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Measurement of Matter

Dalton's Atomic Theory and Laws of Chemical Combination-Group A

Dividing Matter

Matter cannot be divided infinite number of times.

 

For example, if we keep chopping a log of wood into smaller and smaller pieces, then we will reach a point when the wood will not be divisible any further. Minute particles of wood will remain and these will not be visible to the naked eye. This is true for all forms of matter.The same was believed by the early Indian and Greek philosophers. In India, around 500 BC, an Indian philosopher named Maharishi Kanad called matter as padarth and these smallest particles (atoms) as ‘parmanu’. The word ‘atom’ is derived from the Greek word ‘atomos’ which means ‘indivisible’. It was the Greek philosopher Democritus who coined the term. However, for these ancient thinkers, the idea of the minute indivisible particle was a purely philosophical consideration. 

 

 

By the end of the eighteenth century, scientists had begun to distinguish between elements and compounds. Two French chemists named Antoine Lavoisier and Joseph Proust observed that elements combine in definite proportions to form compounds. On the basis of this observation, each of them proposed an important law of chemical combination. The laws proposed by them helped Dalton formulate his atomic theory.

Dalton’s Atomic Theory 

In the early nineteenth century, an English chemist named John Dalton proposed a theory about atoms. Known as ‘Dalton’s atomic theory’, it proved to be one of the most important theories of science. The various laws of chemical combination also supported Dalton’s theory. Dalton asserted that ‘atoms are the smallest particles of matter, which cannot be divided further’. He published his atomic theory in 1808 in his book A New System of Chemical Philosophy. The postulates of Dalton’s atomic theory are as follows:

All matter is made up of very tiny particles. These particles are called atoms. An atom cannot be divided further, i.e., atoms are indivisible. Atoms can be neither created nor destroyed in a chemical reaction. All atoms of an compounds . In a given compound, the relative numbers and types of atoms are constant.

Know Your Scientist

John Dalton (1766−1844) was born into the poor family of a weaver in Eaglesfield, England. He was colour-blind from childhood. He became a teacher when he was barely twelve years old. By the time he was nineteen, he had become the principal of a school. In 1793, Dalton left for Manchester to teach physics, chemistry and mathematics at a college. Elected a member of the Manchester Literary and Philosophy Society in 1794, he became its president in 1817 and remained in that position until his death. During his early career, he identified the hereditary nature of red−green colour blindness. In 1803, he postulated the law of partial pressures (known as Dalton’s law of partial pressures). He was the first scientist to explain the behaviour of atoms in terms of relative atomic weight. He also proposed symbolic notations for various elements.

Dividing Matter

Matter cannot be divided infinite number of times.

 

For example, if we keep chopping a log of wood into smaller and smaller pieces, then we will reach a point when the wood will not be divisible any further. Minute particles of wood will remain and these will not be visible to the naked eye. This is true for all forms of matter.The same was believed by the early Indian and Greek philosophers. In India, around 500 BC, an Indian philosopher named Maharishi Kanad called matter as padarth and these smallest particles (atoms) as ‘parmanu’. The word ‘atom’ is derived from the Greek word ‘atomos’ which means ‘indivisible’. It was the Greek philosopher Democritus who coined the term. However, for these ancient thinkers, the idea of the minute indivisible particle was a purely philosophical consideration. 

 

 

By the end of the eighteenth century, scientists had begun to distinguish between elements and compounds. Two French chemists named Antoine Lavoisier and Joseph Proust observed that elements combine in definite proportions to form compounds. On the basis of this observation, each of them proposed an important law of chemical combination. The laws proposed by them helped Dalton formulate his atomic theory.

Dalton’s Atomic Theory 

In the early nineteenth century, an English chemist named John Dalton proposed a theory about atoms. Known as ‘Dalton’s atomic theory’, it proved to be one of the most important theories of science. The various laws of chemical combination also supported Dalton’s theory. Dalton asserted that ‘atoms are the smallest particles of matter, which cannot be divided further’. He published his atomic theory in 1808 in his book A New System of Chemical Philosophy. The postulates of Dalton’s atomic theory are as follows:

All matter is made up of very tiny particles. These particles are called atoms. An atom cannot be divided further, i.e., atoms are indivisible. Atoms can be neither created nor destroyed in a chemical reaction. All atoms of an compounds . In a given compound, the relative numbers and types of atoms are constant.

Know Your Scientist

John Dalton (1766−1844) was born into the poor family of a weaver in Eaglesfield, England. He was colour-blind from childhood. He became a teacher when he was barely twelve years old. By the time he was nineteen, he had become the principal of a school. In 1793, Dalton left for Manchester to teach physics, chemistry and mathematics at a college. Elected a member of the Manchester Literary and Philosophy Society in 1794, he became its president in 1817 and remained in that position until his death. During his early career, he identified the hereditary nature of red−green colour blindness. In 1803, he postulated the law of partial pressures (known as Dalton’s law of partial pressures). He was the first scientist to explain the behaviour of atoms in terms of relative atomic weight. He also proposed symbolic notations for various elements.

Valency:

We know that the outermost shell of an atom can hold a maximum of eight electrons. The elements, whose atoms have a completely filled outermost shell, have very little chemical activity. Such elements are said to have zero combining capacity or valency. For e.g., helium, neon, argon. (The elements of the 18th group in the periodic table). These elements have either completely filled outermost shells or have 8 electrons in their outermost shell. Hence, their valency is zero. They are called inert or noble gases.  

The combining capacity of atoms of the elements is their tendency to react with other atoms of the same or different molecules to attain a filled outermost shell. The outermost shell, which has eight electrons, is said to possess an octet and every atom tends to achieve an octet in its outermost shell. This is done by gaining, losing, or sharing its electrons. The number of electrons gained, lost, or shared by an atom to complete its octet is called the combining capacity or valency of that atom.

Both hydrogen and sodium contain one electron each in their outermost shells. Thus, both can lose one electron. Hence, their valency is one.

It is not always true that the number of electrons present in the outermost shell of an atom represents its valency. For example, in fluorine, there are seven electrons in the outermost shell, but the valency of fluorine is one. This is because it is energetically suitable for fluorine atom to accept one electron, rather than donating seven electrons. Hence, its valency is obtained by subtracting seven electrons from the octet.

Concept of valency

We know that the combining power or the combining capacity of an atom or an element is called its valency. The number of atoms of other elements with which one atom of an element combines is decided by the valency of that element.

For example, both hydrogen and chlorine have a valency of 1. Therefore, one atom of hydrogen reacts with one atom of chlorine to form one molecule of hydrogen chloride.

The valency of an ion is equal to the charge on it. The valencies of some commo…

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