react with 3 g of hydrogen gas.
, Classified Notes
Sol. Ratio in which hydrogen and oxygen
combine = 1:8
1 g of hydrogen cmbines with oxygen = 8 g
3 g of hydrogen will combine with oxygen = 8 x
3 = 24 g.
LAW OF MULTIPLE PROPORTIONS:
It was given by Dalton in 1808. According to it, when one element combines with the other element to form two or more
different compounds, the mass of one element, which combines with a constant mass of the other,
bear a simple ratio to one another. Example:
Carbon and oxygen when combine, can from two oxides that are CO (carbon monoxide),
CO2 (carbon dioxide).
In CO, 12g carbon combine with 16g of oxygen. In CO2, 12g carbon combine with 32g of oxygen. Thus, we can see the
mass of oxygen which combine with a constant mass of carbon (12g) bear simple ratio of 16:32 or 1:2.
DALTON'S ATOMIC THEORY
On the basis of laws of chemical combination
John Dalton proposed atomic theory in 1808.
The main postulates of the Dalton's atomic theory are :
1. All matter is made up of very tiny particles called atoms.
2. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction,
3. Atoms of a given element are identical in mass size and chemical properties.
4. Atoms of different element have different mass size and chemical properties.
5. Atoms combine in the ratio of small whole numbers to form compounds.
6. The relative number and kinds of atoms are constant in a given compound.
7. Atoms of same element can combine in more than one ratio to form more than one compound. For example,
hydrogen and oxygen combine to give water and hydrogen peroxide. in water, two atoms of hydrogen combine with one
atom of oxygen while in hydrogen peroxide, two atoms of hydrogen combine with two atoms of oxygen. for example:
The postulates of Dalton's atomic theory that "atoms can neither be created nor destroyed", was the result of law of
conservation of mass and the postulates of Dalton's atomic theory that "the element consist of atom having fixed mass",
and that the number and kind of atom in a given compound is fixed came from the law of constant proportions.
Drawbacks of Dalton's Atomic Theory:
Some of the drawbacks of the Dalton's atomic theory of matter are given below:
According to Dalton's atomic theory, atoms were thought to be indivisible. But it is now known that atoms can be
further divided into still smaller particle called electrons, protons and neutrons
. Dalton's atomic theory said that all the atoms of an element have exactly the same mass. But it is now known that
atoms of the same element can have slightly different masses, as in case of isotopes.
Dalton's atomic theory said that atoms of different elements have different masses.
But it is now known that even atoms of different elements can have the same mass as in case of isobars.
